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 jchiargc00.tex to appear in ApJ, July 2000
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%http://arXiv.org/abs/astro-ph/?0002421 
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%\documentclass[manuscript]{aastex}
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\newcommand{\av}{\ensuremath{A_V}}
\newcommand{\rv}{\ensuremath{R_V}}
\newcommand{\ebv}{\ensuremath{E_{B-V}}}
\newcommand{\chisq}{\ensuremath{\chi^2}}
\newcommand{\cm}{$\!{\rm cm}^{-1}$}
\newcommand{\lmm}{lines\,mm\ensuremath{^{-1}}}
\newcommand{\etal}{et\,al.}
\newcommand{\cgs}{CGS\,4}
\newcommand{\cotwo}{CO$_2$}
\newcommand{\hltau}{HL~Tau}
% Objects
\newcommand{\cyg}{Cyg OB2 \#12}
\newcommand{\mon}{Mon\,R2 IRS2}
\newcommand{\ngc}{NGC\,7538 IRS9}
\newcommand{\gl}{GL\,2136}
\newcommand{\rcra}{R\,CrA}
\newcommand{\sgra}{Sgr\,A*}
\newcommand{\sgraw}{Sgr\,A\,W-IRS3}
% Molecules, groups
\newcommand{\ammonia}{NH$_3$}
\newcommand{\formald}{H$_2$CO}
\newcommand{\methane}{CH$_4$}
\newcommand{\methanol}{CH$_3$OH}
\newcommand{\methylene}{-CH$_2$-}
\newcommand{\methyl}{-CH$_3$}
\newcommand{\oxygen}{O$_2$}
%
\newcommand{\nco}{$N$(CO)}
\newcommand{\nwater}{$N$(H$_2$O)}
\newcommand{\nmeth}{$N$(CH$_3$OH)}
\newcommand{\oph}{$\rho$~Oph}
\newcommand{\avthresh}{$A_{\rm th}$}
\newcommand{\redchisq}{\ensuremath{\chi^2_{\nu}}}
\newcommand{\dia}{$sp^3$}
\newcommand{\tco}{\ensuremath{\tau_{4.67}}}
\newcommand{\twater}{\ensuremath{\tau_{3.1}}}
\newcommand{\tdia}{$\tau_{3.47}$}
\newcommand{\tsil}{$\tau_{9.7}$}
\newcommand{\water}{H\ensuremath{_2}O}

\begin{document}

\title{The Composition and Distribution of Dust Along the Line of Sight
Towards
the Galactic Center\normalsize{\altaffilmark{1}\altaffiltext{1}{Based on
observations  made with ISO, an ESA project with instruments funded by ESA
member states (especially the PI countries: France, Germany, the Netherlands,
and the United Kingdom) and with the participation of ISAS and NASA.}}}

\author{J.E.\ Chiar\altaffilmark{2}, A.G.G.M. Tielens\altaffilmark{3}, D.C.B.
Whittet\altaffilmark{4}, W.A. Schutte\altaffilmark{5}, A.C.A.
Boogert\altaffilmark{3,6}, D. Lutz\altaffilmark{7},
E.F. van Dishoeck\altaffilmark{5}, M.P. Bernstein\altaffilmark{8}}
\altaffiltext{2}{NASA/Ames Research Center, Mail Stop 245-3, Moffett
Field, CA 94035 and SETI Institute, Mountain View, CA
94043. \texttt{mailto:chiar@misty.arc.nasa.gov}}
\altaffiltext{3}{Kapteyn
Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands}
\altaffiltext{4}{Rensselaer Polytechnic Institute, Department of Physics,
Applied Physics and Astronomy, Troy, NY 12180}
\altaffiltext{5}{Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The
Netherlands}
\altaffiltext{6}{present address: California Institute of Technology, Downs
Laboratory of Physics, Mail Code 320-47, Pasadena, CA 91125}
\altaffiltext{7}{Max-Planck-Institut f\"ur extraterrestrische Physik,
Postfach
1603, 85740 Garching, Germany}
\altaffiltext{8}{NASA/Ames Research Center, Mail Stop 245-6, Moffett Field,
CA
94035 and SETI Institute, Mountain View, CA 94043}


\begin{abstract}
We discuss the composition of dust and ice along the line of  sight to the
Galactic Center (GC) based on analysis of  mid-infrared spectra (2.4--13
$\mu$m) from the Short  Wavelength Spectrometer on the Infrared Space
Observatory  (ISO).  We have analyzed dust absorption features arising in
the  molecular cloud material and the diffuse interstellar medium  along
the lines of sight toward Sagittarius A* and the  Quintuplet sources, GCS3
and GCS4. It is evident from the  depth of the 3.0 $\mu$m H$_2$O and the
4.27 $\mu$m CO$_2$  ice features that there is more molecular cloud
material  along the line of sight toward Sgr A* than GCS3 and 4. In  fact,
Sgr A* has a rich infrared ice spectrum with evidence  for the presence of
solid \methane, NH$_3$, and  possibly HCOOH. 

Hydrocarbon dust in the diffuse interstellar medium along  the line of
sight to the GC is characterized by absorption  features centered at 3.4
$\mu$m, 6.85 $\mu$m, and 7.25  $\mu$m. Ground-based studies have identified
the 3.4 $\mu$m  feature with aliphatic hydrocarbons, and ISO has given us 
the first meaningful observations of the corresponding modes  at longer
wavelengths. The integrated strengths of these  three features suggest
that hydrogenated  amorphous carbon is their carrier. We attribute an 
absorption feature centered at 3.28 $\mu$m in the GCS3  spectrum to the
C-H stretch in aromatic hydrocarbons.  This feature is not detected, and
its C-C stretch counterpart appears to be weaker, in the \sgra\ spectrum. 
One  of the key questions which now arises is whether   aromatics are a
widespread component of the diffuse  interstellar medium, analogous to
aliphatic  hydrocarbons. 

\end{abstract}

\keywords{dust, extinction --- ISM: molecules --- Galaxy: center ---
infrared: ISM: lines and bands --- infrared: stars}

\end{document}