From staguhn@Newton.ph1.Uni-Koeln.DE Thu Jul  3 04:10:44 1997
Date: Thu, 03 Jul 1997 10:10:40 +0200
From: staguhn@Newton.ph1.Uni-Koeln.DE
To: gcnews@astro.umd.edu
Subject: RE: GCFLASH - Vol. 5, No. 7 (Jul 1, 1997)
%manuscript format
\documentstyle[12pt,aasms4]{article}

%definitions go here
\def \deg{$^\circ$ }
\def \threePtwoone{\mbox{$^{3}P_{2}\!\rightarrow\!\!^{3}P_{1}$ }}
\def \threePonezero{\mbox{$^{3}P_{1}\!\rightarrow\!\!^{3}P_{0}$ }}
\def \twCO{\mbox{$^{12}$CO}}
\def \thCO{\mbox{$^{13}$CO}}
\def \kms{ \mbox{km s$^{-1}$} }
\def \Kkms{\mbox{Kkms$^{-1}$}}
\def \grad{\mbox{$^{\circ}$}}
\def\lsim{\mathrel{\mathop{<}_{\textstyle\sim}\limits}}
\def\gsim{\mathrel{\mathop{>}_{\textstyle\sim}\limits}}

%title page comment
\slugcomment{Submitted to {\it The Astrophysical Journal}, October 1996; 
Accepted July 1997}
%running headline
\lefthead{Staguhn et al.}
\righthead{[CI] and CO Absorption Toward Galactic Center}

\begin{document}

\title{Observations of [CI] and CO Absorption in Cold, Low Density
Cloud Material Towards the Galactic Center Broad Line Emission}

\author{J. Staguhn\altaffilmark{1}, J. Stutzki\altaffilmark{1}, 
R. A. Chamberlin\altaffilmark{2,3}, S. P. Balm\altaffilmark{4,5},
A. A. Stark\altaffilmark{4}, A. P. Lane\altaffilmark{4}, 
R. Schieder\altaffilmark{1}, \& G. Winnewisser\altaffilmark{1}}  

\altaffilmark{1}{Universit\"at zu K\"oln, I. Physikalisches Institut,\
Z\"ulpicher Stra{\ss}e 77, 50937 K\"oln, Germany 
(staguhn, stutzki, schieder, winnewisser@ph1.uni-koeln.de)}

\altaffilmark{2}{Astronomy Department, Boston University, 725 Commonwealth 
Avenue, Boston, MA 02215 (cham@bu.edu)}
                                       
\altaffilmark{3}{California Institute of Technology,  Submillimeter 
Observatory, 111 Nowelo St., Hilo, 
HI 96720 (cham@ulu.submm.caltech.edu)}

\altaffilmark{4}{Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, MA 02138 (aas, adair@cfa.harvard.edu)}

\altaffilmark{5}{Department of Physics and Astronomy, 8971 Math Sciences 
Building, University of California, Los Angeles, CA 90095-1562 
(balm@astro.ucla.edu)}

\begin{abstract}

We report the detection of a deep \threePonezero [CI] absorption feature 
at v$_{lsr}$ = 12 km s$^{-1}$ with a linewidth of 6\kms towards extended 
line emission at a distance of $11'$ from Sgr C.  The 492~GHz observations were 
made with the 
Antarctic Submillimeter Telescope and Remote Observatory (AST/RO).
The absorption feature allows the derivation of lower limits for the CI 
column density in the cold foreground material. 
The feature is unlikely to be caused by self-absorption within the
[CI] emitting cloud because it is observed over a region at
least $4'$ across and is also seen in emission $22'$ north of the 
Galactic plane in $^{12}$CO J=2--1.     
In order to determine the temperature and the abundance ratio of CI to 
CO in the foreground gas, 
we compare the observations with $^{12}$CO and $^{13}$CO J=1--0 
observations obtained with the Bell Labs 7~m antenna and with $^{12}$CO 
and $^{13}$CO J=2--1 observations made with the KOSMA 3~m telescope. 
All these observations have about the same beam size.
On the assumption that the background emission is not spatially
associated with the absorbing cloud(s), a consistent model for the observed 
line intensities yields an
excitation temperature of 3.5~K for $^{13}$CO and 5~K for [CI], implying
low volume densities $n({\rm H_2}$) $\lsim$ 10$^3$ cm$^{-3}$. 
The measured abundance ratio of CI to $^{13}$CO is $\sim 34$.
This value is consistent with photochemical model calculations which predict 
an abundance ratio of CI to $^{12}$CO of $\sim$1 and a $^{12}$CO to 
$^{13}$CO ratio of $\sim$30
(reduced in comparison to the intrinsic $^{12}$C to $^{13}$C
isotopic ratio of 60 by fractionation).
%If fractionation is taken into account, so that 
%$^{12}$CO to $^{13}$CO $\approx$ 30, the
%abundance ratio of CI to $^{12}$CO
%$\approx$ 1, consistent with photochemical model calculations. 
The observed $^{13}$CO column density corresponds to an $A_V$ of $4.6^{\rm m}$, 
i.e., the hydrogen column density  
$N$(H) is $\sim 9 \times 10^{21}$ cm$^{-2}$.
This, together with the observed [CI] linewidth, indicates that the  
absorption is likely due to several translucent clouds. 

We compare our results with
line fluxes derived from the large-scale, low resolution COBE FIRAS
spectral line survey of [CI] \threePonezero and [CI] 
\threePtwoone emission in the galactic plane.
Taking into account beam filling, the {\em lower limit} for the column 
density of cold ($T_{ex} \le 10$~K) CI which is traced by our absorption
observations is at least a factor of two higher than 
the column density of the warmer CI ($T_{ex} \ge 20$~K) detected 
in emission by COBE.
Our results suggest that a substantial fraction of atomic carbon in
the interstellar medium may be difficult to detect in 
[CI] emission, due to its low excitation.

\end{abstract}
\end{document}
 



----- End Included Message -----