Comparing the Ortho-To Ratios of H_2 and H_3^+ in Diffuse Interstellar Clouds

Details Comparing the Ortho-To Ratios of H_2 and H_3^+ in Diffuse Interstellar Clouds Comparing the Ortho-To Ratios of H_2 and H_3^+ in Diffuse Interstellar Clouds

Jun 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010mss..confewg05i&link_type=abstract

"International Symposium On Molecular Spectroscopy, 65th Meeting, Held 21-25 June, 2010 at Ohio State University. http://molspec

Astronomy and Astrophysics

Astronomy

Astronomical Species And Processes

Scientific paper

The ratio between the populations of the two lowest rotational levels of H_2, J=0 and J=1, can be used to determine the temperature of interstellar gas (referred to as T01). Likewise, a temperature can be inferred from the populations of the (J,K)=(1,0) and (J,K)=(1,1) states of H_3^+. However, the average temperatures derived from these methods (T01≈60 K, T({H}_3^+)≈30 K) do not agree. Theories predict that the deviation from a Boltzmann distribution in both species is due to collisions between H_2 and H_3^+ which can change the spin alignment. Recent laboratory results confirm this deviation from a thermal distribution, and provide a relationship between the (1,0)/(1,1) ratio of H_3^+ and the (1)/(0) ratio of H_2. We have made observations searching for H_3^+ in several sight lines with measured H_2 abundances for the purpose of determining this relationship in interstellar clouds. With such a relationship, we then show that IR observations probing the (1,0) and (1,1) states of H_3^+ can be used to estimate the H_2 temperature in highly extincted sight lines where UV spectroscopy is not possible.

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