AAS 196th Meeting, June 2000
Session 40. High Resolution Spectroscopy at Visible and Ultraviolet Wavelengths
Topical Contributed Display, Wednesday, June 7, 2000, 10:00am-7:00pm, Empire Hall South

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[40.05] Observational Constraints for Models of CH+ Production in Diffuse Interstellar Clouds

J. Zsargó, S.R. Federman (Univ. of Toledo)

Our goal is to place observational constraints on models describing CH+ production in diffuse interstellar clouds. High quality archival spectra of interstellar absorption from C~{\small I} toward 9 stars, taken with the Goddard High Resolution Spectrograph on HST, were analyzed. Directions with known CH+ absorption, but only upper limits on absorption from C2 and CN, were considered for our study. This restriction allows us to focus on regions where CH+ chemistry dominates the production of carbon-bearing molecules.

The relative populations among the fine structure levels in the ground state of C~{\small I} were used to extract the thermal pressure for the clouds along these lines of sight. Profile synthesis of several multiplets yielded column densities and Doppler parameters. Very accurate values were deduced when high resolution spectra could be utilized, but the Doppler parameters were not characterized as well when only medium resolution spectra were available. Equilibrium excitation analysis of the C~{\small I} fine structure levels, using the measured column densities as well as the temperature from H2 excitation, yielded values for density. These densities, in conjunction with measurements of CH, CH+, C2, and CN column densities, provided estimates for the amount of CH associated with CH+ synthesis, an important constraint on theories for CH+ production.

A simple chemical model for gas containing non-equilibrium synthesis of CH+ was developed for the purpose of predicting column densities for molecular species generated from large abundances of CH+. Results for CH, CO, and HCO+ will be presented.

We acknowledge support from Grant AR-08352.01-97A from Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS5-26555.

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