The EDIBLES survey

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

¹ Sackler Laboratory for Astrophysics, Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: bacalla@strw.leidenuniv.nl
² ACRI-ST, 260 Route du Pin Montard, 06904, Sophia Antipolis, France
³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
⁴ Department of Physics and Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
⁵ SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, CA 94043, USA
⁶ Observatoire de Paris, Sorbonne Université, Université PSL, CNRS, LERMA, 92190 Meudon, France
⁷ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁸ School of Astronomy, Institute for Research in Fundamental Sciences, 19395-5531 Tehran, Iran
⁹ Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, PR China

Received: 17 March 2018
Accepted: 19 November 2018

Abstract

We report cosmic ray ionization rates toward ten reddened stars studied within the framework of the EDIBLES (ESO Diffuse Interstellar Bands Large Exploration Survey) program, using the VLT-UVES. For each sightline, between two and ten 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 toward 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 ζ_p in the targeted diffuse interstellar clouds. For the ten selected targets, we obtain a range of values for ζ_p 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/submillimeter-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.