Using CH3+ Observations to Constrain the Environment of CH+ Formation

Details Using CH3+ Observations to Constrain the Environment of CH+ Formation Using CH3+ Observations to Constrain the Environment of CH+ Formation

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21541506i&link_type=abstract

American Astronomical Society, AAS Meeting #215, #415.06; Bulletin of the American Astronomical Society, Vol. 42, p.261

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While CH+ was first discovered in the interstellar medium nearly 70 years ago, the formation of this simple molecule has continued to be an outstanding problem for chemical models. Many such models have been unable to explain the large observed abundances of CH+ in diffuse cloud sight lines. We propose the use of CH3+ observations - a molecule chemically linked to CH+ - in constraining the environment where both species form. A steady state analysis of CH3+ and CH2+ leads to a relationship between the abundances of CH+ and CH3+ which only depends on the temperature, T, and molecular hydrogen fraction, fH2, as free parameters. We present observations of CH+ and CH3+ along the diffuse molecular cloud sight line Cyg OB2 No. 12 made using HIRES at Keck and CGS4 at UKIRT, respectively. The column densities and upper limits we derive from these observations are then used to constrain the (fH2, T) parameter space where CH+ and CH3+ can form. From this analysis, we find that typical diffuse molecular clouds (i.e. T ˜ 60 K, fH2 > 0.2) are excluded as an environment where CH+ formation occurs. However, most current theories invoke non-equilibrium chemistry to drive the reaction C+ + H2 → CH+ + H, endothermic by 4640 K. If our analysis includes a higher effective temperature due to collisions between neutrals and accelerated ions, the CH3+ partition function predicts that the population will be spread over several excited rotational levels. In this case, new CH3+ observations with higher signal-to-noise ratios are required to constrain models where CH+ formation is driven by collisions between accelerated C+ ions and H2.

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