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A FUSE Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds | | Jason Tumlinson
; J. Michael Shull
; Brian L. Rachford
; Matthew K. Browning
; Theodore P. Snow
; Alex W. Fullerton
; Edward B. Jenkins
; Blair D. Savage
; Paul A. Crowther
; H. Warren Moos
; Kenneth R. Sembach
; George Sonneborn
; Donald G. York
; | | Date: |
11 Oct 2001 | | Subject: | astro-ph | | Affiliation: | University of Colorado), Alex W. Fullerton (UVic,JHU), Edward B. Jenkins (Princeton), Blair D. Savage (Wisconsin), Paul A. Crowther (UCL), H. Warren Moos (JHU), Kenneth R. Sembach (STScI), George Sonneborn (NASA/GSFC), Donald G. York (Chicago | | Abstract: | We describe a moderate-resolution FUSE survey of H2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. Our survey is sensitive to N(H2) >= 10^14 cm^-2; the highest column densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions = 0.010 (+0.005, -0.002) for the SMC and = 0.012 (+0.006, -0.003) for the LMC, compared with = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Lya in pencil-beam sight lines as measures of N(HI). These results imply that the diffuse H2 masses of the LMC and SMC are 8 x 10^6 Msun and 2 x 10^6 Msun, respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 x 10^-18 cm^3 s^-1, and incident radiation fields ranging from 10 - 100 times the Galactic mean value. We find that these high-radiation, low-formation-rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H2 column densities in low rotational states (J = 0 and 1) to derive a mean kinetic and/or rotational temperature = 82 +/- 21 K for clouds with N(H2) >= 10^16 cm^-2, similar to Galactic gas. We discuss the implications of this work for theories of star formation in low-metallicity environments. [Abstract abridged] | | Source: | arXiv, astro-ph/0110262 | | Services: | Forum | Review | PDF | Favorites | |
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