The Subaru Seminar is
usually held in Room 104 of the Hilo Base Facility, adjacent
to the main lobby. Everyone is welcome to attend. If you are
interested in giving a seminar, please contact Subaru
seminar organizers (Tomonori Usuda, Kumiko S. Usuda, Masahiko Hayashi, Masayuki Akiyama)
August 3, Wednesday at 11:00am
" Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+;
Takeshi Oka (University of Chicago)
Using an absorption line from the metastable (J,K)=(3,3) level of H3+ together with other lines of H3+
and CO observed along several sightlines, we have discovered a vast amount of high temperature (T ~ 250 K)
and low density (n ~ 100 cm-3) gas with a large velocity dispersion in the Centeral Molecular Zone (CMZ)
of the Galaxy, i.e., within 200 pc of the center. Approximately three-fourths of the H3+ along the line
of sight to the brightest source we observed, the Quintuplet object GCS 3-2, is inferred to be in the CMZ,
with the remaining H3+ located in intervening spiral arms.
About half of the gas in the CMZ has velocities near ~-100 km s-1 indicating that it is associated with
the 180 pc radius Expanding Molecular Ring which approximately forms the boundary of the CMZ.
The other half, with velocities of ~-50 km s-1 and ~0 km s-1, is probably closer to the center.
CO is not very abundant in these clouds.
Hot and diffuse gas in which the (3,3) level is populated was not observed toward several dense clouds
and diffuse clouds in the Galactic disk where large column densities of colder H3+ have been reported
Thus the newly discovered environment appears to be unique to the CMZ.
The large observed H3+ column densities in the CMZ suggests an ionization rate an order of magnitude
higher than in the diffuse interstellar medium in the Galactic disk.
Our finding that the H3+ in the CMZ is almost entirely in diffuse clouds indicates that the reported
volume filling (f>=0.1) for n>= 10E4 cm-3 clouds in the CMZ is an overestimate by at least an order
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