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<div><font face="Times New Roman" color="#000000">2nd Annual Dasari
Lecture<br>
<br>
<b>Takeshi Oka<br>
University of Chicago</b></font><br>
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<div><font face="Times New Roman" color="#000000"><i><b>H3+ in the
central molecular zone of the Galactic center</b></i></font><br>
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<div><font face="Times New Roman" color="#000000">Tuesday, October 14,
2008</font></div>
<div><font face="Times New Roman" color="#000000">12:00 - 1:00
p.m.<br>
Grier Room (34-401)<br>
<br>
<br>
</font><font face="Times" color="#000000">With a super-massive black
hole at the core, the region near the Galactic center (GC) is a center
of astrophysical activities and is rich in extraordinary phenomena.
The inner region with a radius ~ 200 pc, is called the Central
Molecular Zone (CMZ) since it is especially rich in molecules.
It has been studied by radio-wave, infrared, and X-rays over many
years but the region is still full of mysteries. The infrared
spectrum of H3+ which was discovered in the laboratory in 19801 and
detected in interstellar space in 19962 is a novel and powerful probe
to study the region. The Doppler shift of the spectrum resulting
from the motion of clouds provides us fresh information on the gas in
the area with unprecedented clarity. Initially, the spectrum
demonstrated that H3+ is exceptionally rich in the CMZ, but its
Doppler profile was too complex for us to understand. The
discovery in 20023 of a broad spectrum of H3+ in the (<i>J</i>,<i>
K</i>) = (3,3) metastable rotational level was a revelation.
Since then spectra were analyzed using the<i> R</i>(3,3)l metastable
line as the Rosetta stone. Our observations have revealed a vast
amount of high temperature (~ 250 K) andlow density (~ 100 cm-3) gas
that pervades the space between dense clouds.4, 5 This is a new
category of gas in addition to the previous known three categories of
gas, that is, (i) the dense and mostly cold molecular gas observed by
radio emission of molecules, (ii) the hot ionized gas (104 - 106 K)
observed by radio recombination lines and radio wave scattering, and
(iii) the ultra hot plasma gas (107 - 108 K) reported by X-ray
observers<br>
<br>
1 Oka,<i> Phys. Rev. Lett.</i> 45, 531 (1980) 2 Geballe and
Oka,<i> Nature</i> 384, 334 (1996) 3 Goto, McCall, Geballe,
Usuda, Kobayashi, Terada, and Oka,<i> Publ. Astron. Soc. Japan</i>,
54, 951 (2002) 4 Oka, Geballe, Goto, Usuda, and McCall,<i>
ApJ</i>, 632, 882 (2005) </font><font face="Times New Roman"
color="#000000"> 5 Goto, Usuda, Nagata, Geballe, McCall, Indriolo,
Sutu, Henning, Morong, and Oka,<i> Astrophys. J</i>, (2008) in
press.<i> arXiv</i>:0807.4522</font><br>
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