Dissociative Recombination of Rotationally Cold OH+ and Its Implications for the Cosmic Ray Ionization Rate in Diffuse Clouds

• Published in: Astrophysical journal. Letters Vol. 955; no. 2; p. L26
• Main Authors: Kálosi, Ábel, Gamer, Lisa, Grieser, Manfred, von Hahn, Robert, Isberner, Leonard W., Jäger, Julia I., Kreckel, Holger, Neufeld, David A., Paul, Daniel, Savin, Daniel W., Schippers, Stefan, Schmidt, Viviane C., Wolf, Andreas, Wolfire, Mark G., Novotný, Oldřich
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01-10-2023; IOP Publishing
• Subjects: Astrochemistry; Cloud temperatures; Coefficients; Cosmic ray ionization; Cosmic ray propagation; Cosmic ray showers; Cosmic rays; Cryogenic storage; Density ratio; Diffuse interstellar clouds; Diffuse molecular clouds; Electron beams; Electron recombination reactions; Interstellar clouds; Interstellar matter; Ion beams; Ionization; Laboratory astrophysics; Molecular clouds; Reaction rates; Shielding; Storage rings (particle accelerators)
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/acf71d

Abstract
Observations of OH
+
are used to infer the interstellar cosmic ray ionization rate in diffuse atomic clouds, thereby constraining the propagation of cosmic rays through and the shielding by interstellar clouds, as well as the low energy cosmic ray spectrum. In regions where the H
2
-to-H number density ratio is low, dissociative recombination (DR) is the dominant destruction process for OH
+
and the DR rate coefficient is important for predicting the OH
+
abundance and inferring the cosmic ray ionization rate. We have experimentally studied DR of electronically and vibrationally relaxed OH
+
in its lowest rotational levels, using an electron–ion merged-beams setup at the Cryogenic Storage Ring. From these measurements, we have derived a kinetic temperature rate coefficient applicable to diffuse cloud chemical models, i.e., for OH
+
in its electronic, vibrational, and rotational ground level. At typical diffuse cloud temperatures, our kinetic temperature rate coefficient is a factor of ∼5 times larger than the previous experimentally derived value and a factor of ∼33 times larger than the value calculated by theory. Our combined experimental and modeling results point to a significant increase for the cosmic ray ionization rate inferred from observations of OH
+
and H
2
O
+
, corresponding to a geometric mean of (6.6 ± 1.0) × 10
−16
s
−1
, which is more than a factor of 2 larger than the previously inferred values of the cosmic ray ionization rate in diffuse atomic clouds. Combined with observations of diffuse and dense molecular clouds, these findings indicate a greater degree of cosmic ray shielding in interstellar clouds than has been previously inferred.