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The EDIBLES survey IV. Cosmic ray ionization rates in diffuse clouds from near-ultraviolet observations of interstellar OH+

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Authors : Xavier L. Bacalla, Harold Linnartz, Nick L. J. Cox, Jan Cami, Evelyne Roueff, Jonathan V. Smoker, Amin Farhang, Jordy Bouwman, and Dongfeng Zhao
Abstract : We report cosmic ray ionization rates towards ten reddened stars studied within the framework of the EDIBLES (ESO Diffuse Inter- stellar Bands Large Exploration Survey) program, using the VLT-UVES. For each sightline, between 2 and 10 individual rotational lines of OH+ have been detected in its (0,0) and (1,0) A3Π − X3Σ− electronic band system. This allows constraining of OH+ column densities towards different objects. Results are also presented for 28 additional sightlines for which only one or rather weak signals are found. An analysis of these data makes it possible to derive the primary cosmic ray ionization rate ζp in the targeted diffuse interstellar clouds. For the ten selected targets, we obtain a range of values for ζp equal to (3.9 − 16.4) × 10−16 s−1. These values are higher than the numbers derived in previous detections of interstellar OH+ in the far-infrared / sub-millimeter-wave regions and in other near-ultraviolet studies. This difference is a result of using new OH+ oscillator strength values and a more complete picture of all relevant
OH+ formation and destruction routes (including the effect of proton recombinations on PAHs), and the relatively high N(OH+) seen toward those ten targets.
Journal : Astronomy & Astrophysics ArXiv