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Infrared Response of H2 to X-Rays in Dense CloudsThe excitation by X-rays and cosmic rays of molecular hydrogen in interstellar clouds is analyzed. We carried out detailed calculations of entry efficiencies in rovibrational levels of H2 following impact with fast electrons produced by X-ray ionization of the gas. The competing effect of collisional excitation, and quenching by the ambient gas is examined in detail. Up to date values for H-H2 collisional rate coefficients are adopted, and some derivations of H2-H2 rovibrational rate coefficients from existing literature data are proposed. Several models as a function of temperature, density, and ionization rate are presented. We found that H2 infrared emission in X-ray dominated regions (XDR) is potentially observable for temperatures and ionization rates lower than certain critical values (typically T < 1000 K and zeta/n(sub H) < 10(exp -15) cc/s where zeta is the ionization rate). At higher temperatures, collisional excitation by the ambient gas dominates the population of low vibrational levels, and at higher values of zeta/n(sub H) the abundance of H2 is negligible. If such conditions are satisfied, the resulting infrared emission spectrum can be used as a diagnostic of nearby X-ray sources such as in cooling flows in galaxy clusters, quasars, Seyfert galaxies and supernova remnants. The intensity ratio of the 2-1S(1) and 1-0S(1) lines measured for the Seyfert galaxy NGC 1275 is consistent with X-ray pumping.
Document ID
20000086190
Acquisition Source
Headquarters
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Tine, S.
(Nevada Univ. Las Vegas, NV United States)
Lepp, S.
(Nevada Univ. Las Vegas, NV United States)
Gredel, R.
(European Southern Observatory Santiago, Chile)
Dalgarno, A.
(Harvard-Smithsonian Center for Astrophysics Cambridge, MA United States)
Date Acquired
August 19, 2013
Publication Date
May 20, 1997
Publication Information
Publication: Astrophysical Journal
Publisher: American Astronomical Society
Volume: 481
Subject Category
Space Radiation
Funding Number(s)
CONTRACT_GRANT: NSF OSR-93-53227
CONTRACT_GRANT: NAGW-1561
CONTRACT_GRANT: NAGW-3147
CONTRACT_GRANT: NSF AST-95-31790
Distribution Limits
Public
Copyright
Other

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