[MOS] Dasari Lecture Oct. 14

[Zina Queen]

2nd Annual Dasari Lecture

Takeshi Oka
University of Chicago

H3+ in the central molecular zone of the Galactic center

Tuesday, October 14, 2008
12:00 - 1:00 p.m.
Grier Room (34-401)


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 
(J, K) = (3,3)  metastable rotational level was a revelation.  Since 
then spectra were analyzed using the R(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

1 Oka, Phys. Rev. Lett. 45, 531 (1980)  2 Geballe and Oka, Nature 
384, 334 (1996)  3 Goto, McCall, Geballe, Usuda, Kobayashi, Terada, 
and Oka, Publ. Astron. Soc. Japan, 54, 951 (2002)   4 Oka, Geballe, 
Goto, Usuda, and McCall, ApJ, 632, 882 (2005)  5 Goto, Usuda, Nagata, 
Geballe, McCall, Indriolo, Sutu, Henning, Morong, and Oka, Astrophys. 
J, (2008) in press. arXiv:0807.4522
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