Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010mwac.meet..t04c&link_type=abstract
"Midwest Astrochemistry Meeting 2010, held 5-6 November at the University of Illinois at Urbana-Champaign. http://midwest.astroc
Astronomy and Astrophysics
Astrophysics
Scientific paper
Since its first interstellar detection in the mid 1990s, H3+ has proved to be a powerful probe of astrophysical conditions. In cold molecular clouds, only the lowest two rotational energy levels of H3+ are populated: the (J,K) = (1,1) para state and the (1,0) ortho state. The relative populations of these levels can be used to calculate an excitation temperature, Tex. In dense molecular clouds, Tex agrees well with the estimated cloud kinetic temperature. However, in diffuse molecular clouds, Tex is found to be 30 K, while the kinetic temperature, as determined from the excitation temperature T01 (given by the relative populations of the J = 0 and J = 1 rotational levels of H2) is on average 60-70 K in these environments. Because the (1,1) state of H3+ is the lower-energy state, this indicates that there is more para-H3+ in diffuse molecular clouds than would be expected based on the cloud kinetic temperature.
To understand the excess para-H3+ (or the lower-than-expected Tex), we have constructed a chemical model that takes into account the nuclear spin dependence of H3+ formation, proton scrambling via collisions with H2, and dissociative recombination with electrons. At the heart of this model is the reaction H3+ + H2 → H2 + H3+ , which can proceed by one of three pathways: the identity, proton hop, and hydrogen exchange. The branching fractions for the three pathways, Sid, Shop, and Sexch, influence the nature and extent of the proton scrambling. We use the microcanonical statistical model of Park and Light (J. Chem. Phys. 126, 044305, 1997) to calculate nuclear-spin-dependent rate coefficients that describe the extent of proton scrambling as a function of temperature and the aforementioned branching fractions. As a result, our model predicts, based on these parameters, what the observed Tex should be for a certain cloud kinetic temperature.
Our model suggests that on its own, the H3+ + H2 reaction would effectively thermalize the ortho:para H3+ ratio (i.e., Tex = T01). However, dissociative recombination is taken into account, this thermalization is incomplete, and the resultant competition between thermalization and dissociative recombination seems to reproduce the astronomical observations for Sid ~ 0.8-0.9. However, the details of the spin-dependence of H3+ dissociative recombination are important, and at present, experimentally uncertain. More information about this process is needed to determine the validity of our modeling results.
Crabtree Kyle N.
Indriolo Nick
Kreckel Holder
McCall, Benjamin Affiliations: AA(Department of Chemistry, Un University of Illinois at Urbana-Champaign) AE(Departments of Ch J.
Tom Brian A.
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