Far Ultraviolet Spectroscopic Explorer Observations of Molecular Hydrogen in Translucent Interstellar Clouds: The Line of Sight toward HD 73882

We report the results of the initial Far Ultraviolet Spectroscopic Explorer observations of molecular hydrogen (H2) in translucent clouds. These clouds have greater optical depth than any of the diffuse clouds previously observed for far-UV H2 absorption and provide new insights into the physics and chemistry of such regions. Our initial results involve observations of HD 73882, a well-studied southern hemisphere star lying behind substantial interstellar material (EB-V = 0.72; AV = 2.44). We find a total H2 column density N(H2) = 1.2 × 1021 cm-2 about 3 times larger than the values for diffuse clouds previously measured in the far-UV. The gas kinetic temperature indicated by the ratio N(J = 1)/N(J = 0) is 58 ± 10 K. With the aid of ground-based data to calculate an appropriate multicomponent curve of growth, we have determined column densities for all rotational levels up to J = 7. The J ≥ 2 states can be reasonably fitted with a rotational excitation temperature of 307 ± 23 K. Both the kinetic and rotational temperatures are similar to those found in previous investigations of diffuse clouds. The ratios of carbonaceous molecules to hydrogen molecules are also similar to ratios in diffuse clouds, suggesting a similar chemistry for this line of sight.

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

[2]  È. Roueff,et al.  Total transition probability and spontaneous radiative dissociation of B, C, B′ and D states of molecular hydrogen , 2000 .

[3]  Alexander G. G. M. Tielens,et al.  Photodissociation Regions in the Interstellar Medium of Galaxies , 1999 .

[4]  J. Black,et al.  Carbon and Oxygen Depletion and Extinction in the Translucent Cloud toward HD 24534 (X Persei) , 1998 .

[5]  Shengjun Wu,et al.  Absolute generalized oscillator strengths for the vibronic bands of A1Pi, B1Sigma+, C1Sigma+, and E1Pi transitions of carbon monoxide , 1997 .

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

[7]  Kenneth R. Sembach,et al.  INTERSTELLAR ABUNDANCES FROM ABSORPTION-LINE OBSERVATIONS WITH THE HUBBLE SPACE TELESCOPE , 1996 .

[8]  S. Federman,et al.  Cosmic Ray--induced Chemistry toward Perseus OB2 , 1996 .

[9]  I. Crawford.  Ultra-high-resolution observations of the intrinsic line profiles of interstellar CH, CH+ and CN , 1995 .

[10]  David L. Lambert,et al.  Vibrationally excited H2, HCl and NO+ in the diffuse clouds toward zeta Ophiuchi , 1995 .

[11]  D. Morton,et al.  A Compilation of Electronic Transitions in the CO Molecule and the Interpretation of Some Puzzling Interstellar Absorption Features , 1994 .

[12]  T. Geballe,et al.  Detection of absorption by H2 in molecular clouds: A direct measurement of the H2:CO ratio , 1994 .

[13]  A. Tielens,et al.  THE PHOTOELECTRIC HEATING MECHANISM FOR VERY SMALL GRAPHITIC GRAINS AND POLYCYCLIC AROMATIC HYDROCARBONS , 1994 .

[14]  Yaron Sheffer,et al.  Interstellar Carbon Monoxide toward zeta Ophiuchi , 1994 .

[15]  T. Snow.  A Possible Connection between Enhanced CH + and Intense Far-Ultraviolet Radiation Fields , 1993 .

[16]  J. Black,et al.  CO J = 3-2 observations of translucent and high-latitude molecular clouds , 1991 .

[17]  Edward L. Fitzpatrick,et al.  An Analysis of the Shapes of Ultraviolet Extinction Curves. III. an Atlas of Ultraviolet Extinction Curves , 1990 .

[18]  J. Black,et al.  Interstellar C2, CH, and CN in translucent molecular clouds , 1989 .

[19]  J. Mathis,et al.  The determination of ultraviolet extinction from the optical and near-infrared , 1988 .

[20]  D. Massa,et al.  An analysis of the shapes of ultraviolet extinction curves. II - The far-UV extinction , 1988 .

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

[22]  John H. Black,et al.  Detection of the (3, 0) Phillips band of interstellar C2 toward zeta Ophiuchi , 1986 .

[23]  D. Massa,et al.  An analysis of the shapes of ultraviolet extinction curves. I - The 2175 A bump , 1986 .

[24]  G. P. Forêts,et al.  Theoretical studies of interstellar molecular shocks – III. The formation of CH+ in diffuse clouds , 1986 .

[25]  D. Massa,et al.  Peculiar ultraviolet interstellar extinction , 1983 .

[26]  B. Savage,et al.  Ultraviolet interstellar extinction toward 1367 stars observed by ANS , 1981 .

[27]  R. Kurucz.  Model atmospheres for G, F, A, B, and O stars , 1979 .

[28]  B. Savage,et al.  A survey of interstellar H I from L-alpha absorption measurements. II , 1978 .

[29]  B. Draine.  Photoelectric heating of interstellar gas , 1978 .

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

[31]  Uwe Fink,et al.  Detection of molecular hydrogen quadrupole emission in the Orion Nebula , 1976 .

[32]  F. J. Hawkins,et al.  The Interstellar Medium in the Line of Sight to X Persei and 3U 0352 + 30 , 1976 .

[33]  R. Bohlin.  Copernicus observations of interstellar absorption at Lyman alpha , 1975 .

[34]  D. Morton,et al.  Interstellar absorption lines in the spectrum of Zeta Ophiuchi , 1975 .

[35]  D. S. Mathewson,et al.  Wavelength dependence of interstellar polarization and ratio of total to selective extinction. , 1975 .

[36]  Edward B. Jenkins,et al.  Ultraviolet studies of the interstellar gas , 1975 .

[37]  W. D. Cochran,et al.  Rotational Excitation of Interstellar H_{2} , 1973 .

[38]  J. Black,et al.  The Cosmic Abundance of Deuterium , 1973 .

[39]  A. Rodgers.  Three-Colour Photometry in the Southern Coalsack , 1960 .