A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds

We describe a moderate-resolution Far Ultraviolet Spectroscopic Explorer (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 Å. Our survey is sensitive to N(H2) ≥ 1014 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 for the SMC and = 0.012 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 Lyα in pencil beam sight lines as measures of N(H I). These results imply that the diffuse H2 masses of the LMC and SMC are 8 × 106 and 2 × 106 M☉, 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 × 10-18 cm3 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 kinetic and/or rotational temperatures of diffuse interstellar gas, and we find that the distribution of rotational temperatures is similar to Galactic gas, with ⟨T01⟩ = 82 ± 21 K for clouds with N(H2) ≥ 1016.5 cm-2. There is only a weak correlation between detected H2 and far-infrared fluxes as determined by IRAS, perhaps as a result of differences in the survey techniques. We find that the surface density of H2 probed by our pencil beam sight lines is far lower than that predicted from the surface brightness of dust in IRAS maps. We discuss the implications of this work for theories of star formation in low-metallicity environments.

[1]  D. York,et al.  FUSE Observations of Molecular Hydrogen in Translucent Interstellar Clouds , 2001, astro-ph/0103136.

[2]  J. Ge,et al.  H2, C I, Metallicity, and Dust Depletion in the z = 2.34 Damped Lyα Absorption System toward QSO 1232+0815 , 2001 .

[3]  Linda J. Smith,et al.  The Ultraviolet and Optical Spectra of Metal‐deficient O Stars in the Small Magellanic Cloud , 2000 .

[4]  D. York,et al.  Far Ultraviolet Spectroscopic Explorer Observations of Diffuse Interstellar Molecular Hydrogen , 2000 .

[5]  J. B. Joyce,et al.  On-Orbit Performance of the Far Ultraviolet Spectroscopic Explorer Satellite , 2000, astro-ph/0005531.

[6]  et al,et al.  Overview of the Far Ultraviolet Spectroscopic Explorer Mission , 2000, astro-ph/0005529.

[7]  D. York,et al.  Far Ultraviolet Spectroscopic Explorer Observations of Interstellar Gas toward the Large Magellanic Cloud Star Sk –67°05 , 2000 .

[8]  D. York,et al.  Far Ultraviolet Spectroscopic Explorer Observations of Molecular Hydrogen in Translucent Interstellar Clouds: The Line of Sight toward HD 73882 , 2000, astro-ph/0005090.

[9]  P. Wozniak,et al.  The Properties of Molecular Hydrogen toward the Orion Belt Stars from Observations by the Interstellar Medium Absorption Profile Spectrograph , 2000, astro-ph/0003132.

[10]  J. Edelstein,et al.  ORFEUS II Echelle Observations of Molecular Hydrogen in the Galactic Halo , 2000 .

[11]  S. Asayama,et al.  First Results of a CO Survey of the Large Magellanic Cloud with NANTEN; Giant Molecular Clouds as Formation Sites of Populous Clusters , 1999 .

[12]  M. Dopita,et al.  H I Shells in the Large Magellanic Cloud , 1999 .

[13]  M. Azzopardi,et al.  The fourth catalogue of Population I Wolf-Rayet stars in the Large Magellanic Cloud ? , 1999 .

[14]  D. York,et al.  Interstellar Abundances in the Magellanic Clouds. II. The Line of Sight to SN 1987A in the Large Magellanic Cloud , 1999 .

[15]  R. Sault,et al.  The large‐scale HI structure of the Small Magellanic Cloud , 1999 .

[16]  D. Schlegel,et al.  Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds , 1998 .

[17]  M. Barlow,et al.  Quantitative classification of WC and WO stars , 1998 .

[18]  D. York,et al.  Interstellar Abundances in the Magellanic Clouds. I. GHRS Observations of the Small Magellanic Cloud Star Sk 108 , 1997 .

[19]  J. Blades,et al.  Spectral Classification of the 30 Doradus Stellar Populations , 1997 .

[20]  S. Bowyer,et al.  ORFEUS-I Observations of Molecular Hydrogen in the Galactic Disk , 1997, astro-ph/9709027.

[21]  E. Jenkins,et al.  Molecular Hydrogen in the Direction of ζ Orionis A , 1996, astro-ph/9609144.

[22]  Anthony F. J. Moffat,et al.  A three-dimensional classification for WN stars , 1996 .

[23]  R. Kudritzki,et al.  THE PHYSICS OF MASSIVE OB STARS IN DIFFERENT PARENT GALAXIES. I. ULTRAVIOLET AND OPTICAL SPECTRAL MORPHOLOGY IN THE MAGELLANIC CLOUDS , 1995 .

[24]  L. Auer,et al.  Remarkable long-term changes in the small Magellanic Cloud Wolf-Rayet system HD 5980 , 1994 .

[25]  P. Massey,et al.  Spectrophotometry of Wolf-Rayet stars - Intrinsic colors and absolute magnitudes , 1988 .

[26]  Philip R. Maloney,et al.  I(CO)/N(H2) conversions and molecular gas abundances in spiral and irregular galaxies , 1988 .

[27]  C. Foltz,et al.  Molecules at early epochs. II. H/sub 2/ and CO toward PHL 957 , 1987 .

[28]  N. Walborn,et al.  NEW O3 GIANTS IN THE LARGE MAGELLANIC CLOUD. , 1987 .

[29]  John H. Black,et al.  Comprehensive models of diffuse interstellar clouds : physical conditions and molecular abundances , 1986 .

[30]  P. Conti,et al.  Studies of massive stars in the Magellanic Clouds. I - New spectral classifications of OB types in the LMC , 1986 .

[31]  J. Blades,et al.  An O3 star in the Small Magellanic Cloud H II region NGC 346 , 1986 .

[32]  E. Fitzpatrick The Properties of OB Supergiants in the Large Magellanic Cloud , 1985 .

[33]  E. Fitzpatrick Ultraviolet interstellar absorption toward stars in the Small Magellanic Cloud. III - The structure and kinematics of the Small Magellanic Cloud , 1985 .

[34]  J. Shull,et al.  Galactic interstellar abundance surveys with IUE. I. Neutral hydrogen , 1985 .

[35]  N. Walborn Metal-deficient O9-B0 supergiants in the Small Magellanic Cloud , 1983 .

[36]  T. Graauw,et al.  Observations of C-12O/J=2-1/ emission in the Large and Small Magellanic Clouds. , 1982 .

[37]  D. Crampton,et al.  OB stars in the Small Magellanic Cloud , 1982 .

[38]  D. Hollenbach,et al.  Molecule formation and infrared emission in fast interstellar shocks. I Physical processes , 1979 .

[39]  R. C. Bohlin,et al.  A survey of interstellar molecular hydrogen. I , 1977 .

[40]  N. Walborn Spectral classification of O and B0 supergiants in the Magellanic Clouds , 1976 .

[41]  M. Jura Interstellar clouds containing optically thin H2 , 1975 .

[42]  W. D. Cochran,et al.  Column densities of interstellar molecular hydrogen , 1974 .

[43]  M. Jura,et al.  Formation and destruction rates of interstellar H2 , 1974 .

[44]  George R. Carruthers,et al.  Rocket observation of interstellar molecular hydrogen , 1970 .

[45]  K. Gunderson,et al.  Molecular Hydrogen along Two Lines of Sight through the Large Magellanic Cloud , 1998 .

[46]  A. Moffat,et al.  Wolf-Rayet stars in the Magellanic clouds. VI: Spectroscopic orbits of WC binaries and implications for W-R evolution , 1990 .

[47]  Daniel Taupin,et al.  Probabilities data reduction and error analysis in the physical sciences/ by Daniel Taupin , 1988 .

[48]  S. Beckwith,et al.  INTERSTELLAR MOLECULAR HYDROGEN , 1982 .

[49]  E. Salpeter,et al.  Molecular Hydrogen in H i Regions , 1971 .