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Metal-ion Absorption in Conductively Evaporating Clouds

Gnat, Orly and Sternberg, Amiel and McKee, Christopher F. (2010) Metal-ion Absorption in Conductively Evaporating Clouds. Astrophysical Journal, 718 (2). pp. 1315-1331. ISSN 0004-637X.

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We present computations of the ionization structure and metal-absorption properties of thermally conductive interface layers that surround evaporating warm spherical clouds embedded in a hot medium. We rely on the analytical steady-state formalism of Dalton and Balbus to calculate the temperature profile in the evaporating gas, and we explicitly solve the time-dependent ionization equations for H, He, C, N, O, Si, and S in the conductive interface. We include photoionization by an external field. We estimate how departures from equilibrium ionization affect the resonance-line cooling efficiencies in the evaporating gas, and determine the conditions for which radiative losses may be neglected in the solution for the evaporation dynamics and temperature profile. Our results indicate that nonequilibrium cooling significantly increases the value of the saturation parameter σ_0 at which radiative losses begin to affect the flow dynamics. As applications, we calculate the ion fractions and projected column densities arising in the evaporating layers surrounding dwarf-galaxy-scale objects that are also photoionized by metagalactic radiation. We compare our results to the UV metal-absorption column densities observed in local highly ionized metal absorbers, located in the Galactic corona or intergalactic medium. Conductive interfaces significantly enhance the formation of high ions such as C^(3+), N^(4+), and O^(5+) relative to purely photoionized clouds, especially for clouds embedded in a high-pressure corona. However, the enhanced columns are still too low to account for the O VI columns (~10^(14) cm^(–2)) observed in the local high-velocity metal-ion absorbers. We find that column densities larger than ~10^(13) cm^(–2) cannot be produced in evaporating clouds. Our results do support the conclusion of Savage and Lehner that absorption due to evaporating O VI likely occurs in the local interstellar medium, with characteristic columns of ~10^(13) cm^(–2).

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Additional Information:© 2010 The American Astronomical Society. Received 2010 February 3; accepted 2010 June 3; published 2010 July 14. We thank Blair Savage and Mike Shull for useful comments. This research was supported by the US-Israel Binational Science Foundation (BSF) grant 2002317, by the Deutsche Forschungsgemeinschaft (DFG) via German-Israeli Project Cooperation grant STE1869/1-1.GE625/15-1, and by the National Science Foundation through grant AST0908553 (CFM). O.G. acknowledges support provided by NASA through Chandra Postdoctoral Fellowship grant number PF8-90053 awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060.
Funding AgencyGrant Number
US-Israel Binational Science Foundation (BSF)2002317
Deutsche Forschungsgemeinschaft (DFG)UNSPECIFIED
German-Israeli Project CooperationSTE1869/1-1.GE625/15-1
NASA Chandra Postdoctoral FellowshipPF8-90053
Subject Keywords:atomic processes – conduction – ISM: general – plasmas – quasars: absorption lines
Issue or Number:2
Classification Code:PACS: 98.58.Ge; 98.58.Kh; 98.62.Gq; 98.62.Py; 98.52.Wz
Record Number:CaltechAUTHORS:20100804-135446392
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Official Citation:Orly Gnat et al 2010 ApJ 718 1315 doi: 10.1088/0004-637X/718/2/1315
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:19277
Deposited By: Tony Diaz
Deposited On:04 Aug 2010 21:22
Last Modified:03 Oct 2019 01:54

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