Using H_3^+ Observations to Estimate the Interstellar H_2 Temperature

Details Using H_3^+ Observations to Estimate the Interstellar H_2 Temperature Using H_3^+ Observations to Estimate the Interstellar H_2 Temperature

Feb 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010noao.prop..203i&link_type=abstract

NOAO Proposal ID #2010A-0203

Physics

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 T_01). 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 (T_01~60 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. Exploiting this relationship, we plan to use H_3^+ observations as a proxy in determining T_10 in highly extincted sight lines where H_2 cannot be observed in the UV. To first test this method though, we propose to observe H_3^+ in sight lines where H_2 measurements have already been made.

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