Harvey Liszt, April 28, 2009 @ 4pm CDT

[bjmccall]

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Dr. Harvey Liszt, National Radio Astronomy Observatory

Title: Looking at diffuse clouds from both sides, now.

Abstract:

Optical/uv/near-IR absorption lines against nearby bright stars reveal the presence of small molecules in the nearby interstellar gas, even when the extinction by dust is relatively small; H2, H3+, CH+, C2, C3, CO, CN, CH, OH and NH are seen and some 25%-45% of the local neutral hydrogen is actually in H2. Abaorption line work at radio frequencies increases the set of detected species to small chemical families such as CN, HCN, and HNC; CH, C2H and C3H2; OH, CO and HCO+; CS, SO, HCS+, H2S; and to H2CO and NH3, often with relative abundances surprisingly like those seen in dark dense clouds such as TMC-1. The origins of these molecules are generally not understood although HCO+ and HCS+ should readily recombine to form the observed CO and CS.

I will summarize the chemical systematics and what is known of the physical state of the molecule-bearing diffuse gas, and extend the discussion to actual imaging of the host clouds in extinction and in mm-wave emission lines of CO and HCO+. Viewing the clouds' internal gas flows on the plane of the sky gives a very different perspective on absorption line work, and has important consequences for the CO sky and the use of CO emission as a surrogate for H2. CO emission from diffuse clouds can be very bright (15 K), is much more common than previously known and is easily mistaken as arising in dense gas.