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\begin{document}

\noindent
\centerline{\Large {\bf Section D:  Science Investigation}}

\section{Overview}

\subsection{Executive Summary}
\label{summary}

\fbox{\parbox{3.05in}{
{\bf  The fundamental process in the evolution of galaxies is the
transformation of the interstellar medium (ISM) into stars and the
subsequent return of material back to the ISM.  Yet many phases
of this lifecycle, such as the formation and destruction of
molecular clouds, are poorly observed.  STIM, the Space
Terahertz Interstellar Mapper, will for the first time provide
maps with sufficient angular and velocity resolution, coupled
with expansive spatial coverage, to reveal these processes on a
galactic scale.  STIM, a free-flying observatory with an eight-
beam heterodyne receiver system, will produce maps in the
1.9 THz line of ionized carbon, the dominant emission line of
interstellar medium, and the 1.46 THz line of ionized nitrogen,
the third brightest far-infrared line and a key diagnostic of the
ionization state of the ISM.  STIM has a highly experienced Principal
Investigator, Project Manager, and investigation team, and it uses
Herschel-demonstrated receiver technology, an existing, flight
qualified helium cryostat, and a well-proven spacecraft design.  STIM
directly addresses the NASA research objectives 3D.2 (the
evolution of galaxies) and 3D.3 (star formation). }}}

\medskip

STIM will survey more than 280 square degrees of the Galactic
plane in ionized carbon (\cii) line emission at 1.9~THz (158 $\mu$m), the brightest spectral
line in the Galaxy; and ionized nitrogen (\nii) line emission at
1.46~THz (205 $\mu$m), a tracer of the star formation rate.
At $\sim$1.5$'$ angular resolution and $<$1 km/s velocity resolution,
{\bf STIM will detect every interstellar cloud with \boldmath $A_V \ge
  0.4$ \unboldmath in
the surveyed region}, and, through excitation and kinematic
diagnostics provided by
\cii\ and \nii\ line emission, {\bf will illustrate how atomic and molecular
clouds are formed and dispersed in the Galaxy}.
STIM will make 3-dimensional maps of the structure, dynamics, turbulence,
energy balance, and pressure
of the Milky Way's Interstellar Medium (ISM), as well as the
star formation rate.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=1.2,angle=0]{../figures/surveys-topview.eps}
\caption{\label{GLIMPSE} { \it The proposed regions to be surveyed by
    STIM in its Galactic Plane Survey (GPS) and Targeted Deep Survey
    (TDS) modes.  The GPS 120$^\circ$ longitudinal
    swath of the Galactic Plane reveals major
    components of the Milky Way ISM, such as the Galactic Center, molecular ring,
    spiral arms, and interarm regions. The targeted surveys cover
    Outer Galaxy targets like the Orion GMC and nearby external
    galaxies like the LMC. }}
\end{figure}


\subsubsection{Summary of Science Objectives}

STIM will provide a comprehensive understanding of the inner workings of
our Galaxy by exploring the connection between star formation and the
life cycle of interstellar clouds.  We will study the formation
of molecular clouds from diffuse atomic gas, the feedback of high mass
star formation on the lives of atomic and molecular clouds, and the
effect of these
processes upon the global structure and evolution of the Galaxy.
The detailed
understanding of star formation and evolution of stars and gas in the
Galaxy is directly relevant to star formation in other galaxies.  The
nature of the feedback mechanism of massive star formation with its
interstellar environment is pivotal to the evolution of galaxies.
{\bf Within NASA's research objectives on galaxy evolution and star
formation, STIM addresses the following high priority goals}:

\begin{enumerate}
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\item Determine the anatomy and topology of interstellar gas
\item Study the creation and disruption of star-forming
  clouds in the Galaxy.
\item Determine the parameters that affect the star formation
  rate in a galaxy.
\item Measure the energy balance and mass flow within the Galactic Center
\item  Provide templates for star formation and (inter)stellar
feedback in other galaxies.
\end{enumerate}


\subsubsection{Summary of Mission Approach}
\label{obsmodes}

STIM will utilize two heterodyne
receiver arrays to produce a total of eight 1.5$'$ pixels in the focal plane, each with 1024
spectral channels.  These array components will be directly leveraged from the
Herschel HIFI instrument design, to maximize flight heritage and minimize
cost and risk.  STIM will
map more than 280 square degrees of the inner Galaxy including the
Galactic Center, Galactic molecular ring (Figure~\ref{GLIMPSE}) as
well as several arm and interarm regions. STIM will detect and
resolve both spectrally and spatially all Giant Molecular Clouds (GMCs),
all significant \hii\ regions, and all cold neutral medium (CNM) atomic clouds
with $A_V \ge 0.4$ mag in the surveyed region.

The STIM heterodyne receivers provide sub-km/s velocity discrimination
and sufficient bandwidth to detect and resolve line emission from
every Galactic cloud in the surveyed region.  The data products will include:
\begin{enumerate}
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\item  A high fidelity database of spatially and velocity resolved
far-infrared \cii\ 1.9~THz (158 \micron) and \nii\ 1.46~THz (205 \micron) fine-structure line
emission in the Galaxy.

\item  A combination of STIM's data products with existing line and continuum
surveys to characterize the structure and dynamics of interstellar clouds
and their relation to star formation.
\end{enumerate}

\noindent The data are produced in large scale (Galactic Plane Survey) and
selective (Targeted Deep Survey) modes:
\begin{itemize}
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\item {\bf GPS: Galactic Plane Survey} spanning the scale height of
  neutral interstellar gas in the Plane, and providing an expanded
  panoramic survey of the central $\sim$1~kpc of the Milky Way:
            $-60^\circ > l > 60^\circ$; $-1^\circ < b < 1^\circ$,
  extended to $-2.2^\circ < b < 2.2^\circ$ for the Galactic Center
  region, $|l| < 10^\circ$.
\item {\bf TDS: Targeted Deep Surveys} of selected regions in the
  Outer Galaxy, including a survey of the entire
  {\bf Orion GMC} and a {\bf Magellanic Cloud Survey} of selected
  regions in the LMC and SMC, and a square-degree survey of the nearby spiral galaxy M33.

\end{itemize}

STIM's launch date, mission lifetime, and capabilities make it ideal
for placing the Milky Way in the context of external galaxies that
will be observed in the same spectral lines with Herschel.


\subsection{Science Background}

\fbox{\parbox{3.05in}{
\bf Via spatially and spectroscopically resolved \cii\ and \nii\ line
emission, STIM probes uniquely the pivotal
formative and disruptive stages in the life cycles of interstellar clouds.
It provides new insight into the relationship between interstellar clouds
and the stars that form from them, a central component of galactic
evolution. }}

\medskip

Neutral interstellar gas is the dominant mass component of the ISM,
and tends to exist as
two phases in rough thermal pressure equilibrium: a diffuse warm neutral
medium (WNM) with hydrogen densities at the solar circle of {\it
  n}$\sim$0.3~cm$^{-3}$ and {\it T}$\sim$8000~K, and a denser, cold
neutral medium (CNM) with {\it
  n}$\sim$40~cm$^{-3}$ and T$\sim$70~K \citep{kh87,wolf03}.
Turbulence provides a broader spectrum of conditions
\citep{mac05,gaz05}, but thermal balance drives neutral gas toward these phases.
With sufficient shielding column, $N > 10^{20} - 10^{21}$~cm$^{-2}$
of hydrogen nuclei, the CNM clouds begin to harbor molecular interiors. Above $N \sim
10^{22}$ cm$^{-2}$ they
become fully-molecular, gravitationally bound and stars may form in their
interiors \citep{mckee89}.  The largest condensations take the form of GMCs
 with large masses $10^5 - 10^6 \; {\rm M}_\odot$ and are responsible for
most of the star formation in the Galaxy. These ISM components are shown
schematically in Figure~\ref{ISM-phases}.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.33,angle=0]{../figures/ISM_phases.eps}
\caption{\label{ISM-phases} {\it Schematic representation of ISM
    components.  STIM detects and maps in the Galaxy the higher column
    density CNM component, the \htwo/\cplus\ component, the
    photodissociation region (PDR) surfaces of molecular clouds, the
    \hii\ component and (with \hi) the WNM/CNM ratio.
}}
\end{figure}

Ultraviolet radiation from massive stars not only heats the CNM and WNM
and determines the relative portions of these phases, but it also ionizes
gas and heats it to 10$^4$ K, producing \hii\ regions.
The destructive impact of UV radiation on the surfaces of neighboring
clouds is a key part of the stellar/interstellar feedback that governs
galactic evolution.
This stellar feedback likely determines the fraction of cloud gas which is
converted to stars.  CNM clouds are destroyed by the interstellar UV field
produced by the global (many hundreds of pc) OB stellar population.  If global
star formation rates are high, the interstellar field is high, thereby
lowering the CNM population by converting them to ionized or WNM gas.  This conversion
then throttles the rate of GMC formation, and thus the global star formation rate.
These stellar feedback effects are likely to regulate star formation rates in
galaxies.


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.78,angle=0]{../figures/PDRplot.eps}
\caption{\label{PDRplot} {\it {\bf The uniqueness of STIM's \cii\ and
      \nii\ surveys.}  A model depiction of the intensity of
    diagnostic lines of carbon and nitrogen species as viewed through a UV-illuminated
    cloud from depths of $A_V$=0 to 20 mag.   Vertical purple lines overlay the
    HII-to-HI-to-H$_2$ boundaries found at the edges of dense interstellar
    clouds.  This figure demonstrates that \cii\ will probe H$_2$ clouds
with little CO, and depicts the need to use \nii\ to disentangle the portion of
\cii\ emission stemming from ionized gas.}}
\end{figure}


\subsection{Unique Roles of the Far Infrared Carbon and Nitrogen Lines}
\label{lines}

\fbox{\parbox{3.05in}{
\bf The far infrared fine structure lines of ionized carbon and nitrogen
represent the crucial ``missing links'' in understanding the global evolution of
interstellar gas in the Galaxy. }}

\medskip

Vast surveys in the neutral hydrogen 21~cm line and the 2.6~mm
CO~J=1-0 line have been performed and constitute our best
understanding of the Galactic atomic and molecular gas, respectively.
However the interstellar components associated with the crucial
assembly and destruction of clouds are poorly
probed by such surveys!   The ``missing links'' in the analysis of the
Galactic ISM are the fine structure lines of ionic carbon and
nitrogen; not only are they among the most powerful coolants of the
ISM (together they account for almost 1\% of the bolometric
luminosity of the Galaxy), but they probe crucial aspects of interstellar
clouds that are missed by existing surveys (Figure~\ref{PDRplot}).   {\bf
 \cii\ emission barometrically selects clouds of atomic hydrogen that
are often difficult to distinguish from WNM in maps of 21~cm \hi\ emission, and
identifies molecular clouds that are devoid of CO emission.
\nii\ line emission probes diffuse ionized gas invisible to optical and radio
recombination line surveys.  All of these points are crucial
to identifying the yet unobserved links between the evolutionary
stages of interstellar clouds.}

Since \cii\ line emission efficiently probes both neutral and ionized
gas (Figure~\ref{PDRplot}), it is
necessary to disentangle the two components in order to cleanly interpret the
line emission.   The 1.46~THz \nii\ line stems from the same
low-density ionized gas as \cii; therefore
it provides a way to separate the multiple
excitation components that comprise the \cii\ emission.  In its own
right, the \nii\ line provides a sensitive extinction-free probe of ionizing photons
in the Galaxy, a measure of the star formation rate necessary to
build a Schmidt Law for the Milky Way, a goal of this mission.

\subsection{Overview of STIM Capability}

\fbox{\parbox{3.05in}{
\bf STIM is a uniquely capable mission that brings many orders of magnitude
more angular and spectral resolution to the study of \cii\ and
\nii\ in the Galaxy than the full Galactic Plane survey performed
by the FIRAS instrument on COBE.  It will be the first mission to
spatially {\it and} spectrally resolve individual clouds
and their constituent interstellar components on a galactic scale.}}

\medskip

STIM achieves 3$\sigma$ intensity limits of
$3.5\times10^{-5}$~(t/1 sec)$^{-1/2}$ and $1.5 \times
10^{-5}$~(t/1 sec)$^{-1/2}$ erg s$^{-1}$ cm$^{-2}$ sr$^{-1}$
in the \cii\ and \nii\ lines, respectively.
With this sensitivity, STIM can detect the GMCs, CNM clouds,
and \hii\ regions discussed above.  The CNM clouds typically
have sizes r$\sim$3~pc and subtend $>1'$ of angle at a distance of
8.5~kpc, filling the STIM beam.  These CNM clouds will therefore be
both spatially and spectrally resolved.  STIM will resolve in \cii\ the
surfaces of all GMCs illuminated by the local (or brighter) interstellar radiation field and
the \cii\ and \nii\ emission from \hii\ regions with Emission
Measure (EM) $> 50$~cm$^{-6}$~pc. \cii\ originates in both neutral and ionized gas,
whereas \nii\ arises solely from ionized gas. STIM \nii\ observations
will provide the most sensitive and
detailed maps of star formation rates in the Galaxy, and are crucial for
separating the ionized and neutral components of \cii\  emission.

\smallskip

{\noindent \bf The main features of the STIM survey are: }
\begin{itemize}
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\item  High spatial resolution, $\sim$1.5 arcmin (3 pc at $d=8$~kpc).
\item  Very high spectral resolution, $<$1 km/s.
\item  High dynamic range: $10^5$ spatially and $10^3$ spectrally
\item  More than $10^6$ spatial pixels
\item  High sensitivity:  STIM
  will catalogue all neutral clouds with columns
 $N>5\times10^{20}$~cm$^{-2}$ and all ionized
 clouds with EM $> 50$~cm$^{-6}$~pc.   Convolved to the 7$^\circ$
 angular resolution and 3000~\kms\ spectral resolution of FIRAS/COBE,
 the STIM baseline surveys are 10$^5$ times more sensitive!
\end{itemize}

\noindent These capabilities will be put in broader context in Section~\ref{requirements}.


\section{Science Goals \& Objectives}

\subsection{Revealing the Anatomy of Interstellar Gas}

\fbox{\parbox{3.05in}{
\bf STIM will provide an unprecedented 3-dimensional global map of the distribution of clouds
of  ionized gas,  atomic gas, and molecular clouds (via their dense atomic surfaces)
as a function of radius ($R$) and height ($z$) in the
Galaxy.  It will allow measurement of the gas heating rate and
interstellar pressure to be made throughout the Galaxy.
It will reveal poorly understood interstellar components;  cold atomic
(CNM) clouds that are not always discernable from \hi\ emission and molecular
clouds that are devoid of CO emission. }}

\medskip

STIM's unique blend of sensitivity, angular resolution and spectral
resolving power allows it to map and diagnose {\it all components of the warm and
cold interstellar medium}.  STIM will survey the Galactic Center,
spiral arms, interarm regions, and the molecular ring,
where much of the star formation occurs in the Galaxy.
Galactic rotation allows velocity separation of the clouds along the line of sight.
We can compute the density of
clouds (i.e., the number of clouds per kpc$^3$) and their size distribution
as functions of $R$ and $z$, and see how clouds are clumped together in spiral
arms or supershells. In regions of cloud clustering, the superb velocity
resolution of STIM will measure the random motions of clouds, and diagnose
{\it large scale turbulence}.

COBE FIRAS observations show that the ionized component of the ISM
radiates strongly in both \cii\ 158 $\mu$m  and \nii\ 205 $\mu$m
\citep{wright91}.  To distinguish the origin(s) of \cii\ emission,
velocity-resolved measurements of the small scale distribution of
the ionized gas must be made in \nii\ and compared to the \cii\
distribution. {\bf STIM will conclusively determine the origin of
the \cii\ emission from various regions in the Galaxy}, and will
enable the portion of the \cii\ emission coming from the CNM neutral
gas to be unambiguously determined.

If the CNM clouds are seen in \hi, which determines their column and
mass, the ratio of CNM \cii\ to \hi\ intensity provides a measure of
the \cii\ emissivity per H atom which rises monotonically with gas
density and thermal gas pressure.  {\bf STIM will survey over a
large portion of the Galactic Plane, thereby enabling the
construction of the first barometric maps of the Galactic disk}.
These maps will be used to determine the ambient thermal pressure in
different environments (e.g., the spiral arms versus interarm
regions, turbulent versus quiescent regions). The pressure maps and
the maps of cloud distributions and properties can be correlated
with star formation rates to understand stellar/interstellar
feedback mechanisms.

Where extended emission is seen in \hi\ with no \cii\ counterpart, we
can attribute the \hi\ emission to
extended low density gas -- either WNM or thermally unstable gas with
densities below that of CNM (Figure~\ref{simulatedspectra}). To
achieve the required sensitivity for this extended gas,
we will smooth the data to larger ($\sim$10
km/s) velocity and spatial ($\sim10'$) bins. In this way, STIM can map the
CNM/WNM mass fraction in the Galaxy, and determine how much of the neutral
gas is in clouds rather than in warm or unstable components.

In addition, the \cii\ line
dominates the cooling of CNM clouds. Therefore, we directly obtain
the gas heating rate of clouds as a function of radius throughout the
Galaxy.  Besides the fundamental interest in tracing the energy flow in
the Galaxy, the observations also can test our theoretical hypothesis that
the heating is provided by the grain photoelectric heating mechanism in
diffuse clouds.  This hypothesis has been tested (positively) in denser
clouds illuminated by stronger fields, but it is not yet certain whether in
weak UV fields other mechanisms (i.e. ambipolar diffusion of the
magnetic field) may be important.  The test involves
comparing the heating with the observed incident radiation field and the
gas density in a sample of clouds.



\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.7,angle=0]{../figures/simulation_cube.eps}
\caption{\label{padoan} {\it The ISM is a complex environment with
    structures on all scales and velocity dispersions.  The
    surfaces of clouds corresponding to $n = 30$~cm$^{-3}$ are shown
    within a 6 pc region from a 3-D magnetohydrodynamical simulation
    of cloud formation \citep{juvela03}.
    Neither COBE nor BICE had the
    spatial and spectral resolution to unravel the structure of the
    interstellar clouds or their formation process.
}}
\end{figure}


\subsection{Witnessing the Formation \& Destruction of Clouds}

\fbox{\parbox{3.05in}{
{\bf The formation of interstellar clouds is a prerequisite for star formation,
yet the process has not yet been directly
observed! STIM is designed with the unique
combination of sensitivity and resolution in the \cii\ line needed to observe
cold atomic clouds assembling into giant molecular clouds (GMCs).}
}}

\medskip

The overall topology of the cold, neutral medium is currently unknown.
\hi\ 21~cm line emission does not distinguish contributions from warm and cold
atomic material.  Yet, the configuration and velocities
of the cold, neutral clouds are central to all theories that describe
the formation of giant molecular clouds.
Currently, our principal diagnostic of these clouds is absorption
of the \hi\ 21cm line and optical lines (Ca) toward continuum sources.
Such pencil-beam measurements do not reveal the gas distribution and
kinematics of these critical regions.  With its ability to spectroscopically image
the extended \cii\ emission, STIM will define the topology and dynamics
of cold, neutral clouds to provide definitive constraints to all models
of star forming clouds.

Turbulence may play an important role in the formation and evolution of
interstellar clouds. In a standard scenario where CNM clouds
are formed from WNM gas by thermal instability, we can picture the role of
turbulence in two ways: large scale instabilities, density waves and
supernovae drive compressional
motions that can trigger the thermal instability \citep{deavillez05}.
Alternatively, regions undergoing thermal instability experience a
 dynamic radiative transition from the WNM to the CNM, due to the large
density contrast between these two phases. This transition is known to
generate turbulence and to set up the CNM into a complex network of pancakes
and filaments (Kritsuk \& Norman, 2002, also Figure~\ref{padoan}).
Because of this dynamic nature of both the triggering and evolution of the
thermal instability, departures from thermal pressure equilibrium may be
widespread in the ISM \citep{heiles03}, and the notion of a
dynamic multiphase ISM has been proposed, where turbulent diffusion regulates
phase exchange processes. Only a careful study of both the spatial structure
and kinematics of gas in transition between phases can tell us the role of
turbulence and dynamic pressure in the life-cycle of the ISM.

The spectroscopic images of \cii\ emission can be used
to derive properties of turbulent flows within the cold, neutral
clouds, and within molecular clouds
that can be directly compared to those of the underlying
CO emitting material \citep{heyer04}. The cooling in
shear layers is expected to be dominated by \cii\
cooling at temperatures lower than $\sim$200 K and is $> 10$ times
brighter than the CO emission \citep{falgarone07}.
Thus, \cii\ is a better tracer of turbulent shears than is CO
and will yield the rate of turbulent dissipation in shear layers.

Theories of molecular cloud formation are guided and constrained
by observations of the atomic and molecular gas components.
Four mechanisms have been proposed to consolidate gas into GMC complexes
(Figure~\ref{m33}): (1) self-gravitating instabilities (Kim \& Ostriker 2002,2007)
within  the diffuse gas component (often in a spiral arm where density is highest
and the Jeans time is shortest), (2) collisional agglomeration of small,
long-lived molecular clouds, (3) accumulation of material within high
pressure environments such as shells and rings generated by OB
associations, and (4) compression in the randomly converging parts of a
turbulent medium. STIM can distinguish these processes from each other and
consider new cloud formation schemes by:

\begin{itemize}
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\item Accounting for {\bf all} of the molecular hydrogen mass (the \htwo/\cplus\ clouds as
well as the \htwo/CO clouds) when computing global measures of the
interstellar medium.

\item  Making a more complete, better characterized catalogue of interstellar
clouds than CO or \hi\ surveys.

\item Constructing spatial and kinematic comparisons with sufficient
resolution, spatial coverage and dynamic range to probe a wide range of
interstellar phases and environments.
\end{itemize}


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.66,angle=0]{../figures/m33.eps}
\caption{\label{m33} {\it The location of GMCs in the
    nearby spiral galaxy M33 are overlaid upon an integrated intensity
    map of the \hi\ 21~cm line \citep{engar03}.  These observations show that
    GMCs are formed from large structures of atomic gas, and
    foreshadow the detailed study of GMC formation that STIM will provide.
}}
\end{figure}

Currently, associating diffuse gas (\hi) with molecular gas (CO) is
difficult owing to the large differences of emitting volumes of the
\hi\ line and the CO line (see Figure~\ref{hi-vs-co}). Unlike \cii,
a significant fraction of the \hi\ emission can come from low
density WNM gas. \cii\ emission barometrically picks out clouds of
atomic CNM gas and \htwo\ clouds with little CO. Regions of GMC
formation may therefore be tracked by higher than average cloud
densities (number of clouds per kpc$^3$), or regions with individual
clouds with higher than average columns or pressures.  With STIM's
velocity resolution, these regions can be linked to super-rings or
spiral arms or convergent parts of a turbulent medium. STIM will
identify the sequence of phase transitions as the gas transits
through the spiral potential, and {\bf will witness the process of
cloud formation} directly from the atomic substrate or from small
\htwo\ clouds.  For example, dust lanes along the inner edges of
spiral arms often show neither \hi\ nor CO emission
\citep{wikl90,tilan91}, and are therefore likely to be in an
intermediate phase; sufficiently dense and self-shielded to harbor
\htwo\, but not CO (Grenier et al.\ 2005, see also
Figure~\ref{ISM-phases}). These clouds will be seen in \cii\ by
STIM.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.8,angle=0]{../figures/hi-vs-co.eps}
\caption{\label{hi-vs-co} {\it This 3-color image is constructed from
    the VLA Galactic Plane Survey and the BU-FCRAO Galactic Ring
    Survey. Blue corresponds to the HI 21cm line integrated between
    V$_{LSR}$ of 50-55 km/s.  The red and green images correspond to
    $^{13}$CO J=1-0 emission
    over the same velocity interval but with integrated intensities of
    0-5~K~\kms\ (red) and 4-10~K~\kms\ (green). Even within these
    narrow
    velocity intervals, it is difficult to associate atomic gas with
    star-forming molecular material owing to the long path lengths
    contributing to the HI 21cm line emission relative to the CO
    emitting columns. The \cii\ line provides a more direct connection of
    the cold, neutral atomic gas to star forming molecular clouds. }}
\end{figure}

{\bf The high spectral resolution of STIM enables crucial kinematic
studies of the Galaxy to be made}.  STIM will determine the kinematics and
thermal pressures of most supershells, fossil superrings, and molecular
clouds just condensing via gravitational instability of old superrings.
STIM will detect many of the CNM clouds formed out of WNM in
the shells, and the larger column density clouds, which may harbor
\htwo.  With these detections STIM will
determine the role of OB association-driven supershells and
superrings in the production of molecular clouds and the cycling of gas
between the various phases of the ISM  \citep{elme89,mccray87}.

{\bf STIM witnesses the disruption of clouds}:
\cii\  and \nii\ measure the
photoevaporating atomic or ionized gas driven from clouds with
UV-illuminated surfaces, thereby converting the clouds to WNM or to
diffuse \hii\ regions. Thus, STIM can directly determine the rate of mass
loss from all catalogued clouds, and their destruction timescales.


\subsection{Measuring the Star Formation Rate in the Galaxy}

\fbox{\parbox{3.05in}{
\bf STIM, using the \cii\ and \nii\ maps, will provide an
extinction-free measure of the star formation rate and its
relationship to the gas density.}}

\medskip

Star formation within galaxies is commonly described by two empirical
relationships: the variation of the star formation rate per unit area with
the gas surface density (atomic + molecular), $\Sigma_{\rm SFR} \sim
\Sigma_{\rm gas}^n$ \citep{schm59} and a surface
density threshold below which star formation is suppressed
\citep{kenn89,martin01}.  The
Schmidt Law has been evaluated from the radial profiles of ${\rm H}\alpha$,
\hi, and CO
emissions for tens of galaxies.  The mean value of the Schmidt index, $n$,
is $1.3 \pm 0.3$ \citep{kenn89}, valid for kpc scales.
This empirical relationship is
used in most models of galaxy evolution with surprising success given its
simplicity.  Oddly, there has been little effort to evaluate the Schmidt
Law in the Milky Way owing to the difficulty in deriving the star
formation rate as a function of radius within the plane. Previous
studies have been typically
limited to tangent point locations where IRAS point sources can be
linked to molecular line emission from high density tracers \citep{luna06}.

The STIM survey of \cii\ and \nii\ emission provides the optimum
set of data to calculate the Schmidt Law in the Milky Way.  The \nii\
line is an excellent tracer of the star
formation rate as it directly measures ionizing luminosity with unmatched
sensitivity, angular and spectral resolution, and is unaffected by
extinction.  The \cii\ line, in conjunction with \hi\ 21-cm and CO line
emissions, provide the first coherent map of the neutral interstellar gas
surface density and its variation with radius.

{\bf STIM may help us understand the origin of the Schmidt-Kennicutt Law}. For example,
it will correlate the thermal pressures on the surfaces of GMCs (which may
relate to the star formation rate inside) with surface densities of \hi\ and
CO. It may uncover regions around OB associations devoid of GMC-forming CNM clouds.
The current high rate of star formation in associations may impoverish
large regions of the clouds needed to start the star formation cycle in
the future.   Such measurements are pivotal to models of
star formation feedback \& global galactic evolution.


\subsection{Determining the Energy Balance \& Mass Flow within the Inner Galaxy}

\fbox{\parbox{3.05in}{
\bf The compressed and turbulent interstellar
medium of galactic nuclei provides a
completely different environment for star formation
than the relatively quiescent disk. STIM will explore the Galactic Center
with sufficient spatial ($\sim$3 pc) and spectral
resolution to separate large clouds and trace material as it falls into and
through the Center region.  STIM will study the origin of the mysteriously
low \cii/FIR flux ratio \citep{naka98}, all of the
center's massive cloud complexes, and the
bar.  High-velocity resolution observations will provide a new kinematic
map of the gas, ultraviolet fields, and star formation  in the nucleus.}}

\medskip

The central region of the Galaxy has two major features that will be
the primary objects of study with the \cii\ and \nii\ data from STIM:
(1) the tilted nuclear disk, mostly atomic and well studied in \hi, but
with some molecular component as well \citep{burton78,liszt78,burton92}, and
(2) the central molecular zone, containing $\sim$4 x 10$^7$ M$_\odot$
of molecular gas, representing the strongest concentration of gas in
the Galaxy.   ISO observations of the Galactic Center region
\citep{goi04} demonstrated the utility of far-infrared line
diagnostics, but those observations had neither the velocity
resolution nor the areal coverage to reveal the full dynamical
situation.

 Current models and observations of Galactic gas dynamics show \citep{binney91,morris96}
that gas migrates inward through the Galaxy as a result of angular
 momentum loss, perhaps largely caused by the bar, and therefore it
 passes through the nuclear disk following X1 orbits, which are
elongated along the bar.  As it approaches a radius of $\sim$ 200 pc,
the orbits become self-intersecting, so the gas shocks and is
compressed, thereby becoming predominantly molecular.  The shocks
lead to angular momentum loss, which causes the molecular gas to
enter X2 orbits, elongated perpendicular to the bar.  Most of the
mass of the central molecular zone is on X2 orbits.  The clouds in that zone migrate
inward as a result of dynamical friction and magnetic drag on time
scales of $\sim$ 3 x 10$^8$ years, forming stars as they go
\citep{Stark91, figer04} and ultimately feeding the activity in the central parsec.

With the proposed STIM surveys, we can study this full range of scales,
and investigate the atomic-to-molecular transition that occurs at 200
pc.  A measurement of the dynamics made possible by the high velocity
resolution of STIM will be important for distinguishing the gas on X1
and X2 orbits.

Since \cii\ emission will arise from both the atomic clouds and the
surfaces of the molecular clouds, a comparison between it and CO
emission will reveal both the locus of the shocks, as well as the
preshock and postshock dynamics.
Shocks effectively transfer some fraction of the clouds' orbital kinetic
energy into heat, which then escapes primarily through line emission.
By measuring this emission, we can estimate the amount of orbital
energy that has been dissipated and estimate the rate of mass inflow
from the decaying orbits.  This quantity is a crucial datum for
understanding both the mass budget of the Galactic Center, including the time-
averaged rate of star formation in the Central Molecular Zone, and the evolution of the
Galactic Center magnetic field.

One of the puzzles of the central molecular zone is why the molecular
gas there is so much warmer than elsewhere in the Galaxy.  The
temperature of clouds 
in the central molecular zone is typically about 40-80 K, and it
ranges up to even a few hundred K \citep{morris83, belmont06}.
This is the temperature range in which \cii\
is most efficient at radiative cooling.  There is currently no
universal consensus on what the heating source is, although various
possibilities have been considered, including dissipation of MHD
turbulence, viscous heating associated with cloud shear in the strong
tidal field of the Galactic Center, and heating by copious X-rays.
When combined with the cooling that can be inferred from existing
molecular line surveys, the proposed observations of \cii\ allow for a
thorough characterization of the cooling rate in the Galactic Center.  The cooling
rate provides the heating rate, assuming that most clouds are in
thermal equilibrium.  Then, for the first time, the various
hypotheses for how Galactic Center clouds are heated can be confronted with the
actual heating rates required, as well as with a measurement of how
the heating rate varies with depth into the cloud.

Finally, the data from STIM will be of considerable value for
extending our current characterization of the tilted nuclear disk.
While it has been studied in both CO and \hi\
\citep{burton78,liszt78,burton92}, its content of warm, diffuse
clouds remains poorly known.
Furthermore, the dynamics of  gas in this region provides one of the
best ways of modelling the bar potential of this crucial, central
part of the Galaxy.

\subsection{Constructing a Milky Way Template}

\fbox{\parbox{3.05in}{
\bf STIM maps of the Milky Way Galaxy will provide physical insight to
the ISM processes needed for the interpretation of spatially
unresolved observations of distant galaxies. }}

\medskip

\cii\ 158 \micron, the strongest Galactic cooling line, will be the premier
diagnostic tool for studying external galaxies with future far-infrared
(FIR) observatories (SOFIA, Herschel) and in the submillimeter for
galaxies with large redshifts (Atacama Large Millimeter Array).  In such
spatially unresolved galaxies, however, only global properties can be
measured. To interpret the measurement of extragalactic \cii\,
one must turn to the Milky Way for the spatial resolution needed to
disentangle the various contributors to the total \cii\
emission. At present,
there is debate on the dominant origin of the \cii\ emission in the Galaxy:
diffuse \hii\ regions, CNM clouds, or the surfaces of GMCs.  STIM will
solve this mystery.  The \cii\ and
\nii\ intensities depend on the strength of the UV heating and on the amount
of gas in the appropriate ISM phase. The STIM mission covers a broad range
of density and UV intensity, thus establishing the relationship between
physical properties, \cii, \nii, CO, \hi, FIR emission, and star
formation.   This study will provide the ``Rosetta Stone'' for translating the global
properties of distant galaxies into reliable estimators of star formation
rate and state of the ISM.


\section{ Science Requirements}
\label{requirements}

\fbox{\parbox{3.05in}{
\bf STIM's Science Traceability Matrix is displayed in tabular form in Science
Foldout 1.   The following sections describe the most important points
of the matrix in additional detail.  }}

\subsection{High Resolution Spectroscopy}

\fbox{\parbox{3.05in}{
\bf STIM will have adequate velocity resolution to isolate separate
components in a cloud complex.}}

\medskip

The superposition of many clouds along the line of sight can
be disentangled with spectral line techniques.  Fitting to a model of
Galactic rotation is often the
only way to determine each cloud's distance and location within the
Galaxy.  It is the spectral resolution that gives us a 3D rather than
a projected 2D map of the Galaxy. Moreover,
internal dynamics are revealed from the variance of measured
velocities and profile line widths within interstellar clouds.  With
resolution finer than 1~\kms, a line profile can disentangle processes
such as turbulence, rotation, and local effects such as protostellar
outflows.  These kinematic components play a vital role in the
sculpting of interstellar clouds, and a survey that has the goal of
understanding their evolution {\bf must} be able to separate
the velocity components.



\subsection{Large Scale Mapping}

\fbox{\parbox{3.05in}{
\bf STIM will map a large enough area to sample the full range of Galactic environments.}}

\medskip

Interstellar pressure, elemental abundances, and radiation field
vary as a function of Galactic radius, so it is
necessary to probe from the inner Galaxy out to at least the Solar circle to
obtain a statistically meaningful survey
that encompasses the broad dynamic range of conditions in the
Galaxy.  Large cloud ensembles, as gathered by surveys,
are required to average properties extracted from clouds at varying stages
of evolution.   Large scale mapping places detected interstellar
structures in an environmental context.  Finally, neutral interstellar
gas is affected by many physical processes that operate over
a broad range of spatial scales.
High spatial dynamic range observations are
required to follow the gas distribution and kinematics through
each spatial scale.
To meet these objectives we will carry out an unbiased Galactic
plane survey (GPS), and targeted deep surveys (TDS) of selected areas in the Galactic
plane.

\medskip

\noindent {\it Galactic Plane Survey (GPS)}

This moderate sensitivity survey will sample the Galactic plane at
longitudes $-60^\circ > l > 60^\circ$ and $-1^\circ < b < 1^\circ$
in 24 seconds of cumulative integration time per 1.5$'$ resolution
element.   This survey region includes the Galactic Center, the
molecular ring ($l\! \sim\! -23^\circ$), spiral arm regions ($l\!
\sim\! -50^\circ$), and interarm regions ($l \sim -40^\circ$). In
addition, numerous shells, bubbles, and star formation regions
appear throughout our survey area as seen, for example, on MSX
8\micron\ emission maps.  Thus, our survey probes the Galactic
regions where the full cycle of cloud formation and destruction
occurs. {\bf STIM's angular and velocity resolution will be
$\sim280$ and $\sim10^3$ times that of the COBE survey, allowing the
\cii\ and \nii\ emission regions to be disentangled along a given
line of sight.} The STIM data will be invaluable in interpreting the
recent Spitzer GLIMPSE and MIPSGAL surveys of the Galactic Plane
\citep{benj03, carey06}.

\medskip
\noindent {\it Targeted Deep Surveys (TDS)}

The deep sensitivity surveys will consist of observations of 100-600 seconds of
integration per $1.5'$ beam towards selected regions of the GPS
survey, the Outer Galaxy, and a sampling of the most nearby galaxies,
notably the Large and Small Magellanic Clouds.
The GPS survey only allows detection of clouds with columns of $A_V \ge 0.4$,
and so only uncovers the more massive CNM diffuse clouds.   Our deep
surveys allow a much broader
range of diffuse clouds to be studied, down to a column $A_V \sim 0.1$. In
addition, for fixed $A_V$, CNM clouds in lower pressure regions, or in
regions with lower heating rates are more difficult to detect.  Deep
surveys therefore also extend
our ability to detect clouds along the line of sight in such regions.

STIM's targeted surveys will overlap with important regions of
special interest to Galactic astronomy.   The Outer Galaxy survey will
include the Perseus OB star formation region and molecular cloud
complex that is a part of the Cores to Disks Spitzer Legacy program
and a large map of the Orion Molecular Cloud, which features a full
complement of ancillary data which STIM will be able to draw from and
contribute to.    A small survey of high-latitude clouds will include
the Polaris Loop, featuring dense CO cores, newly formed molecular
clouds, and dynamical structures in \hi\ emission.   The Magellanic
Cloud surveys will allow exploration of the differences in cloud
properties coming from very different dynamical and metallicity
environments.


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.75,angle=0]{../figures/smoothing.eps}
\caption{\label{smoothing} {\it Integrated intensity of
    $^{13}$CO~J=1-0 emission over V$_{LSR}$ of  50-55 km/s from the
    BU-FCRAO Galactic Ring Survey at different multiples (2,4,8) of
    the observed resolution of 45$''$. The corresponding grid size of
    each image reflects Nyquist sampling at each resolution. The
    threshold of the halftone corresponds to 4$\sigma$ of the full
    resolution image. Owing to beam dilution, one marginally detect
    the small, isolated clouds on the left 1/3 side of the image at a
    resolution of 6$'$. }}
\end{figure}


\subsection{Angular Resolution and Fully Sampled Maps}
\label{resolution}

\fbox{\parbox{3.05in}{
\bf STIM will angularly resolve individual CNM clouds at the distance
of the Galactic Center. }}

\medskip

Previous surveys of \nii\ and \cii\ were limited
to very small regions (KAO, ISO) or had low angular resolution (COBE,
BICE) \citep{benn94,naka98}.  STIM will fully sample both species over
large regions of sky
to their diffraction limited resolution of 1.7$'$ and 1.3$'$,
respectively.  Arcminute resolution with proper sampling
is crucial to disentangling
different clouds and cloud components over large distances in the
Galaxy.  At a distance of 10~kpc,  GMCs subtend 10$'$,
CNM clouds or small molecular clouds subtend 1-2$'$, diffuse \hii\
regions about 20$'$.  Such diffuse \hii\
regions tend to dominate \nii\ and \cii\ emission from ionized
gas \citep{mckee97} and are a substantial component of the ionized gas
in the Galaxy.  For these large-scale ISM components, it is possible
to trade some angular resolution for sensitivity.  Figure~\ref{smoothing}
demonstrates that we can spatially
smooth up to 6$'$ resolution when it is necessary to attain the
highest possible sensitivity to faint diffuse gas.


\subsection{High Sensitivity}
\label{sensitivity}

\fbox{\parbox{3.05in}{
\bf STIM will have sufficient sensitivity in \cii\ to detect diffuse
atomic clouds at the distance of the Galactic Center and sufficient
sensitivity in \nii\ to detect diffuse \hii\ regions in the molecular
ring. }}

\medskip

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.8,angle=0]{../figures/sensitivity.eps}
\caption{\label{sensitivity1} { \it Comparison of STIM's sensitivity with
    \cii\ and \nii\ integrated intensities for diffuse interstellar components
    (A$_v$=0.1 to 1 magnitudes)
    which constitute the building blocks for molecular clouds, and
    UV-irradiated molecular cloud surfaces and HII regions (with 10 to
    10$^5$ times the local interstellar radiation field).
    The corresponding sensitivities (3$\sigma$ rms noise) of
    STIM's two survey modes (see section~\ref{obsmodes} for definitions) are
    indicated by the top two horizontal gray lines.  The bottom gray
    line shows the improved sensitivity of smoothing to 5$'$ angular resolution.
}}
\end{figure}



Figure~\ref{sensitivity1} shows the required STIM
sensitivity for detecting \cii\ and \nii\ from photodissociated cloud
surfaces and for distant ($\sim$10 kpc) clouds in the inner Galaxy.
Our most demanding requirements
lie in the search for the formation of molecular clouds
and the measurement of the warm ionized medium in the
Galaxy.   In low
density gas, the brightness temperature of \nii\ emission is
expected to be about half that of the corresponding \cii\ emission.
A flux limit of $10^{-6}$~erg~s$^{-1}$~cm$^{-2}$~sr$^{-1}$
will detect \nii\ in diffuse \hii\ regions as far away as the Molecular
Ring, achievable in 20 seconds of integration
with velocity smoothing to 5~\kms, appropriate for ionized gas.
Spatial smoothing will also be employed for probing large-scale ISM
components as described in Section~\ref{resolution}.
Tracing the formation of GMCs requires the detection of
cold neutral clouds of relatively low
column densities of $\sim 5\times 10^{20}$~cm$^{-2}$.
The expected \cii\ emission at high interstellar pressures would be
$10^{-5}$~erg~s$^{-1}$~cm$^{-2}$~sr$^{-1}$, detectable in 12
seconds with STIM.  The STIM Deep Survey allows the study of smaller
columns of gas, and/or regions of lower interstellar pressure, such as in
the Solar vicinity.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.85,angle=0]{../figures/simulated_spectra.eps}
\caption{\label{simulatedspectra} { \it Observed CO and \hi\ spectra (top) and
    simulated \cii\ spectra for 15 seconds (middle) and 300 seconds
    (bottom) integration. The yellow curves indicate three distant CNM
    clouds with A$_v$ ranging from 0.1 to 0.5 magnitudes.
}}
\end{figure}


Figure~\ref{simulatedspectra} shows a simulation of
\cii , CO and \hi\ spectra along a line of sight through the Galaxy.
Observed CO and \hi\ spectra (top) and
simulated \cii\ spectrum are provided for 15 seconds
(middle) and 300 seconds of total integration (bottom).
The simulations indicate that three distant CNM
clouds of columns (left to right) of 3, 5, and 7 $\times
10^{20}$~cm$^{-2}$ are detectable. The \hi\ has a considerable WNM
component. The blue peaks shown in the figure
are surfaces of molecular clouds.


\subsection{A Terahertz Carbon and Nitrogen Survey:  Complementarity
  with other Missions \& Surveys} 
\label{complementarity}

\fbox{\parbox{3.05in}{
\bf STIM is timely. STIM will provide the best corresponding
interstellar cloud survey to the GLIMPSE and MIPSGAL Spitzer Legacy programs and
contemporary \hi\ and CO line surveys.   It naturally complements
heterodyne instruments on Herschel and SOFIA, and provides
capabilities far beyond the reach of balloon-borne experiments. }}


\subsubsection{Relationship to Existing Data Sets}

The fine structure lines of ionized carbon and nitrogen are essential
to accomplishing the science defined in this
mission, as described in Section~\ref{lines}; they are among the
dominant coolants of the ISM, and are the ``missing links'' in
diagnosing the global state of the ISM and the formative and
destructive evolution of interstellar clouds and star forming
regions.   However, observations performed by STIM are also
highly complementary to the existing large scale surveys performed in
other atomic and molecular species.

\smallskip

\noindent {\bf CO}:
For tenuous clouds of column less than about $N < 2 \times
10^{21}$~cm$^{-2}$, CO is photodissociated and the CO emission
either disappears entirely, or if present, is a very unreliable measure of
the molecular mass.
The more opaque clouds in the $5 \times 10^{20}$ to $3 \times
10^{21}$~cm$^{-2}$ range may be
primarily \htwo, which is extremely difficult to detect directly. These
clouds have most of the carbon in \cplus, which makes \cii\ the only
reliable probe. \cii\ not only detects the CO clouds (via the
emission from their \cplus\ surfaces),
but also the \htwo\ clouds at intermediate column, and the mainly
atomic clouds with $N < 5 \times 10^{20}$~cm$^{-2}$.
Recent J-K extinction maps (Figure~\ref{ir-vs-co})
confirm these components are present.  Thus, the \cii\ detects a broader
range of clouds than the CO, and allows us to see the possible assemblage
of clouds of ever greater column, from atomic clouds, to \htwo\ clouds, to CO
clouds.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.63,angle=0]{../figures/IR-vs-CO.eps}
\caption{\label{ir-vs-co} {\it Near infrared $J-K$ extinction maps
    (top) depict significant interstellar gas not probed by CO (bottom).
    \cii\ line
    emission will barometrically select young molecular clouds that
    have formed \htwo\ but not CO.
}}
\end{figure}

The CO surveys will complement the STIM survey by
helping to identify molecular clouds whose surfaces STIM detects and whose
ionized gas seen in \nii\ may be expanding into the ISM
\citep{dame01,onishi05,jackson06}.

\noindent {\bf H~{\small I}}:
The STIM surveys enhance substantially the interpretation of existing \hi\
surveys.   The most comparable datasets for STIM are
the Southern Galactic Plane Survey with 1$'$ resolution
\citep{mccl05} and the VLA Galactic Plane Survey \citep{stil06}.
These \hi\ emission maps are sensitive only to column, whereas \cii\ is
sensitive to density times column.  The \cii\ therefore picks out the cloud
regions with density $>$ 30 cm$^{-3}$, whereas the \hi\ is often
dominated by the WNM emission (see Figure~\ref{simulatedspectra}).  In
fact, \hi\ clouds can appear in
absorption, in emission or undetected against the confused background and
foreground WNM. When \hi\ and \cii\ are both seen in emission, the ratio
provides the gas density and pressure in the cloud. When extended and
broad \hi\ emission is seen without a \cii\ counterpart, WNM or thermally
unstable gas is identified.

\smallskip

\noindent {\bf [C~{\small I}]}:
Moving from the CNM through the surfaces of molecular clouds to their
cores, the predominant form of carbon transitions from \cplus\ to CO,
with high abundances of C in the transition region. Unbiased surveys of the
[C~{\small I}]
fine structure lines, with parameters similar to the STIM survey, have
recently become feasible \citep{mart04,zhang01}. Maps from STIM coupled with
CO and [C~{\small I}] data, will follow carbon in all its forms in position,
velocity, cooling rate, temperature and pressure as the interstellar
gas evolves.

\smallskip

\noindent {\bf Infrared Continuum Surveys}:
MSX, Infrared Astronomical
Satellite (IRAS), Infrared Space Observatory (ISO) and Spitzer GLIMPSE and MIPSGAL Galactic plane
surveys permit locating dark clouds, supershells, and star
forming regions using the IR continuum.  STIM provides the best corresponding
interstellar cloud survey that will place these 2D imaged
structures in a broader context by identifying their gaseous structural
and kinematic counterparts.


\subsubsection{Complementarity to Other Missions}

STIM builds upon the heritage of three pioneering surveys which provided coarse
pictures of \cii\ and \nii\ emission in the Galaxy.  COBE
(spatial resolution 7$^\circ$, velocity resolution $>$1000 km/s),
BICE (spatial resolution 15$'$,
velocity resolution 175 km/s), and the Infrared Telescope in Space (IRTS,
spatial resolution 10$'$, velocity resolution 750 km/s).   None of these missions
had sufficient spectral or spatial resolution to locate clouds, or
separate one cloud from another along a given line of sight, and thus
could not draw specific conclusions about cloud properties or
distributions, or even the origin of the \cii\ or \nii\ emission \citep{holl99}.
{\it STIM has about the
same sensitivity to surface brightness as these missions, but adds orders
of magnitude in spatial and spectral resolution.}

Besides COBE, BICE and IRTS, other
platforms include the now defunct Kuiper Astronomical Observatory and
Infrared Space Observatory, which did not survey the Galaxy in \cii\
and \nii, but made pointed observations.  The Spitzer Space Telescope
has no spectroscopic capability at these wavelengths.
Herschel's HIFI instrument (2009-2013) and (SOFIA, 2009+)
will have the capability to observe both \cii\ and  \nii\
at high spectral resolution.  The Herschel Guaranteed Time program
will devote significant time observing \cii\ and \nii\ but only in
limited selected targets.   The advantage
of STIM, however, is its ability to provide large scale coverage.
Herschel/HIFI will have a 10$''$ field of view at the \cii\ line, and
the lower resolution PACS instrument will provide a 50$''$ field of
view -- in comparison to STIM's two $1.5' \times 6'$ receiver arrays
which will observe simultaneously.
Furthermore, Herschel \& SOFIA are general
purpose observatories with a variety of instruments and many competing
science goals; they will simply not obtain the large-scale maps that a
dedicated, wide-field mission like STIM can provide.

A scientific and technical precursor to STIM is the Stratospheric
Terahertz Observatory (STO), led by STIM Project Scientist Chris
Walker. STO, a funded NASA suborbital program, is an 80~cm,
balloon-borne observatory designed to map Galactic Plane emission in
a number of far-infrared lines.  STO is scheduled to fly in December
2010.   The first STO flight instrument is designed to be nearly
identical to that of STIM's. Indeed, the STO focal plane unit (FPU)
will serve as an effective demonstrator for the STIM FPU. Being a
Long Duration Balloon mission, STO's observing time is limited to 14
days. During this time it will survey a 30 square degree
longitudinal swath of the Galactic Plane in \cii\ and \nii. An LDB
instrument typically flies at most once every two years. If STO were
to observe only \cii\ and \nii, it would take nearly 20 years for
STO to survey a similar size region of sky as STIM will. Even then,
the map would be limited to that part of the  Galactic Plane
observable from Antarctica. The STO map will serve as a precursor
for STIM and help the Science Team (many of whom are involved in
STO) optimize STIM's observing program.



\section{Data Products and Requirements}

The STIM surveys will
observe \cii\ and \nii\ spectra over large contiguous
areas on the sky; including the Galactic Plane, the Orion Region, and
the Magellanic Clouds.  The total baseline mission data volume is 90
GB, which is easily accomodated by current magnetic disk
storage.   The final products of this imaging
campaign are three dimensional data structures that
describe line intensity as a function of position on
the sky (e.g. $l,b$ coordinates) and spectroscopic
(velocity) axis. The data cubes will be written in
FITS (Flexible Image Transport System) formatted
files. This format is the basis for all widely used
image analysis programs.   The data produced by STIM is virtually
identical to that produced by the Submillimeter Wave Astronomy
Satellite (SWAS); indeed Gary Melnick (SWAS PI) will lead the ground
data system effort at SAO as was done for SWAS.   A description of the
basic processing and archiving of the data may be found in
Section~\ref{archiving}.


\section{Minimum Science Mission}

\fbox{\parbox{3.05in}{ \bf The STIM mission is highly robust to
achieving the Minimum Science Mission.}}

\medskip

The Minimum Science Mission is defined as the minimum acceptable
scientific return for the mission, below which the mission would not
be worth pursuing.  The power of STIM is its unprecedented
combination of wide area coverage, angular resolution, sensitivity,
and velocity resolution.  This combination of capabilities enables
for the first time the investigation of the interstellar gas life
cycle on a galactic scale.  The science objectives are defined by
the requirement to observe the full range of environments in the
Galaxy, including the Galactic Center, molecular ring, arm, and
inter-arm regions.  The baseline 6-month science mission
accomplishes this by mapping 120$^\circ$ of galactic longitude to a
nominal sensitivity sufficient to detect all atomic clouds with
$A_V$ less than 0.4. The principal science goals can still be
accomplished by mapping a smaller region, or the same region with
lower angular resolution, with lower velocity resolution, or with
lower sensitivity.  These descopes address the key technical
uncertainties in the mission, and can be applied during development
to constrain cost growth.

The minimum Galactic plane survey would comprise the central $\pm$
10$^\circ$ in Galactic longitude with a latitude coverage of
$\pm$2.2$^\circ$ and one half of the remaining Galactic plane out to
60$^\circ$ longitude with $\pm$1$^\circ$ of latitude coverage.  This
minimum science mission would survey 160 square degrees of the
Galaxy as opposed to the baseline 288 square degrees.


\subsection{Mission Lifetime}
\label{lifetime} The most significant driver on mission lifetime is
the hold time of the superfluid helium cryostat.  STIM uses an
existing flight qualified cryostat with demonstrated performance.
Nevertheless, unanticipated poorer performance in the cryogenic
system, particularly the radiative cooling system, could result in
reduced lifetime.  The minimum survey, however, can be observed in 4
months.

As will be discussed in Section~\ref{instrumentation}, the
cryostat has 50\% lifetime margin over the baseline mission and more
than 100\% margin over the minimum mission.  The principal value of
the lifetime descope is to minimize the risk of schedule and cost
growth during development.

\subsection{Sensitivity}

The STIM focal plane array must have adequate sensitivity in \cii\
to detect diffuse atomic clouds at the distance of the Galactic
Center and sufficient sensitivity in \nii\ to detect diffuse \hii\
regions in the molecular ring.  Since it is possible to trade off
observing time for sensitivity, a less sensitive focal plane array
will result in a smaller mapped region over the life of the mission.
Using the same minimum galactic plane area as in
Section~\ref{lifetime}, minimum mission receiver noise temperature
is a factor (288/160)$^{1/2}$ higher than nominal.  The sensitivity
calculations for STIM assume a receiver noise temperature of 2000~K,
so the highest acceptable noise temperature would be $\sim2700$~K.

As will described in Section \ref{instrumentation}, recent
measurements of the flight HIFI mixers at this frequency have
demonstrated significantly better system noise temperatures. The
main benefit of accepting this descope would be in preserving
schedule at SRON.

\subsection{Pixel Count}

Adequate Local Oscillator (LO) power is required for each of the
pixels in the focal plane array.  If the JPL LO sources are unable
to produce adequate power, the pixel count would need to be reduced.
A similar mapped area argument can be applied to the acceptable
pixel count.  Since mapping speed is roughly proportional to the
number of pixels in the focal plane, the minimum mission science
goal of mapping 160 square degrees could be accomplished by using
three-pixel arrays.

Tests of the identical HIFI LO chains have shown power outputs of
greater than 10~$\mu$W, or 2.5~$\mu$W per pixel for the baseline
arrays, well in excess of the 1~$\mu$W required per pixel.

\subsection{Angular Resolution}

The need to descope the angular resolution specification could arise
from a number of causes.  For STIM, the most likely cause of
degraded angular resolution would be inadequate pointing knowledge
smearing the reconstructed image beyond the baseline 1.5$'$. Another
possible cause would be a need to restrict data rate to the extent
that smearing during On-the-Fly mapping is significant. Degrading
the resolution by coadding pixels could also compensate for
sensitivity shortfalls in the instrument.  Finally, an unlikely but
conceivable descope would be to reduce the antenna size below 0.5~m
because of difficulties in fabrication or the need for additional
black radiator area.  In these four scenarios, degrading the angular
resolution would significantly relax requirements on other system
resources.

The atomic interstellar medium is best characterized as turbulent
and inhomogeneous (Dickey et al. 2004) with no preferred size scale
for structures, so there is a graceful degradation with angular
resolution. The STIM baseline performance has been set at 1.5$'$ to
match a 3 pc cloud at the 8 kpc distance of the Galactic Center.
Significant structures can still be expected at 3$'$ angular
resolution (Figure~\ref{smoothing}). The savings due to the exercise
of this descope would be in the control of cost growth in the
spacecraft. Current MATLAB models of pointing performance show
margin over the STIM requirement.



\subsection{Minimum Mission Summary}

The Minimum Science for STIM will be a 180 square degree map of the
Galactic plane in the \cii\ and \nii\ lines at a sensitivity adequate
to detect diffuse \hii\ and \hi\ clouds at the distance of the Galactic
Center.  The base performance allows degradation singly of the
following parameters:

\begin{itemize}
\setlength{\topsep}{-1ex}
\setlength{\partopsep}{-1ex}
\setlength{\itemsep}{-0.4ex}
\setlength{\parsep}{-0.4ex}

\item Mission lifetime: 4 months
\item Receiver sensitivity: T$_{rec}$ (DSB) = 2700 K 
\item Pixel count: 3 pixels per band
\item Angular resolution: 3$'$
\end{itemize}

\section{Science Implementation}

\subsection{ Instrumentation Summary}
\label{instrumentation}

\fbox{\parbox{3.05in}{ \bf STIM will benefit tremendously from
flight qualified mixer, low-noise amplifier, and local oscillator
technology that was developed for the Herschel mission. It will also
utilize a flight qualified, superfluid helium cryostat developed by
Ball Aerospace. The digital autocorrelation spectrometers used on
STIM have heritage from ESA (ODIN) and NASA (MLS) missions.}}

\medskip

The observational goal of STIM is to make high spectral ($<$1 km/s)
and angular resolution ($\sim1.5'$) maps of the Galactic plane in
two astrophysically important atomic transitions: \cii\ line
emission at 1.9~THz (158 $\mu$m), and \nii\ line emission at
1.46~THz (205 $\mu$m).   To achieve the angular resolution
requirement we have designed STIM to have an aperture of 50~cm. To
achieve the target spectral resolution, STIM will utilize a
heterodyne receiver system. To provide sufficient mass margin for a
superfluid helium cryostat with a sufficiently long mission
lifetime, STIM will be a free-flying observatory launched into a
28.5$^\circ$, 500~km high orbit from Kennedy Space Center by a
Pegasus-class vehicle.  The instrument portion of STIM consists of
(1) the telescope, (2) eight heterodyne receivers (4 at the 1.9 THz
[C II] line and 4 at the 1.46 THz \nii\ line), (3) an eight channel
autocorrelator system, (4) the instrument control electronics, (5)
the star tracker, and (6) the cryostat (see Foldout Figures, 2.1,
2.2, 2.3, and 2.4). The spacecraft portion of STIM is derived from
the BATC STP-SIV bus and contains (1) the solar arrays and power
regulation system, (2) the attitude control system (ACS), (3) the
spacecraft computer, (4) the solid-state memory for data storage,
and (5) the spacecraft transponder.

A block diagram of the STIM instrument is shown in the Instrument
Foldout: Figure 2.1.  Key instrument parameters are listed in
Instrument Foldout: Table 2.0. The STIM optical system consists of a
50 cm diameter reflector, a secondary reflector, a calibration load
flip mirror, and 10\% reflective dielectric beam splitters for local
oscillator (LO) injection. The telescope provides an f/10 beam that
is split into horizontal and vertical components by a polarizing
grid. Simple reimaging optics convey the combined sky and LO beams
through two cryostat windows to the \cii\ and \nii\ arrays located
on the $\sim$2K cold plate.  The diffraction limited,
full-width-half-maximum (FWHM) beamsize on the sky will be $\sim$1.3
and 1.7$'$  on the \cii\ and \nii\ lines, respectively. Our
observing strategy for the main survey is to make adjacent
On-the-Fly (OTF) strip maps of the Galactic plane.  An ambient
load/cold-sky calibration (CAL) will be performed at the beginning
and end of each strip map. During each strip map (lasting as long as
20 minutes) the calibration load will be regularly observed. With
this mode of operation, secondary chopping is not required. The
calibration load flip mirror actuator will use flex-pivot technology
utilized on ISO and Spitzer. As with SWAS, the primary and secondary
mirrors will be diamond turned aluminum, with a surface finish of
$\le5$~$\mu$m RMS. The telescope support structure will be made of
composite materials.



\subsubsection{System Description}

A cut-away view of STIM showing the placement of the receiver system
is shown in Instrument Foldout: Figures 2.2 and 2.3. The receiver
will consist of two, orthogonally polarized arrays of phonon-cooled,
HEB mixers operating at $\le$4~K. One 4 pixel array is optimized for
the \cii\ (1.90 THz) line, the other for the \nii\ (1.46 THz) line.
The mixers will be pumped by two, fixed tuned solid-state LOs. The
\cii\ and \nii\ LOs chains will produce single Gaussian beams. A
chain of four, 10\% reflective dielectric beam splitters will be
used to divide and inject each LO into the appropriate mixer array.
The LOs and beam splitters are mounted on an optical bench located
between the telescope and cryostat.  The cryostat outer housing,
optical bench, and telescope are passively cooled to 130K by a large
Winston cone pointed at cold space. Smaller Winston cones served a
similar function on SWAS, where they cooled selected receiver
components. Since STIM is a high resolution, heterodyne mission, the
telescope need not be cooled and can also serve as the primary heat
radiator at the base of the Winston cone (See Instrument Foldout:
Figure 2.2). By passively cooling the cryostat outer housing, the
boil-off of helium is decreased and the mission lifetime extended by
more than a factor of 3. Cooling the LO will increase its output
power by about 50\%.

The first stage, low-noise IF amplifiers (LNAs) will follow the
design used in HIFI (see Instrument Fold-Out Figure 2.7) and operate
at a physical temperature of 20~K. In Phase A LNAs capable of
yielding comparable noise performance at a higher physical
temperature ($\sim$40~K) will be evaluated for flight use. The IF
center frequency will be 3.4 GHz, with an instantaneous bandwidth of
2~GHz. As demonstrated on HIFI, this IF frequency and bandwidth can
be supported by NbN, phonon-cooled mixers.  At our highest observing
frequency (1.9 THz, the \cii\ line) a 2 GHz IF bandwidth will
provide 316 km/s of velocity coverage. A velocity coverage of this
order is needed to accommodate the wide velocity dispersion expected
in the data toward the inner parts of the Galaxy. The eight IF
signals from the mixers pass through an ambient temperature IF
processor where they are further amplified and downconverted to
baseband (0-2 GHz). STIM will use eight, 2 GHz wide, 1024 lag,
autocorrelators as the backend spectrometers for the mixer arrays.
We have baselined autocorrelators over acousto-optical spectrometers
because of their significantly lower mass, volume, and cost. A
single-chip,  2~GHz wide correlator has been demonstrated by two
companies; Spaceborne Inc. and Omnisys Instruments.  An ambient
temperature flight instrument electronics box will house (1) the IF
and correlator boards, (2) the LO/HEB/LNA bias board, (3) instrument
computer,(4) a power conditioning board, and (5) the
sythesizer/power amplifier board for the LOs. Each board will reside
in its own shielded compartment.



\subsubsection { Expected Sensitivity}

Recent lab measurements on waveguide and quasi-optical HEBs in the 1
to 2~THz range have yielded double side band (DSB) receiver noise
temperatures in the 1000-2000K range. For our sensitivity
calculations we have conservatively assumed a receiver noise
temperature of 2000K and single-sideband (SSB) system noise
temperature, T$_{sys}$=4000K. With the 24 second integration time
per Nyquist-sampled resolution element characteristic of the
unbiased, Galactic Plane survey (GPS) mode, we will be able to
achieve an rms noise level of $\le 1$~K at a $\le$1~\kms\ velocity
resolution. The STIM instrument characteristics are summarized in
Instrument Fold-Out 2: Table 2.0.



\subsubsection {Component Selection}


\medskip
\noindent {\it Technical Approach to Mixers}
\smallskip

\noindent \fbox{\parbox{3.05in}{ \bf STIM will utilize the same HEB mixer
technology developed at SRON University of Technology Delft, and JPL
in support of the HIFI instrument on Herschel.}}

\medskip

HEB mixers, both in space qualification tests for HIFI and at high
altitude ground-based observatories, have been shown to have the
sensitivity needed for the proposed STIM science investigations. A
plot of the noise temperature of HEB receivers as a function of
frequency is provided in Figure 2.6 of the Instrument Foldout.
Transitioning these superconductive HEB mixers to the 1$\times$4
arrays baselined for STIM is a straightforward repackaging of flight
qualified components. The 1.46~THz array will be provided by JPL and
the 1.9~THz array by SRON.

\medskip
\noindent{\it Technical Approach for Local Oscillator hardware}
\smallskip

\noindent \fbox{\parbox{3.05in}{ \bf JPL has developed and delivered
flight-qualified solid state local oscillator (LO) chains for HIFI
Band 6L and Band 6H that meet STIM requirements.}}

\medskip


HIFI Band 6L covers the 1400-1600 GHz region while Band 6H covers
the 1600-1900 GHz range. These LO chains meet the requirements
placed on them by the STIM mission, and consist of GaAs Monolithic
Microwave Integrated Circuit (MMIC) power amplifier modules and
JPL-designed and fabricated waveguide GaAs planar multiplier
Schottky diode circuits. The power handling capacity of first stage
GHz Schottky diode multipliers has been quadrupled due to the
development of novel multiple-device planar circuit topologies, new
GaAs device fabrication techniques, and extremely high cutoff
frequency planar diodes.

The performance of the LO chain that covers the high end of Band 6H
for HIFI is shown in Figure 2.5 of the Instrument Fold-Out. At 130~K
ambient temperature the chain produces more than 10~$\mu$W at
the frequency of importance for STIM. The 6H Flight Spare (1.9 THz)
chain is currently at JPL in a controlled environment. HIFI is now
undergoing final spacecraft I\&T and is scheduled to launch in
August 2008.  The HIFI project management has agreed to release this
chain to STIM once HIFI is launched.  This chain will be used to
pump the \cii ~mixer array on STIM.

Local oscillator sources to cover the 1400-1600 GHz range (Band 6L)
were also developed and delivered for the HIFI instrument by JPL. An
LO chain following the HIFI Band 6L design will be used to pump the
\nii ~mixer chain on STIM.



\medskip
\noindent{\it Spectrometers}
\smallskip

STIM will utilize eight high-efficiency, low-power digital
correlation spectrometers. The baseline spectrometer (shown in
Figure~2.8 of the Instrument Foldout) can process two, 2~GHz wide
receiver IF's, each with 1024 channels.  We will explore an upgrade to
3~GHz bandwidth during Phase~A.   Four of these units would
more than meet the spectrometer requirements of STIM. The 8$\times$12 mm
Multi-Chip Module shown in the figure will be upgraded in 2008 to a
new single chip spectrometer ASIC, 5$\times$6 mm in size with lower power
consumption and higher bandwidth. With this upgrade, only two of the
units shown in Figure~2.8 will be required to process the output of
all 8 STIM receivers. The spectrometers have been tested up to 25
kRAD total dose with no degradation in performance and less than 1\%
increase in power consumption. They have also been tested in vacuum
and are designed to be operated in space.


\begin{table*}[!hbt]
\centering
\small
\begin{tabular}{|l|l|l|l|}
\hline
{\bf Item } & {\bf Demonstrated LLC} & {\bf Modified LLC cryostat} & {\bf STIM requirement} \\
 & {\bf cryostat performance} & {\bf estimated performance} &  \\
\hline \hline
Loaded Mass & 107 kg & 97 kg & $\leq$ 100 kg \\
\hline
Helium volume & 95 liters & 109 liters & 109 liters \\
\hline Helium life @ shell T & 2.0 months @ 300K  & 8.8 months @ 130
K & $\geq$6.0 months @ 130 K \\
& 19 months @ 60 K &   &  \\
\hline
Detector Temp. (K) & 1.5 K &  2.0 K & $\leq$ 4.0 K \\
\hline
Launch Hold & 60 hours (modeled) & 60 hours & $\geq$ 12 hours (TBD) \\
\hline Resonant Frequencies & 72 Hz axial, 65 Hz lateral & 72 Hz
axial, 65 Hz lateral & $\geq$ 20 Hz (Pegasus) \\
\hline
\end{tabular}
\caption { \label{thermal} {\it Modified LCC Cryostat will meet STIM Requirements}}
\end{table*}


\subsubsection{Cryostat}

\fbox{\parbox{3.05in}{ \bf The STIM mixer arrays and low noise
amplifiers will be kept at cryogenic temperatures in a modified
version of the space-qualified Ball Lightweight Low Cost (LLC) Cryostat.}}

\medskip

This Helium II cryostat, which has been
designed, built and tested by Ball for SMEX missions.  It will be rebuilt
using some of the existing hardware with design modifications to
accommodate the requirements of the STIM instrument. This is a low cost,
low risk approach that is well suited to the STIM mission and will
leverage off the considerable Helium II cryostat heritage at Ball.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.7,angle=0]{../figures/LLC.eps}
\caption{\label{thermal-1} {\it LLC cryostat during cold
    vibration tests.
}}
\end{figure}

Figure~\ref{thermal-1} shows the current LLC Cryostat.  Figure~2.4
in the Instrument Fold-Out 2, shows a cutaway view of the LLC with
the major parts and their planned reuse.

The cryostat tank internal plumbing and liquid level sensing will be
rebuilt to allow it to be filled in either vertical or horizontal
position. The tank design will be modified to increase the helium
volume and lifetime by 15\%, while decreasing the overall system
mass. This will be done by replacing the toroidal tank with a
roughly spherical one. The mass savings that results from
eliminating the cavity tube and thinning the reinforced areas of the
tank near the tube more than compensates for the additional helium
mass.

The STIM mixer arrays and associated optics are small enough to be
mounted on top of the tank where they will operate at approximately
2~K.  The low noise amplifiers will be mounted on the inner vapor
cooled shield where they will operate at approximately 20 K.
Additional modifications will be considered to reduce the vacuum
shell and girth ring mass. Quartz windows will be added to the
vacuum shell and vapor cooled shields to pass the desired signals
from the telescope.  The external plumbing will be modified to meet
the requirements of the launch faring envelope.  The LLC cryostat
design information and the parts to be reused will be donated by
Ball to STIM.

The modified LLC cryostat meets the requirements of the STIM
mission. Table~\ref{thermal} shows the demonstrated performance of
the current LLC cryostat, the modeled performance of the modified
and rebuilt cryostat and the STIM requirements.
Figure~\ref{thermal-2} shows the helium life for the current and
modified cryostat as a function of vacuum shell temperature. The
fill level of the liquid helium II will be monitored on the ground
with a standard superconducting wire probe in the tank. On orbit,
the helium II mass and boil off rate will be determined using
thermal capacitance gauging, which has been used successfully on the
Spitzer cryostat. Three gauging events are planned, with a total
heat input of 37 joules, which will reduce the mission helium life
by less than an hour.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.78,angle=0]{../figures/thermal2.eps}
\caption{\label{thermal-2} {\it Helium life as a function of vacuum
    shell temperature for STIM instrument loads.
}}
\end{figure}


\subsubsection{Cryo-Radiator/Winston Cone}

 \fbox{\parbox{3.05in}{ \bf The cryostat helium lifetime is a function of the cryostat vacuum
shell temperature.  To minimize this temperature, we will use an
innovative approach to radiatively cool the cryostat shell using the
primary antenna reflector as a thermal radiator.}}

\medskip

The primary reflector surface will be designed to radiate well at
the 25~\micron\ thermal wavelength and reflect well at the
150-200~\micron\ science wavelengths. There are a number of good
candidates for surface coatings, including aluminum coated FEP
(Teflon), which has heritage in spaceflight applications.  We have
measured the THz reflectance of this material in our laboratories and 
found it acceptable. These coatings will be further evaluated in Phase A.

A Winston cone will shield the radiator from the Sun and Earth. The
cone will have a base diameter of 54~cm, a height of 70~cm and will
reject almost all the radiation that enters at more than 55$^\circ$
from the cone axis. The cone's internal surface will have a highly
polished, reflecting surface. Since the science observing schedule
will avoid pointing the Winston cone (and primary reflector) any
closer than 55$^\circ$  to the Sun or Earth limb, the radiator will
see only deep space. Winston cone radiators were successfully used
on SWAS as part of the thermal control system.  APART and ZEMAX
modeling of the Winston cone/primary antenna combination indicate no
stray light issues.

The antenna primary reflector/cryo-radiator will be thermally
linked to the optical bench and cryostat vacuum shell with flexible
thermal links. The optical bench and cryostat vacuum shell will be
thermally isolated from the Winston cone, star-tracker and
spacecraft by low conductivity support struts and 25 layers of high
performance MLI.  The outer MLI surface will be covered in silver
coated FEP, to provide low solar absorption and high
infrared emittance.  The instrument thermal model shows that this
configuration will be effective in cooling the cryostat shell
temperature to a temperature of 130 K as shown in the thermal
schematic (Figure~\ref{thermal-3}).  The cryostat model is based
on the LCC cryostat test data and indicates that for a shell
temperature of 130~K and the STIM instrument heat loads, the
helium life will be 8.8 months (Figure~\ref{thermal-2}).

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.52,angle=0]{../figures/thermal3.eps}
\caption{\label{thermal-3} {\it STIM instrument on-orbit thermal
schematic.  Powers are in mW. }}
\end{figure}



\subsection{Mission Concept}
\label{concept}

The objective of the STIM mission is to map the inner 120$^\circ$ of
longitude of the Galactic Plane in the emission lines of \cii\ and
\nii, to map targeted outer Galaxy regions like the Orion Giant
Molecular cloud, and to produce sensitive maps of specially chosen
extragalactic targets such as the Magellanic Clouds and face-on
spiral galaxies like M33.  To provide a high level of mass margin
for the instrument cryostat (and hence a good mission lifetime),
STIM will be launched in a 28.5$^\circ$ inclination low-Earth orbit.

The rate of nodal precession is 6.4$^\circ$ per day, allowing a
complete orbital visibility cycle to be completed in nearly two
months.   Orbital visibilities are limited by a 55$^\circ$ avoidance
angle for the Sun and Earth limb, and by a 40$^\circ$ maximum
incidence angle for the spacecraft solar panels during the
illuminated part of each 90 minute orbit.

The most efficient observing strategy for all STIM surveys is
On-the-Fly mapping (OTF). The satellite is slewed to the Galactic
latitude limit for a particular scan (e.g., at $b=-1^\circ$). The
satellite is then slewed to partially compensate for the orbital
motion so that it accomplishes the scan through the target in the
available half orbit.  STIM will reach the end of the mapping leg
(e.g. at $b=+1^\circ$) just as the Earth limb constraint is reached.
At that point, STIM will slew to the next target in the observing
sequence for the other half-orbit. The optimal observing sequence
will be generated by the same NASA Spike toolkit that has been
successfully used by HST, Spitzer, and FUSE. Given the angular
constraints and the available slew rate for the Ball spacecraft,
STIM will be $>$80\% efficient in performing mapping operations.
Readouts of the spectrometer will occur every 1-2 seconds, which
is consistent with scan speed and the Allan variance time of the
detectors.

Assuming a launch in mid March 2012 and after a 15-day In Orbit
Checkout (IOC) period, science observations commence on April 2012.
The launch cannot precede 15 February as it would place the critical
Galactic Center region inside the 55$^\circ$ Solar avoidance zone.
Visibility of the main Galactic Plane Survey (GPS) region rises to
70\% shortly after the beginning of science observations. On the
``other'' half orbit, high Galactic longitude and latitude targets
like Perseus OB and the Polaris Flare are available.  By May, the
Magellanic Cloud mapping will begin on the ``non-GPS'' portion of
each orbit. Similarly, by August, the Orion molecular cloud mapping
projects will begin.

Table~\ref{regions} summarizes the various mapped regions and the
total amount of time that will be ascribed to each map and the
sensitivity limits per diffraction-limited beam in the first 180
days after science operations.    To minimize risk, STIM will
quickly map much of the GPS region with 12 seconds of integration
time per independent beam within the first 3 months of operation,
and then repeat the mapping coverage to achieve the targeted GPS
sensitivity.



\begin{table}[!hbt]
\centering
\small
\begin{tabular}{|cccc|}
\hline
{\bf Survey } & {\bf Area} & {\bf \#days} & {\bf 3$\sigma$ rms/beam}\\
 & {\footnotesize deg$^2$} &  & {\scriptsize 10$^{-6}$erg/s/cm$^2$/sr}\\
\hline
GPS & 280 & 90 & 7.1 \\
LMC \& SMC  & 10 & 20 & 2.8 \\
OG/Orion & 13 & 40 & 2.3 \\
high-$b$ clouds & 6 & 18 & 2.3 \\
M33 & 1 & 12 & 1.1 \\
\hline
\end{tabular}
\caption { \label{regions} {\it Summary of Observing Plan for the
    first 180 days of science time. }}
\end{table}


\subsection{ Data Analysis and Archiving}
\label{archiving}

Raw STIM data will be downloaded three times daily to the Goddard Space
Flight Center (GSFC) and transferred daily to the SAO data center
where backups and preliminary data quality checks are performed.  In
the first stage of processing, telemetry frames are decompressed,
reformatted, stored to disk, and transferred to
SAO where the data reduction pipeline occurs under the supervision of
the Ground Data System Lead.

The primary data pipeline task is to regrid the irregularly sampled
On-the-Fly (OTF) data onto a regular grid.  The science team has
extensive experience with the processing and analysis of data from
heterodyne focal plane instruments and arrays (7 element
submillimeter array on the SMT and a 32 element 3mm array on the
Five College Radio Astronomy Observatory 14-meter telescope).    The
basic unit of OTF data is a file comprised of all readouts of the
backend spectrometers for all measurements taken between and
including the required reference and off-source measurements which
intertwine and book-end the scanned data, respectively.   An image
field is comprised of many such units.  The headers for all units
are stored in a relational data base server, which allows  for
efficient data retrieval. Raw but calibrated spectra are formed from
the on/off source measurements and system temperatures computed from
on-board temperature loads.   The spectra are Doppler-corrected,
coadded and convolved onto a final output grid.  A baseline is
subtracted from each spectrum and stored for later consideration of
the continuum level. The full data cube is then written to a FITS
formatted file to be distributed to the science team and national
data repositories.   The final archive of regridded, calibrated, floating-point spectra over
$\sim$300 square degrees of sky at 1.5$'$ resolution 
is 2 GB in size, representing a very manageable dataset and a
factor of 45 reduction from the raw data volume. 
A web-based interface to the database will be
developed to enable efficient extraction of sub-fields of the data
cube centered on interesting objects.

\subsection{Science Team}
\label{scienceteam}


 \fbox{\parbox{3.05in}{ \bf The STIM mission is supported by an
outstanding team of scientists  with extensive experience in
observations, modeling, theory and 
interpretation of astrophysical aspects of the interstellar medium,
star formation, and Galactic structure.}}

\medskip

The STIM science team provides world-class capabilities and relevant
experience in all key areas of the mission.  Unless otherwise noted,
NASA support for science team efforts are included in the STIM
budget.

The Principal Investigator Erick Young (UA) is responsible for the
overall conduct and success of the STIM project.  Young brings 29
years of experience with various NASA and ESA space projects,
including nearly 20 years as Deputy PI of the MIPS instrument on
Spitzer.  He led the development of the focal plane arrays for the
Spacelab~II Infrared Telescope, MIPS, and NIRCam.  He also has been
on the science teams of virtually all of the other space infrared
astronomy missions, notably IRAS, NICMOS, ISO, and WFC3.

The Project Scientist and Deputy PI is Christopher Walker (UA).  He
is responsible for overall scientific leadership of the project and
is an integral member of the project management team. Walker is a
world leader in submillimeter instrumentation, and is founder of the
Steward Observatory Radio Astronomy Laboratory (SORAL).  He is
Principal Investigator for the Stratospheric Terahertz Observatory,
the SuperCam 64-element array receiver, and numerous other
instruments.

Craig Kulesa (UA) is the Instrument Scientist.  He
will be responsible for the overall design of the STIM instrument and
ensuring that it meets the performance requirements.  He has extensive
experience in the construction of submillimeter and infrared
instruments.

Paul Goldsmith (JPL) brings his experience as both
Project Scientist on Herschel and as the leading authority in
quasi-optical techniques.  He will participate in the instrument
development as well as lead studies of the interstellar
lifecycle.

Chris Groppi (UA) will serve as Deputy Instrument
Scientist.  He has extensive experience in the testing of heterodyne
systems.  He will assist Kulesa in managing the instrument development
effort.

Mark Heyer (UMass) will lead the science efforts pertaining
to the global ISM structure of the Milky Way.  He has extensive
experience in conducting large scale surveys from his leadership roles
in CO surveys conducted from FCRAO.

Jonathan Kawamura (JPL) will lead
the \nii\ mixer effort at JPL.  He has directly applicable experience
as Herschel Band 6 lead as well as experience in THz
observations.

Christopher Martin (Oberlin) will act as mission
planning lead for STIM.  As the AST/RO winter-over scientist for two
years at the South Pole, Martin became an expert in operating and
scheduling a complex radio telescope.

Gary Melnick (SAO) will be
Ground Data System Lead for STIM.  As PI for SWAS, Melnick has
detailed experience in all aspects of the implementation and operation
of a space science mission.  In particular, he will apply his
experience from SWAS in converting satellite data into useful
scientific products.  As described in SAO proposal P6863-12-07,
Melnick is a federal employee, and his salary is not being charged to
NASA.

Mark Morris (UCLA) is an authority on the Galactic Center, and
he will lead the STIM science program in that area.

David Neufeld (JHU) will contribute to the detailed modeling of ISM
microphysics and dynamics.  His experience includes being Co-I on SWAS.

Paolo Padoan (UCSD) is a noted theoretician working in the area of
star formation, with a particular interest in the role of
turbulence.  He will lead STIM investigations in that area.

Edward Prather (UA) will lead the
STIM EPO effort.  He has extensive experience in this area as Program
Director for the NASA Center for Astronomy Education and as Deputy EPO
Officer at the Astrobiology Institute (LAPLACE).

Antony Stark (SAO)
will be the GDS Deputy.  He will be responsible for the overall
architecture of the GDS.  He brings his experience in conducting
surveys as PI of the AST/RO and co-PI of the South Pole
Telescope.

Mark Wolfire is an authority on the theory of the ISM,
particularly the energy balance in clouds and Photo Dissociation
Regions.  He will lead the STIM theory effort.

Harold Yorke has
worked extensively in both the ISM and star formation theory.  He also
brings valuable space experience as Herschel Project Scientist.  He
will lead STIM studies of interstellar dynamics.


\section{Science Enhancement Options}
\label{enhancements}

Given the short mission lifetime and the rapid availability of data
to the scientific community, we are not proposing any Science
Enhancement Options.


\clearpage

% uncomment the following to suppress page numbering in the References section
\pagestyle{empty}

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