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.

arXiv commentaccepted to ApJ, 89 pages, 13 tables, 26 figures

References in corpus (82)

• Atomic Data for Astrophysics. II. New Analytic Fits for Photoionization Cross Sections of Atoms and Ions
• 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
• The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
• The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks
• A model for atomic and molecular interstellar gas: The Meudon PDR code
• Calibrator Design for the COBE Far Infrared Absolute Spectrophotometer (FIRAS)
• Photodissociation and photoionisation of atoms and molecules of astrophysical interest
• A FUSE Survey of Interstellar Molecular Hydrogen in Translucent Clouds
• 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
• Interstellar Carbon in Translucent Sightlines
• Dielectronic recombination data for dynamic finite-density plasmas I. Goals and methodology
• The Distribution of Thermal Pressures in the Interstellar Medium from a Survey of C I Fine-Structure Excitation
• Molecular Hydrogen in the FUSE Translucent Lines of Sight: The Full Sample
• Radiative recombination data for modelling dynamic finite-density plasmas
• Studies of Diffuse Interstellar Bands. V. Pairwise Correlations of Eight Strong DIBs and Neutral Hydrogen, Molecular Hydrogen, and Color Excess
• The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions
• Ultraviolet Extinction Properties in the Milky Way
• The Diffuse Interstellar Clouds toward 23 Orionis
• 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
• The chemistry of vibrationally excited H2 in the interstellar medium
• FUSE Measurements of Far-Ultraviolet Extinction. III. The Dependence on R(V) and Discrete Feature Limits from 75 Galactic Sightlines
• Dissecting the Circumstellar Environment of Gamma-Ray Burst Progenitors
• VLT/UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
• Electron-Ion Recombination on Grains and Polycyclic Aromatic Hydrocarbons
• The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds
• The Homogeneity of Interstellar Elemental Abundances in the Galactic Disk
• Chemical probes of turbulence in the diffuse medium: the TDR model
• A Procedure for Correcting the Apparent Optical Depths of Moderately Saturated Interstellar Absorption Lines
• Exploring the diffuse interstellar bands with the Sloan Digital Sky Survey
• Interstellar H I and H_2 in the Magellanic Clouds: An Expanded Sample Based on UV Absorption-Line Data
• Cold Milky Way Hi gas in filaments
• Formation, fractionation and excitation of carbon monoxide in diffuse clouds
• Resonance Averaged Photoionization Cross Sections for Astrophysical Models
• High-Resolution Observations of Interstellar Ca I Absorption -- Implications for Depletions and Electron Densities in Diffuse Clouds
• Multifractal Scaling, Geometrical Diversity, and Hierarchical Structure in the Cool Interstellar Medium
• Cloud Structure and Physical Conditions in Star-Forming Regions from Optical Observations. II. Analysis
• Gas-phase recombination, grain neutralization and cosmic-ray ionization in diffuse gas
• Calculation of isotope shifts and relativistic shifts in CI, CII, CIII and CIV
• Interstellar CN and CH+ in Diffuse Molecular Clouds: 12C/13C Ratios and CN Excitation
• Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
• Atomic and Molecular Carbon as a Tracer of Translucent Clouds
• Observations of rotationally resolved C3 in translucent sight lines
• Diffuse Molecular Cloud Densities from UV Measurements of CO Absorption
• Diffuse Interstellar Bands vs. Known Atomic and Molecular Species in the Interstellar Medium of M82 toward SN 2014J
• From interstellar abundances to grain composition: the major dust constituents Mg, Si and Fe
• Origin of CH+ in diffuse molecular clouds warm H2 and ion-neutral drift
• Abundances and Depletions of Interstellar Oxygen
• The Abundance of Mg in the Interstellar Medium
• Ultraviolet Detection of Interstellar 12C17O and the CO Isotopomeric Ratios toward X Persei
• Measurements of the f-Values of the Resonance Transitions of Ni II at 1317.217A and 1370.132A
• Abundances and Physical Conditions in the Interstellar Gas toward HD 192 639
• Interstellar polarization and grain alignment: the role of iron and silicon
• Unusually Weak Diffuse Interstellar Bands toward HD 62542
• The EDIBLES survey IV. Cosmic ray ionization rates in diffuse clouds from near-ultraviolet observations of interstellar OH${}^{+}$
• The Behavior of Selected Diffuse Interstellar Bands with Molecular Fraction in Diffuse Atomic and Molecular Clouds
• OH+ in Diffuse Molecular Clouds
• Density Variations over Subparsec Scales in Diffuse Molecular Gas
• Thermal Pressures in the Interstellar Medium of the Magellanic Clouds
• Interstellar Iron and Silicon Depletions in Translucent Sight Lines
• The CH+ Abundance in Turbulent, Diffuse Molecular Clouds
• Ultraviolet Measurements of Interstellar C2
• Observational Properties of Rotationally Excited Molecular Hydrogen In Translucent Lines of Sight
• Detection of Interstellar C_2 and C_3 in the Small Magellanic Cloud
• Molecular cloud chemistry and the importance of dielectronic recombination
• Hubble Space Telescope Measurements of Vacuum Ultraviolet Lines of Interstellar CH
• Abundances and Depletions of Neutron-Capture Elements in the Interstellar Medium
• Interstellar Ti II in the Milky Way and Magellanic Clouds
• Dielectronic recombination data for dynamic finite-density plasmas XV. The silicon isoelectronic sequence
• High-Resolution Measurements of Intersystem Bands of Carbon Monoxide toward X Persei
• Ab initio Study of Ground-State CS Photodissociation Via Highly Excited Electronic States
• Interstellar H2 toward HD 37903
• Unidentified Features in the Ultraviolet Spectrum of X Per
• Interstellar H2 toward HD 147888
• Modeling anomalous extinction using nanodiamonds
• Lifetimes and Oscillator Strengths for Ultraviolet Transitions in Singly-Ionized Lead
• Thermal Pressure in Diffuse H2 Gas Measured by Herschel [C II] Emission and FUSE UV H2 Absorption
• Ultraviolet HST Spectroscopy of Planck Cold Clumps

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• The Distribution of Metallicities in the Local Galactic Interstellar Medium
• A High Precision Survey of the D/H Ratio in the Nearby Interstellar Medium
• Hunting for the elusive methylene radical
• The EDIBLES Survey. VII. A survey of C2 and C3 in interstellar clouds
• A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium
• Chemical diversity of gas in distant galaxies: The metal and dust enrichment and variations within absorbing galaxies
• Multi-phase gas properties of extremely strong intervening DLAs towards quasars
• Investigating chemical variations between interstellar gas clouds in the Solar neighbourhood
• First detections of CO absorption in the Magellanic Clouds and direct measurement of the CO-to-H${}_2$ ratio
• Connecting the dusty dots: dust depletion and extinction of local interstellar clouds