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%{\Large HIFI Core Science Program \\
%The interstellar medium}



%\vspace*{0.5cm}

%{\sl Draft May 16  2003/JSt - M. Gerin, E. Falgarone}












\newpage



\section{Introduction}




\subsection{Summary}
The present proposal is devoted to studies of the interstellar medium
(ISM) with HIFI %JST add:
within the HIFI core science program. %JST
This includes all ISM phases, from the very
diffuse regions of low hydrogen column densities, to the dense and
warm clouds in photo-dissociation regions 
%JST added:
and with large scale shocks, e.g.\ in the interaction region of SNR with ISM
clouds. %JST
%JST drop, and state both for PDRs and shocks:
%The proposals also deals
%with large scale shocks as in SNRs, but not with shocks associated
%with YSOs which belong to the star formation proposal.
The questions addressed here, both for PDRs and for shocks, are aimed at
understanding the physical processes and chemical networks controlling these
important phenomena: comparison between a selected sample of sources, covering
a range of parameters and environments, with the state of the art 
models will allow to
focus on the well known deficiencies of the models and 
identify where they are missing important new processes. This is the goal of
the ISM core science proposal. It thus does not cover the observations of PDRs
or shocks as diagnostic tools for individual star forming regions; these are
addressed in the core science proposal on star formation. %JST
\\
The proposed observations include studies of the physical and chemical
processes in the interstellar medium, coupled with investigations of the
spatial  structure, using the information from the intensity and
profile of intense cooling lines. The proposed observing program
is dedicated to key issues in ISM physics, such as understanding 
the heating processes, the mystery of the presence of CH$^+$ and
other reactive molecules, the evolution between the different stable
phases. We also propose to expand the
knowledge on interstellar chemistry by searching for new
molecular lines from a wide variety of molecules :
hydrides, %JST add:
small %JST
carbon clusters, 
%JST drop: and chains.%JST
%JST add:
and 
%JST drop:  Also, we propose to identify some of 
the large aromatic species, thought to be the carriers of
the mid infrared emission bands. %JST
%JST add:
HIFI offers unique possibilities for detecting these species by covering for
the first time at high spectral resolution the spectral range which includes
the low rotational transitions of the hydrides and the ro-vibrational
transitions of the low energy vibrational modes of the larger molecules in the
far infrared. %JST
%JST drop:
%, by detecting their low energy
%vibrational modes in the far infrared.%JST

 %{\bf THIS LOOKS LIKE A SUMMARY - IS IT OK ?}

%{\sl
%- give goals of document (scientific justification of each topic; HIFI
%uniqueness of proposed observations; coherent time estimate; identify prep
%work necessary)\\}

\begin{table}
\begin{center}
\caption{Summary of proposed observations}
\begin{tabular}{rcc}
\hline
Topic & Main Lines & Time (hours) \\
\hline
Diffuse ISM & [CII], [NII], [CI] & 378 \\
Hydrides absorption & CH$^+$, NH, H$_2$O, ... & 81 \\
PDRs & H$_2$O, CH$^+$, CO, CII & 70 \\
SNR Shocks & H$_2$O, CO & 35\\
Complex molecules & & 300 \\
\hline
Total & & 864 \\
\hline
\end{tabular}
\end{center}
\end{table}

\subsection{Why Herschel and HIFI}

The study of the interstellar medium (ISM) is critical for understanding
both star formation and the evolution of galaxies.  HIFI is a unique
instrument to accomplish this, since for local sources 
many important spectral
features are obscured by the Earth atmosphere and heterodyne
resolution
is needed to reveal dynamical effects. Herschel opens for
the first time the submillimeter and far infrared wavelength range
totally, with no blockage by any atmospheric feature. While
the far infrared spectrum of some ISM sources is known from
low resolution (spatial and spectral) ISO data , HIFI offers both high
spatial and spectral resolution with high sensitivity throughout
a broad frequency range. In fact, planning of observations must
be done very carefully since no data from any previous observatory
exist in the literature for some frequencies offered by HIFI. For
other frequencies, only a handful of high spectral resolution 
%JST change: data
pioneering observations %JST
have been published up to now, with a 
%JST add:
relatively %JST
poor sensitivity (e.g. KAO heterodyne
data by Betz and Boreiko, see  Gerin et al. 1998).

The earliest data obtained with SOFIA will help in planning HIFI but we
expect that SOFIA data can not compete with HIFI due to the residual
atmosphere and smaller telescope size. The only issue for which
SOFIA may win the competition is mapping of strong lines since SOFIA will
carry detector arrays while HIFI has single pixels.
For this proposal, we have taken the following approach. Since none
of SOFIA or Herschel is ready yet, we have kept  the possibility
of large scale maps with HIFI open. We shall follow the development of the
instrumentation and revise the proposal accordingly, mapping the
strongest lines with SOFIA and devoting HIFI to observations
requiring the superior sensitivity offered by the better operating
environment. 

%{\sl CAN WE INDICATE IN THE PROPOSAL POSSIBLE OBSERVATIONS
%TO BE PERFOMED WITH SOFIA IF TIME IS AVAILABLE BEFORE 2007/2008 ?
%}



Other Herschel instruments have complementary capabilities with
respect to HIFI, and we shall make use of them when they provide
the best sensitivity. PACS and SPIRE will also be  useful for
determining the broad band dust spectra.
  Because of its lower spectral resolution,
PACS cannot provide any infomation on line profiles for ISM sources.
The crossover resolution compared to HIFI, where equal sensitivity per
  spectral and spatial point are achieved, is about 30-100 km/s.
When only the total line intensity is needed, PACS is faster then
HIFI for mapping since 25 pixels can be observed
simultaneously. SPIRE has also a lower spectral resolution than HIFI,
and a somewhat lower sensitivity in spectroscopy mode compared to
PACS. 



%{\sl--  give rules for time estimate used throughout proposal (following Volker's suggestions).}%
\subsection{Rules for time estimates}

In this document all time estimate have been done with the following
rules (proposed by V. Ossenkopf) :\\
\begin{itemize}
\item{The SSB 
system temperature $T_{sys}$ scales as the observing frequency
$\nu$ for bands 1 to 5 : $T_{sys} = 180 {\nu  \over 500 GHz} K $, and
$T_{sys} = 2000 K$ for band 6.}
\item{The observing efficiency is half that of the corresponding ideal mode}
\item{Observations are performed with two mixers tuned at the same
frequency}
\item{The point source sensitivity varies from 450 Jy/K for band 1 to
  610 Jy/K for band 6} 
\end{itemize}

The formula for computing the rms noise level $\sigma$ obtained
after a given integration time $T_{int}$ with a spectral resolution
$\delta \nu$ is therefore :

$$\sigma = {T_{sys} \over \sqrt{2 A \delta \nu T_{int}} }$$
with $A = 0.125 $ for beam switched observations,  $A = 0.25 $ for 
frequency switching ,  $A = 0.3 $ for short OTF scans and
 $A = 0.5 $ for long OTF scans.  The $\sqrt{2}$ term takes into
 account the presence of two mixers at each frequency. 
 



%{\bf then go to the scientific overview}\\
\subsection{Overview}

The formation of new stars in galaxies releases such large amounts of 
radiative and kinetic energy 
%JST change: in 
into %JST
the interstellar medium (ISM)  
that matter continuously cycles between the dense and diffuse phases of the
interstellar medium. The chemical evolution of matter in space, the
evolution of galaxies  and the
formation of stars must be investigated within this broad context.

Gas-phase abundances of elements such
as Si and Fe show that a significant fraction of matter, accreted onto
dust in molecular clouds, is released back to the gas within the low
density phases of the ISM. On the other hand,
the detection of numerous molecular clouds
at high Galactic latitude and of large abundances of molecules in the
cold diffuse ISM show that the condensation of gas 
from the warm to cold phases is easily followed by the
formation of molecules. 
Large molecular abundances in diffuse regions poorly shielded from the
UV radiation field   
are not readily explained with state-of-the-art 
 gas-phase chemistry and alternative formation 
routes may be effective where the required energy is supplied by
shocks or other processes at the origin of turbulence dissipation.
In the interstellar medium, the gas  properties are dependent on
many processes pertaining to  hydrodynamics, to chemistry -- since the
thermal properties depend on the chemical composition--, and to 
solid state and molecular physics. This makes the ISM a
natural laboratory where many 'uncommon' physical processes can be
studied, where extremely reactive molecules can live, 
and where new molecular species  
remain to be discovered in the near future. 



 




The proposal is organised as follows :
Section 2 deals with the diffuse ISM, its dynamical structure and
distribution among different phases. Its anticipated large 
chemical activity is 
addressed in Section 3 
%JST change: which 
with %JST
focus on hydrides.
The 
%JST add:
investigations of the %JST
warm and dense ISM 
%JST: is 
are %JST
described in Section 4 and 5 which consider
photo dissociation regins (PDRs) and SNR shocks respectively. 
Complex molecules, including carbon clusters, chains, cycles and PAHs,
are the subject of Section 6.  

The 
%JST change: proposed 
HIFI core science %JST
proposal 
%JST add:
on the ISM %JST
is closely related to other chapters of the
HIFI core program. The interrelation of the diffuse
ISM phases will be adressed at large scale in the external galaxy
program. The same physical and chemical processes studied in the
PDR and shock section, are important in the context of the evolution
of molecular gas towards star formation, and in the late stage
of stellar evolution when the planetary nebulae stage is reached.
Understanding how the diversity of interstellar molecules and
solid state particles is built  is a common goal of the ISM, star
formation and
solar system science since they target at different evolutionary
stage of the same material.
 
%\end{document}





