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\begin{document}

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\twocolumn[

\Large 
\centerline {The {\bf S}pace {\bf T}erahertz {\bf O}bservatory (STO):}
\centerline {A 10-meter-class Far-Infrared Telescope for Origins Research}
\large
\medskip
\begin{center}
Christopher K. Walker, Craig Kulesa, Erick Young \ \ ({\it University of Arizona}) \\
David Hollenbach \ \   ({\it NASA Ames Research Center}) \\
T.~G. Phillips, Sander Weinreb \ \  ({\it Caltech Institute of Technology}) \\
William Langer, Imran Mehdi, Harold Yorke \ \ ({\it Jet Propulsion Laboratory})\\
Gary Melnick \ \ ({\it Smithsonian Astrophysical Observatory})\\
Paul Goldsmith, Gordon Stacey \ \ ({\it Cornell University})\\
David Fisher, David Glaister \ \ ({\it Ball Aerospace}) \\
Daniel Lester \ \ ({\it University of Texas})\\
Mark Wolfire \ \ ({\it University of Maryland})\\
J\"{u}rgen Stutzki \ \ ({\it University of Cologne})
\end{center}
\medskip

\hline
\vspace{0.75cm}
]

\section{Scientific Investigation}

\subsection{Overview}

Here we describe the scientific motivation and technology for a
10-meter-class Space TeraHertz Observatory (STO)
(Figure~\ref{STO}). STO would 1) conduct origin 
studies of planets, stars, and molecular clouds; 2) trace the life
cycle of the Interstellar Medium (ISM) 
and star formation rate throughout the Galaxy; 3)
determine the deuterium abundance in nearby molecular clouds and Galactic
Center; and 4) observe the distribution of atomic and molecular gas in
nearby and distant galaxies. STO will achieve these goals through high
spectral and angular resolution observations of C, O, N, HD, and
H$_2$O lines in the far-infrared. The science goals of STO can be
achieved either through a dedicated mission or by implementing
heterodyne instrumentation on SAFIR and extending its operational
lifetime. STO science objectives are
closely aligned with many of those found in the {\it Origins Roadmap} and
drive the creation of a new generation of heterodyne instrumentation
that benefits directly from technologies developed for the Herschel
HIFI instrument. 

\medskip


{\noindent
\begin{tabular}{|l|l|} 
\hline
Primary Mirror Diameter  &  9.0m \\ 
Angular resolution & 1.7$''$ -- 16$''$ \\
Spectral resolution & $\lambda/\Delta\lambda = 10^6$ \\
Wavelength range & 60 -- 540~\micron \\
\# of spectroscopic pixels & 288 \\
Lifetime  &  $>$5 yr \\ 
Orbit  &  L2 \\ 
\hline
\end{tabular}
}

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.33,angle=0]{./figures/STO.eps}
\caption{\label{STO} {\small The Space Terahertz Observatory will
    provide a powerful probe of our cosmic origins; the evolution
    of galaxies, stars, planets, and the chemical elements of life.}} 
\end{figure}


\subsection{Impact on Origins Research}

STO addresses all major enterprise objectives in the 2003 Origins Roadmap:

\begin{enumerate}
\setlength{\topsep}{-5ex}
\setlength{\partopsep}{-5ex}
\setlength{\itemsep}{-0.5ex}
\setlength{\parsep}{-0.4ex}
\item[A.] {\it Understanding how today's Universe of galaxies, stars and
  planets came to be.}  By exploring star formation throughout the 
  Galaxy \& nearby galaxies
  and the full life cycle of interstellar clouds, STO will directly
  quantify the feedback mechanisms connecting
  star formation with the interstellar environment; pivotal to the
  evolution of galaxies.


\item[B.] {\it Learning how stars and planetary systems form and evolve.}
  STO will explore the assembly of stars and planets from molecular
  cloud cores, and probe the detailed physics and chemistry of
  pre- and post-planetary disks.  STO probes these environments in the
  spectral light of gaseous water, carbon, nitrogen, and oxygen -- the elements
  of life on Earth.  STO will be sensitive to the gas masses that
  circularize the orbits of terrestrial planets, and that form gas
  giants by core accretion. 

\item[C.] {\it Exploring the diversity of other worlds and searching for
  those that might harbor life.}  STO's unprecedented
  sensitivity to water and oxygen enables the search for
  extrasolar Kuiper Belts and residual cometary material in evolved
  stars, constraining the initial inputs to the chemistry of life on
  Earth.  

\end{enumerate}

\smallskip

\fbox{\parbox{2.9in}{\bf
STO will:
\begin{enumerate}
\setlength{\topsep}{-5ex}
\setlength{\partopsep}{-5ex}
\setlength{\itemsep}{-0.5ex}
\setlength{\parsep}{-0.4ex}
\item Identify and characterize thousands of interstellar clouds, protostars
  and outflows in the Galactic Plane (Origins 2, 3)
\item Measure the abundance \& role of water in the sculpting of hundreds of
star-forming regions and circumstellar disks (Origins 3, 4)
\item Determine the life cycle of Galactic
interstellar gas by studying the creation and disruption of
star forming clouds in the Galaxy. (Origins 2)
\item Determine the parameters
that affect the star formation rate in the Galaxy. (Origins 2, 3)
\item Provide and test 
templates for star formation and stellar/interstellar feedback in
other galaxies. (Origins 2, 3)
\end{enumerate}
}}


\begin{figure}[!hbt]
\centering
\includegraphics[scale=1,angle=0]{./figures/schematic.eps}
\caption{\label{schematic} {\small Schematic representation of ISM
    components.}}
\end{figure}

\subsection{Mission Approach}
The Space Terahertz Observatory (STO) will survey the Galactic Plane at
2-16$''$ angular resolution in far-infrared fine-structure
emission lines of singly ionized carbon (C$^+$ 158 \micron, the Galaxy's
strongest IR emission line), nitrogen (N$^+$ 205 \micron), neutral
oxygen (O$^0$ 63 \micron), and ortho-water (H$_2$O 538 \micron).  It will
obtain sensitive maps of selected clouds in the crucial lines of
deuterated molecular hydrogen (HD 112 \micron),
H$_2$O, carbon, nitrogen, and oxygen.  The STO
heterodyne receivers provide sub-km/s velocity discrimination and
bandwidths that encompass all clouds orbiting in the Galaxy. 
STO maps the structure, dynamics,
energy balance, pressure, and evolution of the Milky Way's
Interstellar Medium (ISM), as well as the Galactic star formation
rate.   STO possesses exceptional
angular resolution that couples the life cycle of Milky Way interstellar clouds
{\it inward} to the formation of individual stars, circumstellar disks, and
planets -- and {\it outward} to the global star-forming properties of
nearby galaxies.  

The data are
produced in both large scale and selective surveys, tabulated below. The
data products from STO will be FITS data 
cubes of spectral line maps, a standard radio astronomy product.

\medskip

\noindent \fbox{\parbox{3in}{\bf
STO Surveys:
\begin{enumerate}
\setlength{\topsep}{-5ex}
\setlength{\partopsep}{-5ex}
\setlength{\itemsep}{-0.5ex}
\setlength{\parsep}{-0.4ex}
\item UGPS: An Unbiased Galactic Plane Survey: $-180^o < l < 180^o$ ; $-1^o
 < b < 1^o$ in [C~II] and [N~II] line emission
\item TDS: Targeted Deep Surveys of selected interstellar clouds 
in [C~II], [N~II], [O~I], HD, and H$_2$O line emission
\item PPD: A survey of 300 Pre-Planetary and Post-Planetary
 Circumstellar Disks (defined by the ``C2D''
 and ``FEPS'' Spitzer Legacy programs) in HD, O$^0$, and H$_2$O lines.
\item NG: A Nearby Galaxy survey, based upon the ``SINGS'' Spitzer
 Legacy program, that maps a few galaxies of late-Hubble
 types in [C~II], [N~II] and [O~I] line emission
\end{enumerate}
}}


\section{Science Goals and Objectives}
\label{intro}

Via resolved C$^+$, N$^+$ and O$^0$ line emission, STO uniquely probes
the pivotal formative and disruptive stages in the {\bf life cycles of
interstellar
clouds} and sheds crucial light on the {\bf formation of stars and
pre-planetary disks}.  It provides new insight into the relationship between
interstellar clouds and the stars that form in them; a central
component of {\bf galactic evolution}.  One of the products is a detailed study
of the Milky Way Galaxy, which is then applied as a template or standard
to {\bf interpret global star formation in other spiral galaxies}.

Neutral interstellar gas is the dominant mass component of the ISM,
and exists as two phases in rough thermal pressure equilibrium: a
diffuse warm neutral medium (WNM) with hydrogen densities at the solar
circle of n$\sim$0.3 cm$^{-3}$ and T$\sim$8000~K, and a denser cold neutral medium
(CNM) with n$\sim$40 cm$^{-3}$ and T$\sim$70~K\citep{wolf03}. 
With sufficient shielding column,
$N>10^{20-21}$ cm$^{-2}$ of hydrogen nuclei, the CNM clouds begin to
include molecular interiors. Above $N\sim10^{22}$ cm$^{-2}$ 
they become fully-molecular,
gravitationally bound, and stars may form in their interiors\citep{mcke89}. The
largest condensations take the form of giant molecular clouds (GMCs)
with large masses $M\sim10^{5-6} M_{\rm sun}$ and are responsible for most of the
star formation in the Galaxy. These ISM components are shown
schematically in Figure~\ref{schematic}.  The spectral probes provided 
by STO span an enormous dynamic range of physical conditions in the
Galaxy %(Figure~\ref{range}) 
and uniquely probe all of these components. 


\begin{figure}[!hbt]
\centering
%\begin{minipage}{6.5in}
%{\parbox{4.6in}{
\includegraphics[scale=0.16,angle=0]{./figures/supercam-sto.eps} %}}
%{\parbox{1.9in}{
\caption{\label{scales} {\small {\bf The importance of high angular
    resolution}. Footprint of STO's 63~\micron\
    heterodyne array on M16 and HH30, in comparison to that of previous
    observatories.  The combination of high angular
    resolution and large-format spectroscopic imaging opens up new
    frontiers for planet formation and external galaxies. }}
%} }}
%\end{minipage}
\end{figure}

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.25,angle=0]{./figures/specres.eps}
\caption{\label{specres} {\small {\bf The importance of high spectral
    resolution.}  Synthetic emission \& absorption of the HD J=1-0
    transition (112~\micron) observed at different resolving powers
    demonstrates that high 
    spectral resolution increases the line-to-continuum contrast 
    vital to the detection of narrow
    interstellar lines embedded in a bright infrared continuum. 
}} 
\end{figure}


Joined with other surveys, STO will:
\begin{enumerate}
\setlength{\topsep}{-5ex}
\setlength{\partopsep}{-5ex}
\setlength{\itemsep}{-0.5ex}
\setlength{\parsep}{-0.4ex} 
\item Constrain planet formation models by spectroscopically probing
disks in oxygen and water lines inaccessible from the ground. With the
high spectral resolution of heterodyne instruments we can detect the
clearing of gas and dust in
circumstellar disks due to planetary accretion or disk instabilities.
Constrain the frequency of ``Kuiper Belt'' like reservoirs of
frozen water in post-main-sequence stars.
\item Map as a function of Galactic
position the size and mass distribution and internal velocity
dispersion of interstellar clouds in the Galaxy.
\item Construct the first barometric map of the Galaxy, the first map of the
gas heating rate, and a more detailed map of the star formation rate.
\item Probe the relation between the mass surface density
(on kpc scales) and the star formation rate, so that we may be able to
understand the empirical Schmidt Law used to estimate the star
formation rate in galaxies. 
\item Reveal clouds clustering and forming in spiral arms
and supershells, and follow the growth of clouds to sufficient column
densities to shield molecules and to become gravitationally bound.
\item  Observe the formation
and destruction of clouds throughout the Galaxy, and directly observe
the feedback caused by supernovae and the ultraviolet
radiation from massive stars. Measure the destruction rate of clouds
via the conversion to warm 
($\sim10^4$~K), diffuse neutral and ionized gas.
%\item Map the mass ratio of CNM to WNM in conjunction with HI surveys,
%  so that we may correlate this ratio 
%with thermal pressure, the ultraviolet radiation field, and the star
%formation rate.
%
\item Construct a Milky Way template connecting the line emission from C$^+$,
  N$^+$, O$^0$, CO, H$_2$O, and far-IR continuum to star formation
  properties and state of the ISM; apply this template to nearby
  star-forming galaxies. 
\end{enumerate}


\subsection{Overview of STO Capabilities}

The combination of high sensitivity, angular and spectral resolution
gives the Space Terahertz Observatory unprecedented power to unlock
important mysteries of our cosmic origins (Figure~\ref{scales}). 

The main features of STO surveying modes are:
\noindent \fbox{\parbox{3in}{\bf
\begin{enumerate}
\setlength{\topsep}{-5ex}
\setlength{\partopsep}{-5ex}
\setlength{\itemsep}{-0.4ex}
\setlength{\parsep}{-0.4ex}
\item High spatial resolution; 1.7$''$--16$''$ 
\item Very high spectral resolution, $< 1$ \kms. 
\item High dynamic range; $\sim 10^4$ spatially and $10^3$ spectrally 
\item More than $10^7$ spatial pixels of data, each with a high
  resolution spectrum
\item High sensitivity: $10^6$ times better than FIRAS/COBE. STO will
  catalog neutral clouds with columns $N > 10^{20}$
  cm$^{-2}$, all ionized clouds with EM $> 10$ cm$^{-6}$ pc,
  circumstellar disk gas masses to $<$ 10$^{-3}$ M$_{\rm Jup}$ for
  nearby star forming regions, and water abundances to $< 10^{-10}$ of
  H$_2$. 
\end{enumerate}
}}
\smallskip

STO will have the ability to detect (at 3$\sigma$) C$^+$ 
emission from CNM clouds with
columns of $N > 10^{20}$ cm$^{-2}$, or $A_V > 0.07$ mag. 
Such clouds typically subtend $>1'$ of angle at $d=$8.5~kpc, and will be
spatially and spectrally resolved. 
Figure~\ref{CII} 
demonstrates the beam sensitivity of STO; for large-scale Galactic
clouds, STO's high angular resolution can be
traded for greater sensitivity, allowing the measurement of even
the most tenuous, elusive phases of the warm and cold interstellar
medium. 



\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.35,angle=-90]{./figures/sensitivity.eps}
\caption{\label{CII} {\small 3$\sigma$ sensitivity of STO to spectral
    lines from Galactic clouds and interstellar components (see
    Section~\ref{intro} for definitions) at 10~kpc, circumstellar disks at
    150~pc, and GMC's in M33.  Much greater sensitivities can be
    reached in the case of nearby Galactic clouds by smoothing the
    data in angular resolution.  }}
\end{figure}


\subsection{Specific Science Goals and Objectives}

Of the numerous science aims outlined in Section 2, five will now be
discussed in more detail. 

\subsubsection{Goal 1:  Protostellar Evolution \& Planetary Systems}

\noindent {\bf H$_2$O and O$^0$:} 

Water is a molecule of unique interest in astrophysics; its abundance
and distribution provide information about the chemistry, composition,
and physical conditions within the gas.  Its powerful diagnostic
capacity stems in part from the varied ways in which it is produced;
(1) standard ion-neutral chemistry, (2) endothermic neutral-neutral
reactions, and (3) processing on the icy surfaces of dust grains
\citep{meln00}. 
Precursor missions to STO, such as SWAS, ODIN, and Herschel, have
provided and will provide a wealth of information about
water on large scales.  By virtue of its higher spatial resolution and
greater sensitivity, STO will build on the knowledge gained from these
prior missions by exploring regions of extreme interest that will
remain largely unexplored even after Herschel flies.  Two such
examples are (1) water in circumstellar disks and (2) the forensics of dying
planetary systems. 

\medskip

\noindent {\it Water and Oxygen as Diagnostics of Planet Formation}

The abundance of water in protostellar disks can yield important clues to the
origin of water available to newly forming planets.  Because it is so
sensitive to its environment, water is a superb tracer of the physical, 
chemical structure and dynamics of (planet-forming) circumstellar disks.  

In a ``standard'' model of a static flared circumstellar disk,
the water emission is predicted to be weak due to the
relatively small volume in which it exists in the gas phase.  However,
if vertical mixing within the disk is significant, ice-covered grains
are regularly transported to warm regions where this water is liberated,
increasing the gaseous water abundance by orders of magnitude.  The
presence of a high abundance of gaseous water can thus alter, and
elucidate, the temperature structure and chemical evolution of the disk. 

In contrast, O$^0$ emission at 63~\micron\ provides an outstanding
probe of radiation-heated gas in protostellar collapse \citep{cecc96}
and toward the ``surfaces'' of
circumstellar disks.  With a critical density of
$5\times10^5$~cm$^{-3}$, [O~I] is useful in probing denser regions
than most fine structure lines. 
Very
small gas masses, as small as $10^{-3}$ $M_{\rm Jup}$ result in
strong [O~I] line luminosities of $10^{-6}$ $L_{\rm sun}$ \citep{gort04}.
%(Figure~\ref{OI}).  
STO's high
angular resolution at 60~\micron\ 
will permit easy detection of these gas masses
in less than one minute toward the nearest star forming regions.
Most critically, the
[O~I] line profiles will elucidate the overall physical structure
of the disk, unveiling disk inhomogeneities such as 
gaps cleared by forming planets.
STO will
be able to detect whether there is enough gas for the formation
of gas giant planets by core accretion in $10^6-10^7$ year old disks
and whether there is enough gas in the terrestrial planet zone
to circularize the orbits of terrestrial planets \citep{koku02}.
Surveys of nearby clouds in 538/179~\micron\ H$_2$O and
63~\micron\ [O~I] emission will provide a catalog of disk systems ripe
for follow-up study.


\medskip

\noindent{\it Water as a Diagnostic of Planetary Death}

To date, radial velocity techniques have been extremely successful at
detecting large ($> 1 M_{\rm Jup}$) objects in relatively close orbits
around more than 100 stars, but unfortunately lack sensitivity to
analogues of the smaller constituents of our Solar System, such as
Kuiper Belt objects.  However, it is possible that the presence of smaller
bodies in orbit around other stars may be detectable using the type of
high spectral resolution, high sensitivity spectroscopy proposed for
STO. 

Specifically, as low-to-intermediate mass stars age and evolve off of
the main sequence, they undergo a dramatic increase in their radius and
luminosity.  Orbiting bodies within 50 to 100~AU
of the star that had previously been undisturbed by the star's
radiation field will now be vaporized.  
If a star is orbited by a Kuiper Belt analog, results obtained from
SWAS (Figure~\ref{SWAS}) 
have shown that -- under the proper circumstances -- it is
possible to infer the presence of these icy bodies by means of water
vapor enrichment in the stellar wind.  Carbon stars are
ideal candidates.   For AGB stars like IRC+10216, which possess
circumstellar envelopes in which carbon is the most abundant heavy
element, it is predicted that the equilibrium chemistry will drive all
of the oxygen into CO with little remaining to form other molecules.
Thus, the detection of water vapor toward a carbon-rich AGB star
raises the possibility that icy bodies are being vaporized.
Toward IRC+10216, SWAS data imply a water
vapor abundance 5 orders of magnitude greater than that predicted by
chemical models of the outflow \citep{meln01}. 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.52,angle=0]{./figures/irc+10216.eps}
\includegraphics[scale=0.33,angle=0]{./figures/IRC-H2O.eps}
\caption{\label{SWAS} {\small (left) Artist's conception of the
evaporating comet hypothesis for the water vapor detection of
\citet{meln01} (right) toward IRC+10216.  This result, obtained in
$\sim$200 hours of integration time with SWAS, is
reproducible in 1 minute of integration time with STO. }} 
\end{figure}

STO will have the capability to survey the nearest carbon stars for
evidence of vaporizing icy bodies.  Further, the excitation conditions
giving rise to a number of low-lying ortho- and para-H$_2$O lines
accessible to STO will make it possible to determine where in the
outflow the water is being injected.  A determination of the presence
and composition of the smaller constituents of other planetary systems
is not possible using radial velocity or transit techniques and is
beyond the capabilities envisioned for the Terrestrial Planet Finder.
{\bf By studying the contents of dying planetary systems spectroscopically,
STO will make unique contributions to our understanding of extra-solar
planetary architectures}. 

\medskip

\noindent {\bf HD and its Ions:} 
No molecule is more crucial to star formation
than molecular hydrogen (H$_2$); it comprises the vast majority
of interstellar material from which all
stars are born.  However only its singly-deuterated
form, HD, has permitted, energetically accessible transitions in
molecular clouds.  
In warm (T$>$50~K) molecular environments, 
the J=1$\rightarrow$0 transition (112
\micron) of HD will accurately measure the mass of H$_2$ 
where $N_{\rm H}\geq 10^{22}$~cm$^{-2}$, and can be referenced 
to [C~II], [C~I], CO and [O~I] emission observed in
photodissociation regions, warm molecular clouds and disks
adjacent to forming stars (Figure~\ref{specres}). 
Supplementary science studies include
calibration of the D/H abundance in fully molecular gas and
detailed measurement of the 
deuteration of star-forming cores that can ultimately
serve as a ``chemical clock'' for their evolution. 
The latter studies will measure absorption lines from the
ground states of HD (112~\micron) \citep{caux02}, H$_2$D$^+$ (218
\micron) \citep{bore93} and
D$^2$H$^+$ (203 \micron) \citep{phil02} 
in targeted sources in combination with
ground-based observations of infrared absorption lines (e.g. H$_2$)
\citep{kule02}.

Herschel and SOFIA will observe HD in clouds and (in part) will
provide the 
sample of sources which STO will examine in dramatically better detail.  
In comparison with Herschel and SOFIA, STO has the unique combination of (1) a
dedicated survey mission that will explore hundreds of star forming
regions and disks in HD, (2) high angular resolution (3$''$ at
112~\micron) that will disentangle the complex structure of star
forming regions and provide up to an order of magnitude more
sensitivity to absorption line measurements of ground-state HD, and
(3) large-format arrays to provide a large simultaneous 
($60''\times 60''$) field of view. 


\subsubsection{Goal 2:  The Life Cycle of Interstellar Clouds}

At a distance of 10 kpc, typical GMCs subtend 5 arcminutes, CNM
clouds $\sim$1 arcminute, and diffuse HII regions, 10 arcminutes. 
STO resolves these 
objects spatially and spectrally, and will determine directly their size
and internal velocity distribution as a function of
Galactocentric radius ($R$) and height ($z$). STO's spatial resolution
allows clusters of clouds to be discovered and their random velocity
dispersion measured.  The cloud to cloud velocity dispersion is
the key parameter which determines when gravitational instabilities
are able to collect clouds over huge ($\sim$1 kpc) regions to form
GMC's.

These regions dominate 205~\micron\ N$^+$ and 158~\micron\ C$^+$
emission\citep{mcke97}. 
Because STO can detect CNM
clouds, GMC's, and diffuse HII regions throughout the Galaxy
(ex. Figure~{\ref{CII}), and
because Galactic rotation generally allows velocity
separation of the clouds along the line of sight, STO will provide an
unprecedented global map of the distribution of clouds in the
Galaxy.  From the survey, which covers the majority of the cloud mass
and the star forming regions of the Galaxy, we can see how clouds are clumped
together in spiral arms or supershells. 
Similarly, 
the N$^+$ observations of diffuse HII clouds provide an unprecedented
spectroscopic survey of the location and rate of star
formation in the Galaxy. The rate of star formation is determined by
using the N$^+$ luminosity to determine the ionizing luminosity of OB
stars, a standard metric for the star formation rate.  Since 
the C$^+$ emissivity
per hydrogen atom rises monotonically with gas density and thermal gas
pressure, the STO survey 
enables the construction of the first barometric maps of the
Galactic disk, determining the ambient thermal pressure in different
environments (e.g., the spiral arms versus interarm regions, the
Galactic Center and higher Galactic latitudes), and 
probing and characterizing a turbulent
medium stirred by e.g. young stellar outflows and supernovae.  These
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
C$^+$ counterpart, we can attribute the HI emission to extended low
density gas -- either WNM or thermally unstable gas with densities
below that of CNM.  Simultaneous N$^+$ measurements will disentangle the
contribution to the C$^+$ emission from ionized gas.  To achieve
the required sensitivity, we will smooth the data to larger (10 \kms)
velocity and spatial (10$'$) bins, as shown in Figure~\ref{CII}. 
In this way, STO can map the CNM/WNM mass fraction in the
Galaxy, and determine how much of the neutral gas is in clouds. 
This ratio can be correlated
to the thermal pressure, to the ultraviolet radiation field, and to
the star formation rate to probe the stellar feedback processes that
regulate star formation. 

\subsubsection{Goal 3: The Formation and Destruction of Clouds}

{\bf The formation of interstellar clouds is a prerequisite for star
formation, yet the process has not yet been observed! STO is designed
with the unique combination of sensitivity and resolution needed to
observe atomic clouds in the process of becoming giant molecular
clouds (GMCs). }

Theories of cloud formation are guided and constrained by observations
of the atomic and molecular gas components. Based primarily on HI and
CO observations, four mechanisms have been proposed to consolidate gas
into GMC complexes (Figure~\ref{m33}): (1)
gravitational-magnetothermal instabilities within the diffuse gas
component, (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. STO's surveys, particularly in the C$^+$, N$^+$ and O$^0$
lines can distinguish these processes by: 
\vspace{-0.1cm}
\begin{enumerate}
\setlength{\topsep}{-5ex}
\setlength{\partopsep}{-5ex}
\setlength{\itemsep}{-0.4ex}
\setlength{\parsep}{-0.4ex}
\item Accounting for all the molecular hydrogen
mass (the H$_2$/C$^+$ clouds as well as the H$_2$/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 of
sufficient resolution, spatial coverage and dynamic range to probe a
wide range of interstellar phases and environments. 
\end{enumerate}

\vspace{-0.1cm}
\noindent In particular, C$^+$ emission barometrically picks out
clouds of atomic gas and H$_2$ clouds with little CO. Regions of GMC
formation may therefore be tracked by a large density of clouds per beam, 
or regions with individual
clouds with higher than average columns or pressures. With STO's
superlative spectral resolution, these regions can be identified with
superrings or spiral arms or convergent parts of a turbulent
medium. STO will follow the CNM clouds and H$_2$ clouds as they transit
the spiral potential, and will witness the process of cloud formation
directly from the atomic substrate or from small H$_2$ clouds. For
example, dust lanes along the inner edges of spiral arms often show
neither HI nor CO emission \citep{wikl90} and are therefore likely to be in an
intermediate phase; sufficiently dense and self-shielded to harbor H$_2$,
but not CO (see Figure~\ref{schematic}) \citep{stac91}. 
These clouds will be seen in C$^+$ line
emission by STO. The high spectral resolution of STO enables crucial
kinematic studies of the Galaxy to be made. The expansion of stellar
outflows and supernova  remnants create supershells that sweep up
surrounding ISM and 
overrun surviving molecular clouds and cloud fragments. The high
pressures in the shells convert swept-up WNM gas to CNM clouds via
thermal instability. The resulting supershell can grow to several
times the typical thickness of the gas layer in a galactic disk,
creating superrings that can contain millions of solar masses of
swept-up gas. Gravitational fragmentation of superrings may be an
important mechanism for the formation of GMC's \citep{mccr87}. 

STO will determine
the kinematics and thermal pressures of most supershells, fossil
superrings, and molecular clouds just condensing via gravitational
instability of old superrings and supershells. STO will detect
the CNM clouds formed out of WNM in the shells, and the H$_2$ clouds which
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.  STO witnesses the disruption of GMCs
and all CNM or C$^+$/H$_2$ clouds with columns greater than about
$10^{20}$ 
cm$^{-2}$, since N$^+$ measures the flux of ionizing photons, and
C$^+$ measures 
their impact upon neighboring cloud surfaces. STO will measure the
resolved photoevaporating atomic or ionized gas driven from clouds
with UV-illuminated surfaces, thereby converting the clouds to WNM or
to diffuse HII regions. Thus, STO can directly determine the rate of
mass loss from all catalogued clouds, and their destruction
timescales. STO's survey will correlate the star formation rate in a
given OB association with the rate of destruction of the nearby
(within 0.1-30 pc) natal GMC. It will demonstrate if CNM clouds are
being effectively destroyed by the enhanced fluxes of UV coming from
relatively nearby (50-200 pc) OB associations. Such
measurements are crucial for models of star formation feedback and
global galactic evolution. 


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.55,angle=0]{./figures/m33.eps}
\caption{\label{m33} {\small The location of GMC's in the nearby
    spiral galaxy M33 are overlaid upon an integrated intensity map of
    the HI 21~cm line \citep{enga03}.  These observations show that
    GMC's are formed from large structure of atomic gas, foreshadowing
    the detailed study of GMC formation that STO will provide in the
    Milky Way and nearby galaxies like M33. }} 
\end{figure}


\subsubsection{Goal 4: The Star Formation Rate in the Galaxy}

Star formation within galaxies is commonly described by two empirical
relationships: the variation of the star formation rate per unit area
with the  (atomic + molecular) gas surface density \citep{schm59,kenn98} and a
surface density threshold below which star formation is
suppressed \citep{mart01}.  The Schmidt Law has been evaluated from the radial
profiles of H and HI, CO emissions for tens of galaxies. The mean
value of the Schmidt index, $n$, is $1.4 \pm 0.15$ \citep{kenn98}, 
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. 

The STO survey of C$^+$ and N$^+$
emission provides the optimum set of data to calculate the Schmidt Law
in the Galaxy. The N$^+$ line is an excellent tracer of the star
formation rate as it measures ionizing luminosity with unmatched
sensitivity, angular and spectral resolution, and is unaffected by
extinction. The C$^+$ line, in conjunction with HI~21cm and CO line
emission, provides the first coherent 
map of the neutral interstellar gas surface density and its variation
with radius. STO's high spectral resolution allows one to assign a
radial location of any emission feature assuming a rotation curve. The
Schmidt Law is constructed from the radial profiles of the star
formation rate derived from N$^+$ emission and the gas surface
density. 
The column density threshold
is inferred from the absence of star formation activity in the outer
radii of galaxies where there is still a significant reservoir of
gas \citep{kenn98}. It has been attributed to the conditions required for the
gravitational instability associated with the Coriolis force to
consolidate CNM clouds into GMC's \citep{kenn98,mart01}. 
The velocity-resolved star
formation rate indicators 
provided by STO will be invaluable in interpreting more traditional
indicators, like the far-infrared continuum.
With its resolution and ability to gauge
thermal ISM pressure, STO evaluates this critical, regulatory process
in the Milky Way. 

\subsubsection{Goal 5: The Milky Way Template}

C$^+$ 158\micron, the strongest Galactic cooling line, will be the premier
diagnostic tool for studying external galaxies 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 C$^+$, one
must turn to the Milky Way for the spatial resolution needed to
disentangle the various contributors to the total C$^+$ emission. 
The STO mission covers a
broad range of density and UV intensity, establishing the relationship
between physical properties, C$^+$, N$^+$, O$^0$, 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. 

The exceptional angular resolution that STO provides will be used to
test and extend the Milky Way template against nearby galaxies where
star forming complexes and GMC's can be resolved (Figure~\ref{m33}).  
The Milky Way
template will first be expanded toward the low-metallicity limit by including
star forming regions in the SMC and LMC.  Application of the template
to nearby star forming galaxies, such as IC 342 and dwarf irregular
NGC~5253 will help elucidate the triggering mechanism(s) and evolution of
the starburst phenomenon, and provide the necessary calibration and
testing needed for application toward more distant, unresolved
IR-luminous galaxies.  These notions will be explored in further
detail in the Concept Study.

\subsection{Comparison of STO with other Far-IR Spectroscopic
  Platforms}

STO is a powerful survey telescope capable of resolving
clouds in crucial spectroscopic species 
and using Galactic rotation to place them along a line of 
sight.  It builds upon the heritage of six pioneering telescopes: the
Cosmic Background Explorer, the Balloon-borne Infrared Carbon Explorer
(BICE), the Infrared Telescope in Space (IRTS), the Submillimeter Wave
Astronomical Satellite (SWAS), the Infrared Space Observatory (ISO)
and the Herschel Space Observatory.  
To the first five, STO adds many orders of magnitude
in sensitivity, spatial, and spectral resolution.  None of the C$^+$
and N$^+$ 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 C$^+$ or N$^+$ emission\citep{petu93}.    SWAS had 
two hundred times less collecting area, 
and Shottky receivers with 30 times the receiver noise
as the cooled SIS mixers that will be used for the
0.6~THz channel in STO.   Compared to Herschel, STO's
integrated heterodyne receiver arrays increase mapping speed by two
orders of magnitude and sensitivity to point sources by an order of
magnitude. 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.23,angle=0]{./figures/FIRAS.eps}
\caption{\label{COBE} {\small COBE FIRAS image of [C~II] (top) and
    [N~II] (bottom) integrated intensity at 7$^o$ angular resolution.
    These data were taken with velocity resolution 3,000 \kms, greater
    than the Galactic velocity dispersion.}} 
\end{figure}

It is illuminating
to put the capabilities of STO in context with the successful Far
Infrared Absolute Spectrophotometer (FIRAS) instrument on
COBE. Convolved to the angular and spectral resolution of FIRAS, STO
is $> 10^6$ times more sensitive, even in its moderate
sensitivity (UGPS) survey mode.  STO's sensitivity is therefore traded
for angular and velocity resolution that optimally couples to the
physical scales of important interstellar components in the
Galaxy. Even at at 5$''$ angular resolution and 0.6
\kms velocity channels, 
the mean level of C$^+$ emission seen by FIRAS in the Galactic plane is
detected by STO at the 5$\sigma$ level in 1 second! 

STO is unique and timely when compared with concurrent airborne and
space observatories.  The Spitzer
Space Telescope (SST, formerly SIRTF) has no spectroscopic capability at these
wavelengths.  A series of balloon missions cannot provide the database
essential for accomplishing STO's science goals. A prohibitive number,
almost one hundred 10-day balloon flights, would be required to
achieve equal sensitivity, and there is no long duration balloon
flight capability in the northern hemisphere. STO is optimized to support
large-format heterodyne arrays, with broad spectral coverage that includes
interstellar water.  The advantage
of STO is its ability to provide large scale coverage, while
simultaneously providing higher angular resolution than both Herschel and
SOFIA.  There will not be sufficient time on these other
facilities to complete even a small fraction of the Galactic survey of
the scale proposed for STO.  We estimate that each facility will map
approximately 1\% of the area of STO's survey during their
lifetimes.  STO's database will provide important diagnostics to be used
with future far-IR continuum surveys, such as those suggested for
SAFIR. 


\begin{figure*}[!hbt]
\centering
\includegraphics[scale=0.34,angle=0]{./figures/flow-of-requirements.eps}
\caption{\label{flow} {\small Flow of requirements}} 
\end{figure*}



\section{Mission Concept}

The scientific objectives of the STO can be achieved by having a
dedicated mission or providing a comparable heterodyne instrument for
SAFIR (Single Aperture Far-Infrared Observatory). Below we briefly
discuss the advantages and disadvantages of each approach. The
relative merits of the two mission concepts will be examined in detail
during the concept study. 

\subsection{Heterodyne Augmentation to SAFIR}

SAFIR is a large (10-m class), cold (4-10K) space telescope for
wavelengths between 20 \micron\ and 1~mm. This wavelength range
encompasses that of STO, opening the possibility of combining the two
missions. Indeed, SAFIR's telescope performance (e.g. surface
quality, pointing) exceed those of STO. The only additional optical
requirement would be to allow for the possibility of a chopping
secondary or tertiary.  The
most significant mission impact would be increasing spacecraft power
to cover the additional load of the heterodyne instrument and increasing the
SAFIR mission lifetime to allow implementation of the STO science
program.  STO science
investigations do not require a cooled aperture. Therefore one mission
design approach could be to conduct investigations requiring a cooled
aperture first and then shut-down the cryogenic systems responsible
for cooling the aperture (and sun shield) for the heterodyne portion
of the mission. The spacecraft power that was used to cool the
aperture would then be used to meet the additional power needs of the
heterodyne instrument package.   

%{\bf During the concept study we will work with the SAFIR study team to
%examine
%design and cost impacts of implementing a heterodyne instrument
%package on SAFIR. } 

\subsection{Dedicated Mission - STO}

All science goals outlined in Section 1 are met with a purely
heterodyne instrument. Heterodyne instruments are far less sensitive
to telescope emissivity than broadband, incoherent detection
systems. Indeed, there is no need to actively cool the
telescope, resulting in a significant
reduction in mission complexity and cost. With a heterodyne-only STO
mission a deployable sunshade may not be needed. The additional
cryogenics required by SAFIR to cool telescope optics are not
required. Thermal blankets on the back of the reflector may be all
that is needed to maintain the figure of the telescope in both
targeted and survey modes. Furthermore, without the need for cooling,
the STO reflector can be made of lightweight, relatively low-cost,
temperature insensitive, carbon composite panels. A telescope design
incorporating these simplifications is shown in Figure~\ref{ball} in
Section~\ref{spacecraft}.  The shortest
wavelength at which STO will observe is 60~$\mu$m, $3\times$ longer than
that of SAFIR. This results in significantly reduced telescope surface
and pointing requirements (see table in Section~\ref{sense}). 
Preliminary investigations
suggest the pointing specification can be achieved using a standard Ball star
tracker and reaction wheels; active control of the telescope surface
is not required. 
The anticipated instrument power (1~kW) and data
rates (0.5~Mbits/sec) can be supported by a JWST style spacecraft
bus.  Locating STO at L2 has several advantages. These include mission
lifetime, the ability to radiatively cool portions of the instrument,
low radiation environment, accessibility, and amenable data rates. 

%{\bf During the concept study our team will carefully examine all
%aspects
%of the STO mission option and perform a cost/risk comparison between
%it and the implementation of a heterodyne instrument on SAFIR.     }

\section{Instrument Roadmap}

Figure~\ref{flow} shows how the major science goals described in the
previous
sections define  
telescope and instrument capabilities and, ultimately, the key
technology development areas. The proposed instrument concept greatly
benefits from the heterodyne receiver technology  
developed for the HIFI instrument on Herschel. In the following
sections we present an instrument concept for STO. The same concept
could also be optimized for implementation on SAFIR. 
Together with a large aperture telescope,  
the instrument would provide a powerful, unique, 
capability for exploring the formation of stars and planetary systems,
the life cycle of the ISM in the  
Milky Way and nearby galaxies, the starburst phenomenon, and [C~II] and
[N~II] emission from more distant galaxies.    


\subsection{System Description}

A schematic of the STO instrument concept is presented in
Figure~\ref{optics}.  The f/19 beam from  
the secondary passes through the apex hole of the 9m segmented
carbon composite  
primary and encounters a fold mirror which directs it to the
instrument cryostat. The fold mirror can be chopped at a ~5~Hz rate.  A
vane chopper located between the fold mirror and cryostat is used to
calibrate the observed intensity scale. Upon entering the cryostat the
telescope beam passes through two frequency selective surfaces (FSS)
and two wire grids that divide the light into 4 frequency
sub-bands. Band 1 is optimized to observe the 557 GHz ground-state
($1_{10}- 1_{01}$) ortho water line. Band 2 covers the 1 to 2.1~THz
band, which includes [C~II], [N~II], and the ($2_{12} - 1_{01}$)
1.69~THz water line.  Band 3 is optimized for the 112~$\mu$m HD line
and Band 4 for the 63 $\mu$m [O~I] line.  A third
wire grid is used to divide Band 1 into orthogonally polarized
components that are independently detected and co-added. Band 1 uses
two, orthogonally polarized, 4x4 arrays of SIS mixers. Bands 2, 3, and
4 utilize single polarization, 8x8 arrays of HEB mixers. Below 2.1~THz
solid-state sources provide  
the $\sim1$~mW of LO power needed to efficiently pump the
arrays. Above 2.1~THz   
FIR or quantum cascade lasers serve as the LO sources. Low-noise,
4-8~GHz MMIC 
amplifiers are used to a boost the intermediate frequency (IF) output
signal of each mixer  
to a power level suitable for processing by 2-4~GHz wide, 2048 lag,
single-chip,  
correlators. The broadband IF output of each mixer will be detected and
made available for continuum measurements of planets (calibration and
pointing) and other astrophysical objects.
The instrument architecture is flexible,
allowing the user to  
configure the instrument to target the optimum lines for a particular
observing program.  
We anticipate it will be possible to observe with $\sim$288 pixels at
a time. The mixer arrays and IF amplifier arrays will be cooled to 4
and 15~K respectively by a low-power, 
low-microphonic cryocooler. The lifetime of all instrument components
will be $\ge$5 years.  
%{\bf During the concept study we will carefully examine the design and cost
%of the instrument described above and develop a technological roadmap to
%meet mission objectives.}  
 
 
\begin{figure}[hbt]
\centering
\includegraphics[scale=1.1,angle=0]{./figures/opticalpath.eps}
\caption{\label{optics} {\small STO optical path}} 
\end{figure}
 
 
\subsection{Expected Sensitivity}
\label{sense}
The Nb SIS device technology used in the Band 1 mixers is very
mature. Lab measurements of waveguide mounted SIS devices routinely
yield receiver double sideband (DSB) noise temperatures of
$\sim$100K, with noise reductions to $\sim$60K expected in the near
future \citep{kooi03}. Above $\sim$1~THz signal loss in the on-chip
tuning structures required to neutralize device capacitance cause SIS
noise temperatures to rise above $\sim$500~K. At these frequencies HEB
mixers become competitive with SIS mixers. Lab measurements on
waveguide and quasi-optical phonon-cooled HEB mixers in the 1.5~THz
range have yielded DSB receiver noise temperatures in the 1000-1400K
range (Tong et al. 2003; Yngvesson 2003). Using devices made with
enhanced NbN HEB processing techniques and ~2~$\mu$W of  LO power,
Baselmans et al. (2004) have recently demonstrated an HEB mixer (in
this instance quasi-optical) with a DSB receiver noise temperatures of
~950K. Unlike SIS devices, HEBs do not have a bandgap limitation or
significant capacitance to tune out. As long as the signal is coupled
to the device with low-loss transmission line (e.g. micromachined
waveguide) the performance of HEB mixers is not expected to vary
significantly with frequency (Yngvesson 2003). For sensitivity
calculations for Bands 2, 3, and 4 we have assumed the receivers will
have DSB noise temperatures of 1000K. This translates into a system
noise temperature of $\sim$2000K. With the 10 sec integration time per
pixel characteristic of the unbiased, galactic survey (UGS) mode, STO
will be able to achieve an rms noise level of  $<$0.25K at a 1~\kms\
velocity resolution in each pixel. The STO instrument characteristics
are summarized in the table below.

%\begin{table}
%\centering
%\caption{\label{instrument} Instrument Summary}
\footnotesize
\noindent \begin{tabular}{|l|l|} 
\hline
{\bf Parameter} & {\bf Value} \\
\hline \hline
Telescope  & 9m -- deployable \\
 & \small carbon composite reflector \\ 
Aperture Efficiency  &  66\% \\ 
Main Beam Efficiency  &  90\% \\ 
Mirror Surface Accuracy  & $< \lambda$/30 \\
 &  total error @60 \micron \\ 
Absolute Pointing  &  $< 0.3''$ (3$\sigma$) \\ 
Jitter  &  $< 0.3''$ (3$\sigma$) \\ 
Receiver Types  &  four $8\times8$ HEB arrays\\ 
		 &  two $4\times4$ SIS arrays\\ 
RX Operating Temp  &  $\sim$4 K \\ 
Target Frequencies &   \\ 
Band 1:  H$_2$O $1_{10} - 1_{01}$  &   557 GHz  (538 \micron) \\ 
Band 2:  H$_2$O $1_{11} - 0_{00}$  &  1.11 GHz  (269 \micron) \\
Band 2:  {[N~II]}   &  1.46 THz  (205 \micron) \\ 
Band 2: H$_2$O $2_{12} - 1_{01}$  &  1.69 THz  (179 \micron) \\ 
Band 3:  {[C~II]}  &  1.90 THz  (158 \micron) \\ 
Band 4:  HD   &  2.68 THz  (112 \micron) \\ 
Band 5:  {[O~I]}  &  4.76 THz  (63 \micron)   \\ 
    System Noise Temp.  &  $<$2000 K (DSB) \\ 
    Backend Spectrometer  &  Autocorrelators \\
    & \small 2-4 GHz BW (2048 lags) \\ 
    Orbit  &  L2 \\ 
    Mission Lifetime  &  $\sim$5 years \\ 
\hline
\end{tabular}
\normalsize
%\end{table}

\subsection{Component Technologies}
 
\subsubsection{Mixers}
 
As discussed in Section~\ref{sense}, SIS and HEB device technologies
have
or are rapidly approaching the noise performance required by STO. The
technological challenge will be in efficiently packaging the devices
into integrated arrays.  
%{\bf In the design study we will investigate
%the
%relative advantages of implementing the arrays using waveguide and
%quasioptical techniques.} 
In Figure~\ref{feed1} we
present 
photographs of laser etched feedhorns at 1.5~THz to
illustrate
the technologies available for the realization of THz arrays.   
 

\begin{figure}[hbt]
\centering
\includegraphics[scale=0.35,angle=0]{./figures/corrugated.eps}
\caption{\label{feed1} {\small Laser Micromachined 1.5~THz
    corrugated feedhorn array}} 
\end{figure}
 
 
\subsubsection{Technical Approach for the LO Hardware}
 
The technical approach for developing and delivering the LO sources up
to 2.1~THz is very similar to the approach being utilized by the
Heterodyne Instrument (HIFI) on the Herschel Space Observatory
(HSO). All of the recent work has concentrated on advancing planar
technology
into the THz range; but only single pixel applications have been
considered.  However, for array deployment, a paradigm shift is
necessary.  Whereas before emphasis was placed on producing a handful
of superbly working prototypes, now the requirement is to produce
large format arrays without sacrificing sensitivity.   
 
An example of an integrated LO topology is diagrammed in
Figure~\ref{topology}. 
It consists of
a ``tray'' containing a linear array of multiplier units. The savings in
system assembly and operational logistics is substantial. A single LO
chain
similar to the type used by Herschel is about 9 cm long by 1.9 cm
wide. The chain ``tray'' of Figure~\ref{topology} 
produces four 1600~GHz outputs, and
is only 0.5 by 0.8~cm. In order to produce a two-dimensional array,
trays such as that in Figure~\ref{topology} can be stacked vertically.  
%{\bf In the design study we will further investigate: 1) the projected
%performance of LO arrays, 2) thermal isolation between stages, and 3)
%waveguide and quasi-optical approaches
%for coupling LO power into the array mixers. }
 
\begin{figure}[hbt]
\centering
\includegraphics[scale=0.3,angle=0]{./figures/topology.eps}
\caption{\label{topology} {\small Heterodyne receiver array "tray"
    concept. Each tray will produce four 800 GHz 2 mW outputs from a
    single 500 mW 100 GHz input. The "trays" can be stacked to give a
    2-D array. }}  
\end{figure}
 
 
Pumping mixers in the 2-4~THz range continues to pose a great
challenge.  Sufficient pump power beyond 2~THz from multiplied sources
is difficult to achieve.  The baseline approach would be to use
FIR lasers which have been space qualified. Sideband generators could 
potentially be used with the FIR lasers to add a degree of tunability. 
Quantum Cascade Lasers (QCLs) are another promising technology. Test 
results suggest they have the potential of
providing a compact low-power solid-state source for driving THz single pixel or
array heterodyne receivers.
 
%\smallskip
%{\bf In the concept study we will further explore the development of
%  arrays of solid state LO sources for the 1--2.1~THz range (STO Bands
%  2 \& 3) and FIR lasers and QCLs for 2.68~THz (STO Band 4) and
%  4.76~THz (STO Band 5). The technology for the STO Band 1 (557~GHz)
%  SIS array is well in hand.} 
 
\subsubsection{ IF Amplifiers}
 
During the past 4 years wideband, very low noise, cryogenic monolithic
microwave integrated circuit (MMIC) amplifiers have been developed with
design and testing at JPL and Caltech and foundry fabrication at TRW
(now Northrop Grumman Space Systems,  NGST) and HRL.  These LNA's match the
requirements needed for densely packed focal plane 
arrays in terms of noise temperature,  chip size,  DC power dissipation,
yield,  and bandwidth. A typical chip achieves noise temperature of $<$
5~K at 12~K when driven from a 50 ohm generator
impedance.  
%{\bf During the concept study we will determine the IF amplifier
%requirements (e.g. frequency range, power consumption at 12~K, input
%match, and noise temperature) for the STO mixer arrays and develop a
%technological roadmap to achieve them.    }
 

\begin{figure}[htb]
\centering
\includegraphics[angle=0,scale=0.36]{./figures/correlator.eps}
\caption{\small \label{labspec} The Spaceborne digital
  autocorrelator provides an ideal ``off-the-shelf'' multichannel
  spectrometer for STO. a) 2~GHz Spaceborne correlator chip.  b)
  Laboratory measurement of spectral response to single frequency input.}
\end{figure}
  
\subsubsection{Array Spectrometers}
 
Rapid advances in autocorrelator and high-speed, analog-to-digital(A/D) converter technology enables multi-pixel systems
to be used on STO.  Modern correlators incorporate a 2-bit/4-level
digitizer and a 2048-lag auto-correlator on a single chip.
 Figure~\ref{labspec} is a
laboratory measurement of the response of a modern 2~GHz 
correlator chip, manufactured by Spaceborne Inc. 
A full~2 GHz bandwidth is achieved.  
A similar result has been achieved by another firm, OmniSys Inc, who
have constructed 
a 2~bit/3~level correlator chip with 1~watt power dissipation.
Within a 7-10 year time frame, the expected power requirements of
these correlator chips is projected to be reduced by an order of
magnitude. 

Within the past year high-speed A/D converters coupled with
FPGAs have also been demonstrated to provide a full 1~GHz of spectral
coverage. This approach has several advantages
over using autocorrelators: 1) essentially no loss in signal-to-noise
ratio due to digitization, 2) as much as an order of magnitude
improvement in spectral resolution, and 3) the capability of being
reconfigured through software once constructed. The power requirements
of these devices (now $\sim$10W) are also expected to drop significantly
with time.  
 
%{\bf In the design study we will determine the operating requirements of
%the STO correlator system (power, mass, processing speeds) and develop
%a technological roadmap to achieve them. }
 
\begin{figure}[htb]
\centering
\includegraphics[angle=0,scale=0.5]{./figures/STO-scope.eps}
\caption{\small \label{ball} The deployed STO spacecraft and telescope.}
\end{figure}

\subsubsection{Cryocoolers}
 
The STO instrument will use long-life, low-power, low-microphonic
cryocoolers to 
achieve the optimum operating environment ($\sim 4$~K) for the THz array
receivers.
An Engineering Unit of a cryocooler is under development 
at Ball for the NASA/JPL ACTDP (Advanced Cryocooler Technology
Development Program).
The foundation of the Ball Cryocooler design is component 
and system development proven in test verification from Ball programs
such as the 
ACTDP Study Phase, the Ball Aerospace SB235, the NASA Explorer 6 K, the
AFRL 10 K, 
and the DOD COOLLAR, in combination with technology proven on external
programs such 
as the RAL 4 K Cooler and JPL Planck programs.  Cooling capacities 
in excess of 25 mW were measured for temperatures down to 5 K. 
%{\bf During the concept study the cryogenic requirements of the STO
%instrument will be
%fully charcterized and a technological roadmap leading from current
%cooler
%technologies
%to those needed for STO developed.}
 
\begin{figure}[htb]
\centering
\includegraphics[angle=0,scale=0.55]{./figures/STO-stowed1.eps}
\hfill
\includegraphics[angle=0,scale=0.44]{./figures/STO-stowed2.eps}
\caption{\small \label{stowed} Side and top views of the STO
  spacecraft. Telescope \& spacecraft fit in the smallest available 5m
  fairing. }
\end{figure}
  

 
\subsection{STO Telescope Description}
\label{spacecraft}
The STO provides a $120''\times 120''$ field of view to
the heterodyne spectrometer arrays at a focal ratio of f/19 from a
9-meter
f/1 primary mirror composed of seven hexagonal segments
(Figure~\ref{ball}).  
 STO's primary mirror is maintained
at a uniform temperature with minimal thermal gradients by thermal
blanketing on the primary mirror support. The secondary and its support
structure are shielded from the Sun by the primary's thermal blanket for
Sun-Earth off-axis angles $\le 45^\circ$.
To package STO for launch
in a 5-meter Delta-IV or Atlas V faring, the 9-meter primary mirror
incorporates a
deployment scheme identical to the James Webb Space Telescope concept;
the primary mirror articulates in two locations to fold in thirds, one
of the three support spiders for the secondary mirror folds in half,
and the other two support spiders hinge at the primary mirror
attachment points (Figure~\ref{stowed}).  Once the primary
mirror is deployed the segments are adjusted in tip, tilt, and piston
by mirror-mounted actuators using mirror edge sensor position feedback
to position them accurately. The surface figure can be independently
verified by measuring the quality of the diffraction limited telescope
beam on astrophysical objects (e.g. planets). Carbon fiber reinforced
polymer composites are
used to replicate the hexagonal segments for the primary and secondary
mirrors, a technique similar to construction techniques used for
ground-based radio astronomy telescope manufacture and a method that
will allow significant cost savings in comparison to the cost of
beryllium mirror manufacture for JWST.  A comparison of STO and JWST performance requirements is provided in the following table.
 
\medskip

{\noindent \small
\begin{tabular}{|l|l|l|} 
\hline
{\bf Parameter} & {\bf  STO } & {\bf JWST} \\
 & {\bf 2004 } & {\bf 2004 (1998)} \\
\hline \hline
Primary Mirror Diameter  &  9.0m  & 6.4m (8.4m) \\ 
Primary Mirror Material  &  Carbon & Beryllium \\ 
 & composite & \\
Sunshade vanes  &  0  &  5 \\ 
Mirror surface figure  &  2 \micron  &  0.023 \micron \\ 
Pointing Requirements  &  0.5$''$  &  0.15$''$ \\ 
Telescope temperature  & 100-300 K  & 40 K \\ 
Segment actuators  &  rigid body  &  rigid body\\
 & & \& radius of \\
 & & curvature \\ 
Number of Instruments  &  1  &  3 \\ 
Mission life  &  5 yr  &  5 yr \\ 
Orbit  &  L2  &  L2 \\ 
Daily Data Volume  &  22 Gb/day  & 33 Gb/day \\ 
Ground Testing  &  warm  & cold \\ 
Mass  & 2500 kg  &  (4100 kg) \\ 
Power  & 1400 W  & (570 W) \\ 
\hline
\end{tabular}
}

\section{Summary}

The Space Terahertz Observatory (STO) is a deployable, uncooled, 10-meter-class, far infrared space telescope optimized to 1) conduct origin 
studies of planets, stars, and molecular clouds; 2) trace the life 
cycle of the Interstellar Medium (ISM) and star formation rate throughout 
the Galaxy; 3) measure the gas content of formative pre-planetary disks; 
and 4) observe the distribution of atomic and molecular gas in both 
nearby and distant galaxies.  STO will achieve these scientific objectives through high
spectral and angular resolution observations of C+, O, N+, HD, and H2O 
lines in the far-infrared. The science goals of STO can be achieved either 
through a dedicated mission with an uncooled 8-10 meter primary or by 
implementing heterodyne instrumentation on SAFIR and extending its 
operational lifetime.


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\end{document}

\medskip
 
\noindent {\bf Issues to be addressed during the concept study:}
{\it Optical Technology Development Items:}
\begin{itemize}
\setlength{\topsep}{-5ex}
\setlength{\partopsep}{-5ex}
\setlength{\itemsep}{-0.55ex}
\setlength{\parsep}{-0.4ex}
\item Scaling CFRP mirror manufacture process to segments $>$2m
\end{itemize}
{\it Potential Optical Trades:}
\begin{itemize}
\setlength{\topsep}{-5ex}
\setlength{\partopsep}{-5ex}
\setlength{\itemsep}{-0.55ex}
\setlength{\parsep}{-0.4ex}
\item JWST-style sunshade vs. primary mirror thermal blanketing
\item Primary mirror diameter 
\item Mirror segment size and surface figure (e.g. spherical or
parabolic
segments)
\item Mirror segment radius of curvature control 
\item Mirror material: CFRP or pyrolytic graphite 
\end{itemize}
{\it Potential Mission Trades:}
\begin{itemize}
\setlength{\topsep}{-5ex}
\setlength{\partopsep}{-5ex}
\setlength{\itemsep}{-0.55ex}
\setlength{\parsep}{-0.5ex}
\item L2 vs. Earth-following
\item S/C jitter compensation with Fine Steering Mirror vs. Star
trackers + SIRU  
\item Thermal blanketing behind primary mirror vs. deployable sun shield
\item Gimbaled vs. fixed solar array panels 
\end{itemize}


\section{Management Approach}

\subsection{Project Management}

We provide a strong, yet flexible management structure (Figure~\ref{orgchart})
that 
enables us to maintain a tight focus on the tasks at hand. The overall 
responsibility for the effort will fall to the Principal Investigator, 
Christopher Walker.  Professor Walker will be the primary arbiter,
responsible for  
the fulfillment of the core technical and programmatic goals in the
STO Mission concepts.  
Our team structure is built around four working groups (WG); Science,
Instrument,  
Mission, and Telescope/Spacecraft. Each of the working group will be
led by a Group Leader. 
The WG Leaders will be responsible 
for overseeing the activities of each working group, organizing and
defining tasks,  
coordinating group participants, and providing weekly progress updates.  
WG leaders will help organize the project workshop activities and play
significant  
roles in the generation of the final report. Institutional  PI's at
JPL, ARC, and  
GSFC will coordinate activities within their organizations. 

The formation of working groups focuses the necessary
expertise and effort 
in the different study areas. Depending on their range of expertise, a
team member may participate in more than one WG.  
We envisage the WG approach as the appropriate method to develop a
mission concept that maximizes scientific return and minimizes cost
through heritage, simplicity, \& use of enabling technologies. 

A secure website will be used to post reports and ensure 
ready access to project materials.

\subsection{Schedule and Milestones}

A timeline of the STO concept study is provided in
Figure~\ref{timeline}.  The schedule includes 
key milestones \& activities designed to maintain focus \& productivity. 

{\it Weekly Status Meetings}: We plan to hold weekly telephone meetings
throughout the project.  
During these meetings, WG Leaders and Institutional PIs will update
the PI and team on  
progress toward completion of the project goals. 

{\it DRM Review Meeting}: During the first month the PI, WG Leaders will
focus on the development  
of a Design Reference Mission (DRM). At the end of the first month we
will hold a two day  
DRM Review open to all team members. The DRM Review will be a tightly focused 
exchange to tune the DRM prior to commencing the detailed studies.

{\it Working Group Leader Meeting}: Mid-way through the study we will hold
a second 2-day  
meeting of the PI with WG leads. The purpose will be to prepare the
team's presentation   
for the Origins Science Mission Concept Workshop and identify focus
areas for the remaining period of the study.


\begin{figure}[htb]
\centering
\includegraphics[angle=0,scale=0.35]{./figures/timeline.eps}
\caption{\small \label{timeline} Timeline for STO Concept Study}
\end{figure}

\clearpage

\thispagestyle{empty}

