H$_3^+$ IN DIFFUSE INTERSTELLAR CLOUDS: A TRACER FOR THE COSMIC-RAY IONIZATION RATE

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Title: H$_3^+$ IN DIFFUSE INTERSTELLAR CLOUDS: A TRACER FOR THE COSMIC-RAY IONIZATION RATE
Creators: Indriolo, Nicholas; McCall, Benjamin J.; Geballe, T. R.; Oka, T.
Issue Date: 2007
Publisher: Ohio State University
Abstract: \hspace{0.25in} In the past several years H$_3^+$ has been detected} in diffuse interstellar clouds where it had been predicted to exist in abundances below observable limits. These detections led to several questions about H$_3^+$ chemistry and the diffuse cloud environment. To answer these questions, further studies of H$_3^+$ were performed to constrain some uncertain parameters such as the electron recombination rate constant}. With new data and some reasonable assumptions it became possible to infer the cosmic-ray ionization rate necessary to produce column densities of H$_3^+$ observed by the CGS4 spectrometer on the United Kingdom Infrared Telescope (UKIRT). However, the value calculated along the sightline to $\zeta$ Per was over an order of magnitude higher than the value found in previous studies using OH}$^,$}. \hspace{0.25in} Recently, we finished analyzing a survey of H$_3^+$ in twenty diffuse cloud sightlines. In total, eight of these sightlines yielded positive detections of H$_3^+$ with another one or two hinting at possible absorption. Using the observed column densities, we calculated the primary cosmic-ray ionization rate and found an average value of $\zeta_p\approx2\times10^{-16}$ sec$^{-1}$. This is about an order of magnitude larger than what has long been considered the canonical ionization rate. Taking this higher ionization rate into account, some newer models} have been proposed to explain both the H$_3^+$ column densities, as well as the abundances of other species. The observation of H$_3^+$ along many diffuse cloud sightlines, coupled with the lack of a clear correlation between position and ionization rate, suggests that an enhanced cosmic-ray ionization rate may be a general property of the diffuse interstellar medium.
Description: Author Institution: Department of Astronomy and Department of Chemistry, University of Illinois at; Urbana-Champaign, Urbana, IL 61801; Gemini Observatory, Hilo, HI 96720; Department of Astronomy \& Astrophysics and Department of Chemistry, University of; Chicago, Chicago, IL 60637
URI: http://hdl.handle.net/1811/31552
Other Identifiers: 2007-TD-12
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