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 gccore_massp.tex proceedings of IAU Symp. 205
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% http://www.nro.nao.ac.jp/~amiya/papers/gcscore_iau205.pdf
% Paper: proceedings of IAU Symp. 205: "Galaxies and Their Constituents at 
the Highest Angular Resolutions"

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\markboth{Mass Spectrum of the Galactic Center Molecular Clouds}{}
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\begin{document}
\title{Mass Spectrum of the Galactic Center Molecular Clouds}
\author{Atsushi Miyazaki}
\affil{Nobeyama Radio Observatory, National Astronomical Observatory,
Minamimaki, Minamisaku, Nagano 384-1305, Japan}
\author{Masato Tsuboi}
\affil{Institute of Astrophysics and Planetary Science, Ibaraki University,
2-1-1 Bunkyou, Mito, Ibaraki 310-8512, Japan}

\begin{abstract}
We have observed three molecular clumps
in the Galactic center region
in the CS {\it J}=2--1 line with the Nobeyama Millimeter Array.
We have determined the mass spectrum and other statistical relations
for the Galactic center cloud cores.
The derived mass spectrum
is somewhat steeper than for galactic cloud cores
based on molecular line observations.
In addition, the spectrum
is steeper than that of the large scale Galactic center clumps.
\end{abstract}


\vspace{-.8cm}
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\section{Introduction}
\vspace{-.15cm}

The physical characteristics of the molecular clouds
in the central region of the Galaxy
differ considerably from those in the disk region
(e.g., Morris \& Serabyn 1996).
The statistical relations, such as the velocity width-radius relation,
the LTE mass-virial mass relation, and mass spectrum,
provide key information on
how stars are formed in such conditions as
in this Galactic center region.
Previously, we identified a total of 159 molecular clumps
based on the CS {\it J}=1--0 survey data in the Galactic center region
with the Nobeyama 45-m radio telescope
(Miyazaki \& Tsuboi 2000; Tsuboi et al. 1999;
ftp://ftp.nro.nao.ac.jp/users/tsuboi/GC\_CS10/).
We determined the statistical relations
of these Galactic center clumps.
The mass spectral index of the Galactic center clumps
in the mass range of $10^{4-6} M_{\odot}$
is approximately equal to that for galactic disk clouds
(Miyazaki \& Tsuboi 2000).
However, it is the smaller clumps (cores) that are likely
to be most relevant to star formation.
Therefore, we performed high-resolution observations
of a few molecular clumps with the Nobeyama Millimeter Array
in order to determine the interior structure of the Galactic center clumps.


\vspace{-.5cm}
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\section{Observation and Analysis}
\vspace{-.15cm}

We performed observation of 3 molecular clumps,
CS0.26+0.02, CS0.76-0.06, and CS1.35+0.26,
in the CS {\it J}=2--1 line with the Nobeyama Millimeter Array (NMA).
The size of synthesized beam was about $9'' \times 4''$
in both 3 clumps.
We identified cloud cores in the clumps
using the ``clumpfind program'' (Williams et al. 1994).
The derived mean parameter of all identified cores is
radius, $R=0.12$ pc, velocity width (FWHM), $\Delta V=3.5$ km~s$^{-1}$,
and LTE mass, $M_{\rm LTE}=38$ M$_{\odot}$.
The parameter range of the identified cores
is roughly equal in each clump.

\begin{figure}
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\caption{\footnotesize
Mass spectrum of the Galactic center cloud cores
based on the interferometer observations in CS {\it J}=2--1 with NMA.  }
\end{figure}


\vspace{-1.0cm}
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\section{Mass Spectrum}
\vspace{-.15cm}

We constructed the mass spectrum
from a sample of 57 cores in the observed 3 clumps,
assuming that the spectral index is the same in all 3 regions (figure 1).
The mass spectrum of the Galactic center cores (index of --1.99)
is somewhat steeper than
those of the galactic cloud cores based on the molecular line observations
(index of $\sim -1.6$; e.g., Kramer et al. 1998)
and that of the large-scale Galactic center molecular clumps
(index of --1.59; Miyazaki \& Tsuboi 2000),
although the statistical significance is too low to draw firm conclusions
as to differences between the Galactic center cores
and either galactic disk cores or the large-scale Galactic center clumps.

Recent dust continuum observations
of small scale cores in the disk region
have given a steeper mass spectrum,
which is similar to the Salpeter local IMF
(Testi \& Sargent 1998; Motte et al. 1998).
These observations, as well as our interferometer observation,
must trace dense parts of the cores.
Thus our steep spectrum may be due to similar physical processes.
On the other hand,
one of the open questions about the star formation
in the Galactic center region
is that the number of massive stars is less than
that extrapolated from the disk region (e.g., Morris \& Serabyn 1996).
The paucity of massive stars in the Galactic center region
may be related to the relatively steep mass spectrum
implied by our observations.


\vspace{-.48cm}
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\begin{references}
\vspace{-.2cm}

{\footnotesize
\reference
Miyazaki, A., \& Tsuboi, M.\ 2000, \apj, 536, 357
\reference
Morris, M., \& Serabyn, E.\ 1996, \araa, 34,645
\reference
Motte, F., Andre, P., \& Neri, R.\ 1998, \aap, 336, 150
\reference
Kramer, C., et al.\ 1998, \aap, 329, 249
\reference
Testi, L., \& Sargent, A.I.\ 1998, \apjl, 508, L91
\reference
Tsuboi, M., Handa, T., \& Ukita, N.\ 1999, \apjs, 120, 1
\reference
Williams, J.P., de Geus, E.J., \& Blitz, L.\ 1994, \apj, 428, 639
}
\end{references}


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