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

Kálosi, Á., Gamer, L., Grieser, M., von Hahn, R., Isberner, L. W., Jaeger, J. I., et al. (2023). Dissociative Recombination of Rotationally Cold OH+ and Its Implications for the Cosmic Ray Ionization Rate in Diffuse Clouds. The Astrophysical Journal Letters, 955(2): L26. doi:10.3847/2041-8213/acf71d.

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Kálosi, Ábel1, Author                 
Gamer, Lisa1, Author                 
Grieser, Manfred1, Author                 
von Hahn, Robert1, Author           
Isberner, Leonard W.1, Author                 
Jaeger, Julia I.1, Author                 
Kreckel, Holger2, Author                 
Neufeld, David A., Author
Paul, Daniel1, Author                 
Savin, Daniel W., Author
Schippers, Stefan, Author
Schmidt, Viviane C.1, Author                 
Wolf, Andreas1, Author                 
Wolfire, Mark G., Author
Novotný, Oldřich1, Author                 
Affiliations:
1Division Prof. Dr. Klaus Blaum, MPI for Nuclear Physics, Max Planck Society, ou_904548              
2Holger Kreckel, ASTROLAB - MPG-Gruppe im Anschluss an ERC Starting Grant, MPI for Nuclear Physics, Max Planck Society, ou_3349435              

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 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 H2-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 H2O+, 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.

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 Dates: 2023-09-28
 Publication Status: Published online
 Pages: 14
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 Rev. Type: -
 Identifiers: DOI: 10.3847/2041-8213/acf71d
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Title: The Astrophysical Journal Letters
  Other : Astrophys. J. Lett.
Source Genre: Journal
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Publ. Info: Chicago, IL : University of Chicago Press for the American Astronomical Society
Pages: - Volume / Issue: 955 (2) Sequence Number: L26 Start / End Page: - Identifier: ISSN: 0004-637X
CoNE: https://pure.mpg.de/cone/journals/resource/954922828215