HD 62542: Probing the Bare, Dense Core of a Translucent Interstellar Cloud

Abstract

We discuss the interstellar absorption from many atomic and molecular species seen in high-resolution $HST$/STIS UV and high-S/N optical spectra of the moderately reddened B3-5~V star HD~62542. This remarkable sight line exhibits both very steep far-UV extinction and a high fraction of hydrogen in molecular form -- with strong absorption from CH, C${}_2$, CN, and CO, but weak absorption from CH${}^{+}$ and most of the commonly observed diffuse interstellar bands. Most of the material resides in a single narrow velocity component -- offering a rare opportunity to probe the primarily molecular core of a single interstellar cloud with little associated diffuse atomic gas. Detailed analyses of the spectra indicate that: (1) the molecular fraction in the main cloud is high [$f$(H${}_2$) $>$ 0.8]; (2) the gas is fairly cold ($T_{\mathrm{k}}$ = 40--43 K, from the rotational excitation of H${}_2$ and C${}_2$); (3) the local hydrogen density $n_{\mathrm{H}}$ $\sim$ 1500 cm${}^{-3}$ (from the C${}_2$ excitation, the fine-structure excitation of C${}^0$, and simple chemical models); (4) the unusually high excitation temperatures for ${}^{12}$CO and ${}^{13}$CO may be largely due to radiative excitation; (5) $N$(C${}^{+}$):$N$(CO):$N$(C) $\sim$ 100:10:1; (6) the depletions of many elements are more severe than those seen in any other sight line, and the detailed pattern of depletions differs from those derived from larger samples of Galactic sight lines; and (7) the various neutral/first ion ratios do not yield consistent estimates for the electron density, even when the effects of grain-assisted recombination and low-temperaure dielectronic recombination are considered.

accepted to ApJ, 89 pages, 13 tables, 26 figures

References in corpus (18)

• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-Through-UV Curve Morphology
• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
• Radiative recombination data for modelling dynamic finite-density plasmas
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• Models of turbulent dissipation regions in the diffuse interstellar medium
• Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
• VLT/UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
• Chemical probes of turbulence in the diffuse medium: the TDR model
• Formation, fractionation and excitation of carbon monoxide in diffuse clouds
• Diffuse Interstellar Bands vs. Known Atomic and Molecular Species in the Interstellar Medium of M82 toward SN 2014J
• The Behavior of Selected Diffuse Interstellar Bands with Molecular Fraction in Diffuse Atomic and Molecular Clouds
• Observational Properties of Rotationally Excited Molecular Hydrogen In Translucent Lines of Sight
• Hubble Space Telescope Measurements of Vacuum Ultraviolet Lines of Interstellar CH
• Unidentified Features in the Ultraviolet Spectrum of X Per
• Ultraviolet HST Spectroscopy of Planck Cold Clumps

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