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From: arancha@damir.iem.csic.es
To: gcnews@aoc.nrao.edu
Subject: AmoBaladron_CNDpaper.tex A&A, Oct 2010, accepted

http://arxiv.org/abs/1011.3271

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\title{Mapping photodissociation and shocks in the vicinity of 
\mbox{Sgr A$^*$}

\author{M. A. Amo-Baladr\'on
      \inst{1}
      \and
      J. Mart\'in-Pintado
      \inst{1}
      \and
      S. Mart\'in
      \inst{2,3}
}
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\institute{Centro de Astrobiolog\'ia (CSIC/INTA),
                Ctra. de Torrej\'on a Ajalvir km 4,
                E-28850, Torrej\'on de Ardoz, Madrid, Spain\\
                \email{arancha@damir.iem.csic.es}
                \and
                European Southern Observatory,
                Alonso de C\'ordova 3107, Vitacura,
                Casilla 19001, Santiago 19, Chile
                \and
                Harvard-Smithsonian Center for Astrophysics,
                60 Garden Street, Cambridge, MA 02138, USA\\
}
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\begin{abstract}

{}
{We study the chemistry in the harsh environments of galactic nuclei 
using the nearest one, the Galactic center (GC).}
{We have obtained maps of the molecular emission within the central 
five arcminutes (\mbox{12 pc})
of the GC in selected molecular tracers: \mbox{SiO(2--1)}, 
\mbox{HNCO(5$_{0,5}$--4$_{0,4}$)}, and the
\mbox{$J =$ 1 $\rightarrow$ 0} transition of \mbox{H$^{13}$CO$^+$}, 
\mbox{HN$^{13}$C}, and \mbox{C$^{18}$O}
at an angular resolution of 30$''$ (\mbox{1.2 pc}).
The mapped region includes the circumnuclear disk (CND) and the two 
surrounding giant
molecular clouds (GMCs) of the \mbox{Sgr A} complex, known as the 20 
and
\mbox{50 km s$^{-1}$} molecular clouds.
Additionally, we simultaneously observed the \mbox{$J =$ 2 
$\rightarrow$ 1} and \mbox{$J =$ 3 $\rightarrow$ 2}
transitions of \mbox{SiO} toward selected positions to estimate the 
physical conditions of the molecular gas
using the large velocity gradient approximation.}
{The \mbox{SiO(2--1)} emission shows all the molecular features 
identified in previous studies,
covering the same velocity range as the \mbox{H$^{13}$CO$^+$(1--0)} 
emission, which also
presents a similar distribution.
In contrast, \mbox{HNCO(5--4)} emission appears in a narrow velocity 
range mostly concentrated
in the 20 and \mbox{50 km s$^{-1}$} GMCs. A similar trend follows 
the \mbox{HN$^{13}$C(1--0)} emission.
The HNCO column densities and fractional abundances present the 
highest contrast, with difference factors
of \mbox{$\geq$60} and 28, respectively. Their highest values are 
found toward the cores of
the GMCs, whereas the lowest ones are measured at the CND.
SiO abundances do not follow this trend, with high values found 
toward the CND, as well as the GMCs.
By comparing our abundances with those of prototypical Galactic 
sources we conclude that HNCO, similar
to SiO, is ejected from grain mantles into gas-phase by 
nondissociative C-shocks. This results
in the high abundances measured toward the CND and the GMCs.
However, the strong UV radiation from the Central cluster utterly 
photodissociates HNCO
as we get closer to the center, whereas SiO seems to be more 
resistant against UV-photons
or it is produced more efficiently by the strong shocks in the CND.
This UV field could be also responsible for the trend found in the 
\mbox{HN$^{13}$C} abundance.}
{We discuss the possible connections between the molecular gas at 
the CND and the GMCs
using the \mbox{HNCO/SiO}, \mbox{SiO/CS}, and \mbox{HNCO/CS} 
intensity ratios as probes
of distance to the Central cluster.
In particular, the \mbox{HNCO/SiO} intensity ratio is proved to be 
an excellent tool for evaluating the
distance to the center of the different gas components.}

\end{abstract}

\maketitle

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