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\begin{document}

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\twocolumn[
\Large 
\centerline {\bf Project Description}
\normalsize
\vspace{0.5cm}
]

We propose to use a combination of archival and new observations at
infrared, radio, and millimeter/submillimeter wavelengths to identify and characterize
``proto-molecular clouds'' in the Galaxy. Such a study would have
a significant impact on our understanding of the initial conditions
of star formation, galactic evolution and the interstellar medium of external galaxies.
The proposed work will:
\begin{enumerate}
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\item Produce a catalog of regions in the Galaxy where
  molecular cloud formation may be occurring, using archival infrared,
  CO and HI data.
\item Directly measure the abundance and excitation
  of  \htwo, C and CO relative to 
infrared dust extinction \& far-IR dust emission for each candidate ``protocloud''.
\item Aim to provide initial statistics to determine which mechanism of
  cloud formation is most prevalent in the Galaxy.
\item Present a ``finder chart'' of cloud formation for future
  submm/THz/far-infrared observatories such as SOFIA, ALMA, and Herschel.  
\item Provide unique opportunities for public outreach and education,
  with regard to interactive online content, to programs for local
  schools, summer camps and (under)graduate student research projects.
\end{enumerate}

\section{Introduction}

From the Milky Way to the highest-redshift protogalaxies, the internal
evolution of galaxies is defined by processes closely related to their interstellar contents: 
\begin{enumerate}
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\item the transformation of neutral, molecular 
gas clouds into stars \& star clusters.

\item the interaction of the interstellar medium
(ISM) with the young stars that are born from it, a regulator of
further star formation. 

\item the return of enriched stellar material to the
  ISM by stellar death, eventually to form future generations of stars.

\end{enumerate}

\noindent The evolution of galaxies is
therefore determined to a large extent by the life cycles of
interstellar clouds: their creation, star-forming properties, and
subsequent destruction by young (hot) stars.  

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.38,angle=0]{./figures/LifeCycle-ISM.eps}
\caption{\label{cycles} {\small Life Cycles of the ISM}} 
\end{figure}

The life cycle of interstellar clouds is summarized pictorially in
Figure~\ref{cycles}. These clouds are largely comprised of
atomic \& molecular hydrogen and atomic helium. These
species are notoriously difficult to detect under normal interstellar
conditions.  Atomic hydrogen is detectable via the
21~cm spin-flip transition and provides the observational basis for
current models of a multi-phase Galactic ISM \citep{kulk87}.  Its emission
is insensitive to gas pressure and does not always discriminate between 
cold (T$\sim$70K) atomic clouds and the warm (T$\sim$8000K) neutral medium
that is thought to pervade the Galaxy. 
Furthermore, neither atomic helium nor molecular hydrogen
(H$_2$) have accessible emission line spectra in the prevailing
physical conditions in cold interstellar clouds.   Thus, it is important
to probe the nature of the ISM via rarer trace elements. 
Carbon, for example, is found in ionized form (C$^+$) in neutral clouds,
eventually becoming atomic (C), then molecular as carbon monoxide (CO)
in dark molecular clouds (Figure~\ref{PDR}).

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.48,angle=0]{./figures/PDRplot.eps}
\caption{\label{PDR} {\small  Steady state depth dependence of hydrogen and carbon
    species abundances in a typical molecular cloud exposed to the
    local interstellar radiation field.   The HI--\htwo\ interface occurs long before
    the \cplus--C--CO interface, leaving diffuse and translucent
    material whose hydrogen is fully molecular, but whose carbon is
    locked up in \cplus.  {\bf The depth difference between the hydrogen
    and carbon fronts INCREASES with the external radiation field
    strength and with decreasing cloud age.}    
}} 
\end{figure}

The latter of these, the CO molecule, is the most prevalent gas-phase
tracer of molecular clouds.   Since its first measurement via
millimeter-wave spectroscopy $\sim$35 years ago, numerous CO surveys now
provide nearly all-sky coverage and are responsible for most of what is known
about the distribution, physical structure and kinematics of molecular
clouds in the Galaxy.   

However, because the abundance of CO stabilizes only at significant cloud depths where
hydrogen is already molecular (Figure~\ref{PDR}), it makes an ineffective
probe of the formative and disruptive stages of a cloud's life cycle.
{\bf Thus, despite great efforts and millions of CO spectra, 
the conclusive birth of a molecular cloud has never been identified! }   Indeed, 
although we are now beginning to understand the formation of stars, the
formation, evolution and destruction of their parent molecular clouds
still remains (literally) shrouded in uncertainty.   

The need to understand the evolution of interstellar clouds in the context 
of the initial conditions of star formation has become acute -- driven
by the advent of detailed infrared studies of external galaxies, the
expectation of soon detecting the ``first light'' from primordial star-forming
galaxies at high redshift, and the ever-increasing numerical
resolution of galaxy simulations requiring better constraints on ISM
physics.  The National Research Council's
most recent {\it Decadal Survey} has identified the study of star
formation as one of the key recommendations for new initiatives in
this decade. Similarly, understanding the processes that give rise to
star and planet formation represent the central theme of NASA's
ongoing Origins program. 

\subsection{Signposts of Molecular Cloud Formation}

Theories of cloud formation are guided and constrained by observations
of the atomic and molecular gas components. Based primarily on HI and
CO observations, several 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 atomic or molecular
clouds, (3) accumulation of material within high pressure environments
such as rings and shells generated by OB associations, and (4)
compression in the randomly converging parts of a turbulent medium.  

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.58,angle=0]{./figures/m33.eps}
\caption{\label{m33} {\small The location of GMCs 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
    GMCs are formed from large structures of atomic gas, foreshadowing
    the detailed study of GMC formation that this study and future 
    spectroscopic THz followup will provide in the
    Milky Way.}} 
\end{figure}


How might these mechanisms be differentiated?
Clearly, a statistically-meaningful assessment of the formation of
molecular clouds requires a survey of the Galactic Plane.
Then, simply characterizing the Galactic environments of regions identified to be
cloud-forming would be sufficient to disentangle the four
principal mechanisms for Galactic cloud formation.  
However, identifying the environmental context of cloud formation requires a high
resolution spectroscopic
tracer.  The role of turbulent flows, spiral arms, winds and shells, and
cloud collisions are all kinematic phenomena which cannot be
disentangled using continuum emission, but can be distinguished with
spectroscopic imaging with $\sim 1$~km/s resolution.

What would a newly formed molecular cloud look like?   Currently the
association of diffuse gas (HI) with molecular gas (CO) is difficult
owing to the large differences of emitting volumes of the HI and CO
lines; that is, HI emission does not differentiate easily between cool
atomic clouds and the warm diffuse ISM.  
However, Figure~\ref{PDR} shows that over a significant depth (which
is larger still for newborn clouds), C and
C$^+$ are the dominant carbon species and best spectroscopic 
probes of H$_2$ clouds with little CO.    
Indeed, dust lanes along the inner edges of spiral
arms often show neither HI nor CO emission and are therefore likely to
be in an intermediate phase; sufficiently dense and self-shielded to
harbor H$_2$, but not CO \citep{wik90}.   {\bf This intermediate phase has received comparatively
little recognition, but may play a crucial role in the sculpting of
star forming clouds and thus entire galaxies!   }

Future THz observatories will be able to survey the
pivotal 158\micron\ fine structure line of \cplus\ that will barometrically distinguish
natal molecular clouds not seen in CO from diffuse atomic gas.  
Joined with CO, HI and infrared surveys, a dedicated THz survey
telescope could advance our knowledge of star formation in the Galaxy
in the following crucial ways -- it would:

\begin{enumerate}
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\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 in the Galactic Plane, 
the first map of the
gas heating rate, and a 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 external galaxies. 
\item Reveal clouds clustering and forming in spiral arms
and supershells, and follow the growth of clouds to eventually shield
molecules and become gravitationally bound.  
\item Answer how the Galactic environment impacts the formation of clouds
  and stars.   What are the specific roles of spiral arms, central
  bars, infall and other influences from outside the Galaxy?
\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.
\item Ultimately construct a Milky Way template connecting the line
  emission from C$^+$, 
  N$^+$, C, CO, and dust continuum to star formation
  properties and state of the ISM. This template could be applied to nearby
  star-forming galaxies. 
\end{enumerate}

The importance of these fundamental measurements to a
comprehensive picture of the cold ISM as it relates to star formation
cannot be overstated!   
However the first generation of THz heterodyne instrumentation will have very limited
mapping capabilities.   For example -- although 
SOFIA and Herschel will be capable of measuring 158\micron\ \cii\ spectra, they
will possess heterodyne instruments with single diffraction-limited beams of
10-15\arcsec diameter, making large-scale mapping untenable.   
Two of the PI's (Walker \& Kulesa) are currently performing a detailed design study under the
auspices of NSF/OPP for HEAT, the High Elevation Antarctic Terahertz
Telescope.   A 0.5-meter telescope to be placed on Dome~A (the summit of the
Antarctic plateau) as early as 2009, HEAT will be dedicated to
submm/THz CO, \ci\, and \cii\ surveys of the Southern Galactic Plane,
but will still have a single 1-3\arcmin\ spectroscopic beams at each
frequency.   

{\bf Clearly, in order to make significant progress forward on the topic of
molecular cloud formation, it is necessary to focus the first THz
observatories on
specific regions of the Galaxy where cloud formation may be taking place.}
These ``finder charts'' would be based
on the wealth of publically-available surveys from ground and space, in
concert with new and unique ground-based instrumentation at infrared
and (sub)millimeter wavelengths.    It is the creation of these
finder charts that is the focus of this proposed study.


\begin{table*}[!hbt]
\centering
\small
\begin{tabular}{|l|c|c|c|}
\hline
{\bf Survey} & {\bf Product} & {\bf Location} & {\bf Resolution} \\
\hline \hline
FCRAO Galactic Ring Survey & \tco\ J=1-0 & Galactic Plane, 1st quadrant & 45\arcsec
\\
FCRAO Outer Galaxy Survey & \co\ J=1-0 & Galactic Plane, 2nd quadrant & 45\arcsec
\\
Bell Labs 7m GPS & \tco\ J=1-0 & Galactic Plane, 1st/2nd quadrants &
3\arcmin \\
CfA/Columbia CO survey & \co\ J=1-0 & Galactic Plane, high-z &
$>$10\arcmin \\
HI Parkes All Sky Survey & HI 21 cm &  Galactic Plane, high-z &
15\arcmin \\
Southern Galactic Plane Survey & HI 21cm & Galactic Plane, 4th
quadrant & 1\arcmin \\
Canadian Galactic Plane Survey & HI 21 cm & Galactic Plane, 1st/2nd
quadrants &  1\arcmin \\
VLA Galactic Plane Survey & HI 21 cm & Galactic Plane, 1st/2nd
quadrants & 1\arcmin \\
Leiden-Dwingeloo HI Survey & HI 21 cm  & northern sky & 30\arcmin \\
GLIMPSE & 3.6-8~\micron\ IR & Galactic Plane & 1-5\arcsec \\
2MASS & 1-2.4~\micron\ IR & All Sky & 3\arcsec\\
\hline
\end{tabular}
\caption{\label{surveys} Publically-available archival surveys which will be used during
  the course of this study.}
\end{table*}


\section{Proposed Research Activity}

\subsection{Generating Findercharts for Cloud Formation from Archival Data}

\subsubsection{Molecular Clouds at High Galactic Latitude}

This study will first hone its techniques by searching for molecular
material in the diffuse ISM, a rich laboratory of molecule formation
and destruction, and the putative medium from which dense molecular
clouds are sculpted.   The most visually stunning representations of
the diffuse ISM are the far-infrared dust continuum maps of
interstellar cirrus as first produced by IRAS \citep{low84}.   Indeed, 
at high Galactic latitude, 
IRAS maps of the sky represent the most effective way to compute the
combined column density of atomic and molecular hydrogen, $N_H = N(H)
+ 2N(H_2)$,  once
corrections for dust temperature, emissivity, and contamination by
zodiacal dust have been applied.   Such maps currently represent the
best method of computing the foreground (Galactic) extinction
toward extragalactic sources \citep{schle98}.   After the IRAS mission, several
studies aimed to assess the molecular component of high latitude
clouds by accounting for the amount of infrared emission attributable
to atomic gas via contemporary HI surveys.   By subtracting the
estimated contribution of HI gas (i.e. its associated dust) to the
infrared continuum, molecular material would
manifest itself as so-called ``infrared excess clouds''
\citep{dese88, reac94, reac98}.   Several studies discovered that
infrared excess clouds were frequently devoid of CO
\citep{blitz90, reac94,meyer96, lee99, onish01} and were interpreted as diffuse
\htwo\ clouds.   Indeed, this is also precisely the signature one would
expect from natal molecular material!    

Thus,  we will extend the ``infrared excess cloud'' analysis of the
aforementioned studies.  From master maps of IRAS 
far-infrared emission (and Spitzer, when available), we will
subtract the contribution of HI gas and the \htwo\ gas traced by \co\
J=1-0 where both survey products are available (Table~\ref{surveys}).   
Significant residual infrared excess represents \htwo\ gas
with a comparatively weak or nonexistent CO counterpart -- {\bf a necessary
signature of a newly-formed molecular cloud} that is either
insufficiently UV-shielded or too chemically young to have yet formed
much CO.    Such residual \htwo\ 
clouds will be subdivided into two categories by the relative amplitude of the
residual emission.   Small excesses are likely to represent diffuse molecular
clouds in which the volume and column densities are too low to support
significant CO abundance and/or line excitation.  Large excesses however, in which the residuals are
comparable to the subtracted CO and HI emission, are more indicative of
formative and transitional clouds en route to ``traditional''
CO-rich dark molecular clouds -- and are of the greatest
interest to this study.  

A preliminary study was performed on the Polaris flare, a spur of
infrared, CO and HI emission in the near vicinity of the North
Celestial Pole.   The analysis was compared to that of
\cite{meyer96}, and the results depicted in Figure~\ref{polaris}.   At
left is plotted the CO J=1-0 emission from \cite{dame01}, at center is
the reference hydrogenic column density map of \cite{schle98}, and at
right is the column density map after subtraction of both the
\cite{meyer96} HI map contents and CO.   The northernmost clumps of emission are
scarcely seen in CO, do not stand out at all above the ambient HI
emission plateau, but are strongly visible in the IRAS maps (at 25, 60
and 100~\micron).   Thus, they remain in the subtracted plot as
\htwo-only clouds.   The northern portion of the cloud, seen at top, has
the appearance of being compressed by an old supernova
remnant.   

\begin{figure*}[!hbt]
\centering
\includegraphics[scale=0.9,angle=0]{./figures/co.eps}
\includegraphics[scale=1,angle=0]{./figures/ir-annotated.eps}
\includegraphics[scale=1.1,angle=0]{./figures/residual.eps}
\caption{\label{polaris} {\small  [LEFT] CO J=1-0 map of the Polaris
    Flare from the Milky Way survey of \cite{dame01}.  [CENTER] Column
    density map derived from the IRAS 100~\micron\ survey \citep{schle98}, with results of
    preliminary analysis annotated; the peak intensity corresponds to
    $A_v = 7$.  [RIGHT] Subtraction of HI and CO
    components from the column density map; leaving a residual of
    \htwo\ gas ($A_v \sim 2$) with little CO -- the signature of molecular cloud
    formation that we will pursue in less diffuse environments
    elsewhere in the Galaxy. }} 
\end{figure*}


\subsection{Molecular Clouds in the Galactic Plane}

The advent of sensitive, large-scale CO J=1-0 observations to complement single-dish and
interferometric HI surveys has enabled the all-sky mapping of molecular
clouds and constitutes most of what is known about the overall
physical structure and distribution of molecular gas in the Galaxy
\citep{dame01, heyer98, lee01, jack06}.   Yet, understanding how dense molecular
clouds are formed from a more diffuse atomic substrate is not 
observationally clear, despite millions of CO spectra!
Figure~\ref{PDR} suggests that the answer lies in the low-visual
extinction ($A_v$) molecular component not traced by CO, since it is in
an intermediate state between diffuse atomic gas and dark clouds.
Thus, we will search for \htwo\ clouds using a similar strategy as
that adopted for high latitude clouds, above.    However, we will
need to start with a ``master hydrogen map'' optimized for studies in
the Galactic Plane.   Although far-infrared continuum emission from
IRAS and Spitzer is still applicable, systematic uncertainties in emissivity and dust
temperature make the translation to gas column density problematic.
However, the GLIMPSE imaging survey of the Galactic Plane from
Spitzer \citep{benj03} and the 2MASS all-sky near-infrared survey
\citep{2mass} allows infrared extinction
maps using the NICE(R) algorithm \citep{lada94,alves01} to be directly computed and translated to gas column density via
canonical extinction laws \citep{rieke85}.   Once derived, the highest
resolution Galactic Plane survey data of HI and CO can then be applied
to and subtracted from these column density maps and a residual computed.   In principle,
this will lead to identification of regions where both CO and HI under-represent the
state of the cold ISM and the existence of \htwo\ clouds with little CO
can be verified.   However, the plane of the Galaxy is kinematically
complex -- it is difficult enough to correlate CO and HI emission profiles,
much less reference both quantities to an infrared-derived extinction
that blindly measures the {\it total} absorbing column of all clouds along a
given line of sight.  

How will we bridge the fundamental differences between
these survey data-types?   First, interpreting color-color diagrams for
the purpose of extinction measurements is much cleaner than at visible wavelengths,
since the locus of points spanning unobscured stars is much more
restricted.  This makes differentiation of {\it intrinsic color} versus
{\it color
excess} more straightforward \citep{lada94}.  This is particularly
true when one adds the thermal
infrared bands from GLIMPSE/Spitzer to the traditional J/H/K bands
from 2MASS.   Thus, lines of sight passing through multiple distinct spiral
arms with dusty obscuration will lead to a color-color diagram with
potentially distinguishable stellar loci that may be referenced kinematically to CO
and HI emission.   Secondly, we will accept that many regions along the
Galactic Plane will be hopelessly confused and will not attempt to
interpret them -- the tangent arms at $l\sim100^o$ and near the
Galactic Center are likely to rank highest among these regions.   We will
concentrate on specific cloud complexes in the Outer Galaxy in coordination
with Co-PI Bieging's ongoing CO surveys being performed at the HHT, and on
regions of the Inner Galaxy with $30^o < |l| < 70^o$ and $|b| <
2^o$ where we can sample both ``northern'' and ``southern'' spiral
arms and interarm regions, to help evaluate the role of the Galactic
environment on potential sites of cloud formation.   Finally, we will definitively
differentiate gas and dust properties along complex lines of sight using ground-based high
resolution infrared absorption line spectroscopy; discussion will
follow in Section~\ref{IR}.

\subsection{Ground-Based Studies of Candidate Cloud Formation Regions}

Pathfinding studies using archival data surveys of CO and HI line
emission coupled with near- and far-infrared continuum maps will
provide a sample of regions suggestive of molecular cloud formation. 
Future THz observatories will be able to observe \cii\ line emission
toward these regions and, with knowledge of the contribution of
hot ionized gas to the \cii\ line intensity, will directly measure the
interstellar pressure of \htwo\ clouds with little CO.   These
measurements will yield not only diffuse clouds with \htwo\ but will
also yield the direct observation of bound molecular clouds forming from an
atomic substrate.   

However, the archival catalog of ``protoclouds'' can be improved in
quality with selected ground-based observations that supplement and calibrate the
rather disparate archival survey products, and provide fundamental new constraints on
molecular cloud structure.   We aim to do this in two diverse ways
using unique instrumentation developed (in part) by this team of
proposers at the University of Arizona:  

\begin{enumerate}
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\item Infrared absorption line spectroscopy will relate dust extinction
  to a directly-measured abundance of CO and \htwo\ for lines of sight
  spanning diffuse, translucent, and dense clouds throughout the Galaxy.
\item (Sub)millimeter wave mapping of higher excitation CO lines at
  the Arizona 10-meter Heinrich Hertz Telescope (HHT) will
  provide a complete excitation analysis that will dramatically
  enhance the interpretation of CO line emission and will improve upon
  the archival datas' angular resolution by up to an order of
  magnitude in many cases.
\item Measurement of carbon recombination lines at both infrared and
  submillimeter wavelengths will allow the fine structure level populations of
  \cplus\ to be measured -- yielding {\it in advance of THz observations} the expected
  brightness and abundance of \cplus\ emission toward the most favorable lines of
  sight.
\end{enumerate}


\begin{figure*}[!hbt]
\centering
\includegraphics[scale=0.44,angle=0]{./figures/aries.eps}
\includegraphics[scale=0.6,angle=0]{./figures/supercam.eps}
\caption{\label{instruments} {\small [LEFT]  The purple imager and
    green spectrometer halves of ARIES, the {\tt Arizona Infrared Imager and
    Echelle Spectrometer}, slated for first light at the adaptive
    optics focus of the 6.5-meter MMT in December 2006.  [RIGHT]
    {\tt Supercam}, a 64-beam heterodyne receiver operating in the 345~GHz
    atmospheric window, slated for the HHT.  Both instruments are
    one-of-a-kind and dovetail into the proposed science program. }} 
\end{figure*}


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.46,angle=0]{./figures/bigCOplot.eps}
\caption{\label{COspec} {\small Spectra of \co\ in the (2,0) R-band; R(0) through
  R(5).   The uppermost spectrum is a typical
comparison spectrum of the B2V star $\beta_1$~Sco that
depicts the atmospheric transmission.  The spectra are the ratioed (corrected)
spectra of obscured (young) stellar objects that demonstrate prominent
\co\ absorption.  The observed velocity shifts are due to the orbital
motion of the Earth along the line of sight. These data were taken
with the NOAO Phoenix spectrometer on the KPNO 2.1-meter telescope.}} 
\end{figure}


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.5,angle=0]{./figures/bigH2plot.eps}
\caption{\label{H2spec} {\small  Direct measurements of  \htwo\ line absorption in dark
  molecular clouds: the top plot represents the atmospheric
  transmission, ratioed astronomical spectra follow.  The quality
  of the spectra obtained toward NGC 2024~IRS~2 at two different
  epochs can be compared to the landmark observations performed by
  \cite{lacy94}.  Asterisks show the V$_{\rm LSR}$ of CO absorption in each source.}} 
\end{figure}


\subsubsection{Infrared Spectroscopy of Protoclouds}
\label{IR}

Although most interstellar molecules have been detected through
millimeter-wave emission-line spectroscopy, this technique is inapplicable
to non-polar molecules like H$_2$ (among others) which
are central to our understanding of the structure and contents of molecular clouds
and interstellar chemistry.  {\bf Thus high-resolution infrared
spectroscopy plays an important but often unrecognized 
role in interstellar studies:  significant non-polar
molecules like \htwo\ can be directly observed alongside ``common'' molecules like CO, and
their abundances and excitation conditions  
can be referred to the same ``pencil-beam'' absorbing column that
gives rise to interstellar extinction.}   With sufficient resolution
and sensitivity, the long-sought relation connecting extinction, \htwo, \co\
and \tco\ can be conclusively and {\it directly} measured in dense
clouds \citep{scov83,  blac90,  lacy94,  kule02, kule06} as it has been in diffuse clouds
\citep{bsd78}.   Figures~\ref{COspec} and \ref{H2spec} demonstrate the absorption
line measurements of \co\ and \htwo\ that this study will
undertake.

ARIES, the Arizona Infrared Imager and Echelle Spectometer, is nearing
completion as an adaptive optics instrument for the Arizona/SAO 6.5-meter MMT
\citep{don99} and has the unique blend of high resolution
($\lambda/\Delta\lambda = 30,000 - 60,000$) and broad
(cross-dispersed) simultaneous wavelength coverage needed for this
project.   The PI of this effort is on the ARIES instrument team.
ARIES will
  be used at the 6.5-meter MMT to perform 1-5~\micron\ absorption line
  spectroscopy of a sample of lines of sight through diffuse, translucent and
  dense (formative) clouds in the Galaxy.   Near the Galactic Plane,
  extinction measurements through complex lines of sight will be
  kinematically distinguished in \htwo\ and \co\ line absorption at 2.1-2.3~\micron\ with
  5~\kms\ resolution, and directly comparable with radio measurements
  of CO and HI emission.    Toward diffuse lines of sight where
  the intrinsically weak \htwo\ lines cannot be measured, \co\ can still be
  observed in its fundamental ro-vibrational bands at 4.6~\micron\ and
  compared with its radio counterpart \citep{mitch89}.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.45,angle=0]{./figures/COmaps.eps}
\caption{\label{synthcloud} {\small Simulated image of a fractal molecular
    cloud in several CO transitions shows the need for submillimeter
    CO lines.  
    The energetic gas that interacts with stars is far 
    better probed by the $3-2$
    lines, however both $1-0$ and $3-2$ lines are needed together to
    extract a comprehensive understanding of cloud properties,
    dynamics \& evolution. }}
\end{figure}


\subsubsection{Submillimeter Spectroscopy of Protoclouds}

Molecular line surveys have been performed over the entire sky in the
light of the 2.6 millimeter 
J=1-0 lines of \co\ and \tco\ and have been used to synthesize our
best understanding of the molecular content of the Galaxy.  Still, our
understanding of Galactic molecular clouds is incomplete, since the J=1-0 lines
are much less sensitive to warm, low-opacity,
high velocity gas such as produced by outflows, photodissociation
regions (PDRs), and shocks.  
This point is illustrated in Figure~\ref{synthcloud},  
with images of a synthetic model cloud constructed in the
integrated light of different spectral lines of CO.  
The model cloud is externally illuminated by a B
star and cloud excitation, temperature and
chemical abundances are determined self-consistently using Monte
Carlo methods.  The integrated spectral line images show that the
heated portion of the cloud is largely missed by the J=1-0 lines,
but captured by the J=3-2 lines.   Reconstruction of the cloud based
on observation of the \tco\ J=1-0 line alone recovers only 60\% of
the total cloud mass, whereas the combination of J=1-0 and
J=3-2 lines recovers 90\% of the H$_2$ mass.  

\begin{figure*}[!hbt]
\centering
\includegraphics[scale=1.3,angle=0]{./figures/recomb.eps}
\caption{\label{recomb} {\small First detected 
    carbon recombination line spectrum at mm-wavelengths, taken toward the northernmost edge of
    the Orion Molecular Cloud.   The 39$\alpha$ lines of hydrogen and
    carbon are observed adjacent to each other in this spectrum at
    106.8~GHz = 2.8 mm, taken at the Arizona (formerly NRAO)
    12-meter.   Analysis of multiple recombination lines allows
    reverse-construction of the \cplus\ level populations and
    158~\micron\ line intensity. 
 }}
\end{figure*}

A more comprehensive view of molecular clouds can
therefore be gleaned from
measurement of the submillimeter lines of CO and its isotopes, in
combination with existing millimeter-wave observations. 
The gas probed by
higher-J transitions is of greatest interest to our posed 
questions -- it is the {\it energetic} gas that 1) participates
in disruptive molecular outflows, 2) senses radiation fields at the
photodissociated surfaces of clouds, and 3) is warmed by 
star-formation in cloud cores.  Higher-J lines provide the
dynamic range of excitation needed to interpret cloud properties from 
existing CO~J=1-0 observations.

Unique new (sub)millimeter instrumentation offers new capabilities for
mapping candidate regions.   Walker and Kulesa are PI and Co-PI
(respectively) of the {\it Supercam} instrument, a 64-beam heterodyne
array for spectroscopic imaging in the 345~GHz
atmospheric window \citep{grop06}.  When Supercam is completed in
2007, it will be the largest submillimeter heterodyne array in the
world and will be capable of mapping CO J=3-2 with unprecedented
speed and sensitivity (e.g. 3 hours per square degree of coverage).
On the 10-meter Heinrich Hertz Telescope (HHT), Supercam will have
angular resolution of 20\arcsec\ per diffraction-limited beam. 

The HHT is now home to the world's most sensitive 230~GHz receiver.
Equipped with prototype Band~6 ALMA mixers from NRAO, the facility
receiver now offers simultaneous detection of the \co\ and
\tco\ J=2-1 lines with many times the sensitivity of the previous
receiver system.  System temperatures of 100K (SSB) have been measured
in moderate weather conditions on Mt. Graham. 

Using such unique instrumentation, we will map candidate cloud
formation regions in \co\ and \tco\ J=2-1 and 3-2 line emission 
much more sensitively, and with much higher angular resolution
than provided by archival survey data.  For
high latitude clouds, these measurements of minute quantities of CO will directly measure the
temperature and density of the cloud regions and confirm whether the
molecular gas is likely to be diffuse and tenuous, or transitioning toward more
dense and self-gravitating clouds.    Toward the Galactic Plane, the
higher excitation conditions of the (sub)millimeter lines will better
diagnose the translucent surfaces of molecular clouds, reveal the full
excitation and column density of CO and thereby better constrain
its formative state.   

Finally, toward regions that seem to possess all of the proper
characteristics of newborn molecular clouds, we will attempt to
ascertain the state of carbon in forms {\it other} than CO.  We will
measure the fine structure line of atomic carbon in its ($^3$P) J=1-0
transition at 492~GHz and measure multiple radio recombination lines
of carbon from 70~GHz (using the Arizona 12-meter) to 230~GHz (using
the HHT).   Just as optical recombination lines of hydrogen
(e.g. H-$\alpha$) probe
{\it hydrogen-ionized regions}, radio recombination lines of carbon can
probe {\it carbon-ionized regions} in which the hydrogen may be fully
molecular but the carbon has had insufficient time or UV-shielding to
transform into CO.    Although the radio recombination lines are at
least one order of magnitude less sensitive than far-infrared
spectroscopy of 158~\micron\ \cii\ emission, both line emission strengths are
density-sensitive and will therefore be able to discern \htwo\ {\it clouds} from {\it
  diffuse \htwo\ gas}.   Thus, it may be possible to ascertain the
state of \cplus\ and the internal pressure of the cloud in advance
of regular THz observations -- at least toward the most favorable
lines of sight.   The PI
has performed test observations of the technique toward the
northernmost portion of the Orion Molecular Cloud using the Arizona
12-meter; the first millimeter-wave detection of carbon recombination
lines is shown in Figure~\ref{recomb}.   The direct measurement of
hydrogen recombination lines in the same bandpass allows the
warm ionized gas component of the \cplus\ emission to be directly
measured and corrected for. 

\section{Summary}

This study aims to advance the observational study of cloud formation in the
Galaxy in advance of the first focused THz observatories which,
coupled with this work, should soon be
able to address these questions even more directly and conclusively.   The results of this
study will be a finder chart of candidate ``protoclouds'' that will
be well-characterized with fundamental and new
observations of dust extinction, CO and \htwo.  These additions will dramatically advance our
knowledge of the molecular ISM in their own right.    This study bears
important implications for 
the theoretical/numerical modeling of galaxy evolution
and to unresolved cloud and star formation in distant galaxies,
highlighting its broad applicability to topics that are both
fundamental and timely.

\section{Educational Outreach and Broader Impact}

The broadest impact of the proposed research however may be drawn from the use of these surveys
as educational and outreach tools.   The visage of the dusty lanes of
the Milky Way has inspired artistic and scientific imaginations
for generations.   Indeed, 
a project that encompasses so much of the entire sky offers numerous
opportunities to capture the imagination of students of all ages and
persuasions.   This inherent fascination is a powerful tool to
attract ``students'' of all ages and callings 
to a better, more literate appreciation of the sciences.   Thus, spreading
enthusiasm for science and training the next generation of scientists
is a significant component of this research program.  Specifically,
this proposal will target (1) web-based and local (ex. grade school) public
outreach, (2) guiding undergraduate students toward a greater
appreciation and hands-on understanding of science and its methods, and (3) training graduate
students through innovative research, and presentation of research results to the general
public and astronomical community.    These notions are outlined
below.

\subsection{Web-based Outreach}

More people rely on the Internet for news, information, and
entertainment than ever before; a trend which is unlikely to change
soon.   Thus, providing online outreach tools that are accessible and
interesting would be a useful, if indirect way of reaching the
widest range of people.    Distributed software should be operable on
multiple platforms and be open source, so that others in the online 
community can embrace and extend what is provided in the
confines of this study.    A practical application would be, for example, to present
a view onto the multi-wavelength Universe using existing planetarium
software.  For example, Stellarium ({\tt http://www.stellarium.org/})
is a visually stunning, 3D,  open source planetarium package for
Linux/BSD Unix/Macintosh/Windows (Figure~\ref{stellarium}.  
Writing a plugin to allow the
user to put on different wavelength ``glasses'' to view the IRAS 100~\micron\ sky
in place of the visible one -- or CO J=1-0 or HI
21~cm emission, is a tractable possibility and could be used to visualize the Galaxy
in new ways with a minimum of distracting software efforts.   


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.8,angle=0]{./figures/stellarium.eps}
\caption{\label{stellarium} {\small Open source planetarium software
    such as {\tt stellarium} represent useful tools with which maps of
    the sky at other wavelengths can be rendered.
 }}
\end{figure}


\subsection{Summer Outreach Programs for Teens}

The PI is a long-time counselor for the University of Arizona Alumni
Association's Astronomy Camp for adults and teens ({\tt
  http://www.astronomycamp.org/}).   In particular, the advanced teen
camps in the summer present a gender-balanced group of $\sim$30 motivated 
high school students with the opportunity
of becoming real astronomers for a week; they live in astronomer's
dorms at the summit of Mt. Lemmon Observatory, devise astronomical
research programs, write telescope proposals, perform observations,
reduce their data and present their results to the group.   Many of
them continue their research long after Camp concludes.  Some of the
earliest teen campers from the early 1990's have completed their Ph.D's in astronomy and
other scientific fields!   In concert with this study, we would
integrate archival data of HI, CO and extinction into a uniform
MySQL database that students could query and implement in their own
projects -- just as the investigators will do for the principal
science program!

\subsection{Undergraduate and Graduate Student Guidance and Training}
The formal education of undergraduate students occurs through both the
traditional classroom setting and through undergraduate research.
However, the crucial role of creative inquiry in the development of a
scientist is often better served outside the classroom.  Since
archival data is already pre-reduced, one can concentrate on more
creative science-oriented problems;  tasks particularly well suited for
undergraduates or even first-year graduate students who are also new to astronomy.   Similarly,
advising graduate students is an essential role of University
educators, because it is
here where the next generation of professional scientists is
prepared. This grant will support a portion of a graduate student's
salary.  Travel to scientific meetings is
particularly important for graduate students -- thus funding is requested to take
a graduate student to at least one national meeting in the 2nd and 3rd years.

\section{Prior NSF Support}

This is the first proposal that PI Kulesa has authored to
NSF/AST.   He is a Co-PI on two ongoing instrumentation programs;
{\it Supercam}, a 64-beam heterodyne array for the HHT (C. Walker, PI)
funded through an NSF MRI grant (2004-2007),
and the High Elevation Antarctic Terahertz Telescope (HEAT), a design
study for an automated 0.5-meter THz telescope for Dome~A, the summit of the
Antarctic plateau, funded (only) for 2006 through NSF's OPP program.  
Both programs are ongoing and play an integral role in the
definition of this proposed scientific endeavor. 


\clearpage

\pagestyle{empty}

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