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Title:
Composition of interstellar clouds in the disk and halo. 2: Gamma2 Velorum
Authors:
Fitzpatrick, Edward L.; Spitzer, Lyman, Jr.
Affiliation:
AA(Princeton Univ. Observatory, Princeton, NJ, US), AB(Princeton Univ. Observatory, Princeton, NJ, US)
Publication:
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 427, no. 1, p. 232-258 (ApJ Homepage)
Publication Date:
05/1994
Category:
Astrophysics
Origin:
STI
NASA/STI Keywords:
Atmospheric Density, Data Reduction, Galaxies, Halos, Interstellar Gas, Stellar Spectra, Stellar Winds, Ultraviolet Spectra, Absorption Spectra, Atmospheric Temperature, Echelle Gratings, Emission Spectra, Hubble Space Telescope, Infrared Astronomy Satellite, Iue, Line Of Sight, Optical Thickness, Radial Velocity, Spectrographs, Thermal Conductivity
DOI:
10.1086/174136
Bibliographic Code:
1994ApJ...427..232F

Abstract

High-resolution observations of gamma2 Vel with the Goddard High-Resolution Spectrograph (GHRS) echelle on the Hubble Space Telescope reveal the presence of seven narrow absorption components, with LSR velocities between -23 and +9 km s-1. Three of these show column density ratios N(S(++))/N(S(+)) and N(P(++))/N(P+)) of about 1 or more, and can be identified as H II regions, while the other four are H I regions, consistent with the O I profile and with the overall H0 column density of 5.9 x 1019 cm-2, given the usual assumptions that S is undepleted while O has a depletion D(O) = -0.3 dex. The depletions of Fe, Si, and Mn, which could be measure accurately for two of the four H I regions (components 6 and 7), differ somewhat from the values of Dws found for slowly moving warm clouds in HD 93521; in particular, for the component at 4.0 km s-1 (No. 6), absolute of D exceeds absolute of Dws by 0.1-0.4 dex, while for that at 9.3 km s-1 (No. 7), absolute of D equals absolute of Dws on the average. The observed ratio of Fe + Mg atoms to Si atoms in the grains of component 6 is 2.04 +/-0.10, consistent with an olivine grain composition; the Fe/Mg ratio is 1.5 +/- 0.2. The electron density in component 6, determined from the C II* feature, is 0.075 +/- 0.013 cm -3, about two-thirds of that found for clouds of this velocity in HD 93521. In the two conspicuous H II regions, components 3 and 4, ne, determined from the Si II* feature, is about 1 cm-3. From the column density of S(+) + S(++) in these two components, the total H II path length is about 40 pc. With the radius of a wind-blown bubble around gamma2 Vel set equal to 60 pc, the effective Stromgren radius is about 100 pc, requiring that T approx. equal to 50,000 K for the Wolf-Rayet component of the gamma2 Vel binary. Since zeta Pup is a comparable source of ionizing radiation, this temperature is an upper limit. The profiles of the strongest H2 absorption features, from Copernicus archives, indicate that the absorbing molecules have a mean velocity identical with that of the strongest H II component (No. 4). We have no explanation for the possible presence of these H2 molecules in a region of ionized H. Alternatively, the H2 profiles can be explained by molecules in the two adjacent (in velocity) H I regions, components 2 and 5, provided their H I gas has densities and temperatures typical of normal cold clouds. The GHRS data show absorption by highly ionized atoms Si(3+) and C(3+), N(4+) in broad features, in addition to the narrow-line absorption by Si(3+) and C(3+) observed in the dominant H II components, Nos. 3 and 4. The broad C(3+) and N(4+) features have widths corresponding to T in the range (4-8) x 105 K, consistent with the broad O(5+) line shown in Copernicus data. Despite some observational uncertainties, the ratios of column densities in the broad C(3+), N(4+), and O(5+) features agree to +/- 0.1 dex with theoretical values for warm gas, heating and evaporating by thermal conduction from an adjacent hot region. Outward evaporation from an isolated cloud in a hot ambient gas cannot be distinguished, on the basis of these data, from inward evaporation of a warm shell, compressed by an expanding, hot stellar-wind bubble. For several halo stars, the C IV/O VI ratio has a quite different average value, perhaps consistent with cooling of infalling hot gas instead of conductive heating and evaporation.

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