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From: Miwa Goto <mgoto@mpia-hd.mpg.de>
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%astro-ph/0807.4522
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% \shorttitle{Survey of H$_3^+$ toward the Galactic Center II.}
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%  \shortauthors{Goto et al.}

\begin{document}

\title{Absorption Line Survey of H$_3^+$ toward the Galactic Center
  Sources II. \\ Eight Infrared Sources within 30~pc of the Galactic
  Center}

\author{Miwa Goto(1)}
\affil{Max-Planck-Institut f\"ur Astronomie}

\author{T. Usuda(2)}
\affil{Subaru Telescope}

\author{T. Nagata(3)}
\affil{Kyoto University}

\author{T. R. Geballe(4)}
\affil{Gemini Observatory}

\author{B. J. McCall(5)}
\author{N. Indriolo(5)}
\affil{University of Illinois at Urbana-Champaign}

\author{H. Suto(6)}
\affil{NAOJ}

\author{Th. Henning(1)}
\affil{Max-Planck-Institut f\"ur Astronomie}

\author{C. P. Morong(7)}
\author{Takeshi Oka}
\affil{University of Chicago}

\begin{abstract}

Infrared absorption lines of $\mathrm{H}_{3}^{+}$, including the
metastable $R$(3,3)$^l$ line, have been observed toward eight bright
infrared sources associated with hot and massive stars located in and
between the Galactic Center Cluster and the Quintuplet Cluster 30~pc
to the east. The absorption lines with high velocity dispersion arise
in the Galaxy's Central Molecular Zone (CMZ) as well as in foreground
spiral arms. The temperature and density of the gas in the CMZ, as
determined from the relative strengths of the $\mathrm{H}_{3}^{+}$
lines, are $T$=200~--~300~K and $n\leq$50~--~200~cm$^{-3}$. The
detection of high column densities of $\mathrm{H}_{3}^{+}$ toward all
eight stars implies that this warm and diffuse gaseous environment is
widespread in the CMZ. The products of the ionization rate and path
length for these sight lines are 1000 and 10 times higher than in
dense and diffuse clouds in the Galactic disk, respectively,
indicating that the ionization rate, $\zeta$, is not less than
10$^{-15}$~s$^{-1}$ and that $L$ is at least on the order of 50~pc.
The warm and diffuse gas is an important component of the CMZ, in
addition to the three previously known gaseous environments: (1) cold
molecular clouds observed by radio emission of CO and other molecules,
(2) hot ($T~=$~10$^4$~--~10$^6$~K) and highly ionized diffuse gas
($n_e~=$~10~--~100~cm$^{-3}$) seen in radio recombination lines, far
infrared atomic lines, and radio-wave scattering, and (3) ultra-hot
($T~=$~10$^7$~--~10$^8$~K) X-ray emitting plasma. Its prevalence
significantly changes the understanding of the environment of the
CMZ. The sight line toward GC~IRS~3 is unique in showing an additional
$\mathrm{H}_{3}^{+}$ absorption component, which is interpreted as due
to either a cloud associated with circumnuclear disk or the
``50~km~s$^{-1}$ cloud'' known from radio observations. An infrared
pumping scheme is examined as a mechanism to populate the (3,3)
metastable level in this cloud.

\end{abstract}

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