Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood

Abstract

We examine 20 diffuse and translucent Galactic sight lines and extract the column densities of the ^12CO and ^13CO isotopologues from their ultraviolet A--X absorption bands detected in archival Space Telescope Imaging Spectrograph data with lambda/Deltalambda geq 46,000. Five more targets with Goddard High-Resolution Spectrograph data are added to the sample that more than doubles the number of sight lines with published Hubble Space Telescope observations of ^13CO. Most sight lines have 12-to-13 isotopic ratios that are not significantly different from the local value of 70 for ^12C/^13C, which is based on mm-wave observations of rotational lines in emission from CO and H_2CO inside dense molecular clouds, as well as on results from optical measurements of CH^+. Five of the 25 sight lines are found to be fractionated toward lower 12-to-13 values, while three sight lines in the sample are fractionated toward higher ratios, signaling the predominance of either isotopic charge exchange or selective photodissociation, respectively. There are no obvious trends of the ^12CO-to-^13CO ratio with physical conditions such as gas temperature or density, yet ^12CO/^13CO does vary in a complicated manner with the column density of either CO isotopologue, owing to varying levels of competition between isotopic charge exchange and selective photodissociation in the fractionation of CO. Finally, rotational temperatures of H_2 show that all sight lines with detected amounts of ^13CO pass through gas that is on average colder by 20 K than the gas without ^13CO. This colder gas is also sampled by CN and C_2 molecules, the latter indicating gas kinetic temperatures of only 28 K, enough to facilitate an efficient charge exchange reaction that lowers the value of ^12CO/^13CO.

arXiv comment1-column emulateapj, 23 pages, 9 figures

References in corpus (24)

• A FUSE Survey of Interstellar Molecular Hydrogen in Translucent Clouds
• The Distribution of Thermal Pressures in the Interstellar Medium from a Survey of C I Fine-Structure Excitation
• Ultraviolet Extinction Properties in the Milky Way
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
• The Homogeneity of Interstellar Oxygen in the Galactic Disk
• High Resolution 4.7 um Keck/NIRSPEC Spectra of Protostars. II: Detection of the 13CO Isotope in Icy Grain Mantles
• Cloud Structure and Physical Conditions in Star-Forming Regions from Optical Observations. II. Analysis
• Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
• Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines
• Abundances and Depletions of Interstellar Oxygen
• Detection of the ^3P_2 - ^3P_1 Submillimeter Transition of 13CI in the Interstellar Medium: Implication for Chemical Fractionation
• The 12C/13C Isotopic Ratio in Photodissociated Gas in M42
• Oscillator Strengths for B-X, C-X, and E-X Transitions in Carbon Monoxide
• Ultraviolet Detection of Interstellar 12C17O and the CO Isotopomeric Ratios toward X Persei
• CO Isotopes in Planetary Nebulae
• ${}^{12}$C/${}^{13}$C ratio in planetary nebulae from the IUE archives
• Interstellar 12C/13C ratios through CH+ ll 3957,4232 absorption in local clouds: incomplete mixing in the ISM
• Oscillator strengths for transitions to Rydberg levels in ${}^{12}{C}^{16}O$, ${}^{13}{C}^{16}O$ and ${}^{13}{C}^{18}O$ between 967 and 972 A
• FUSE Measurements of Rydberg Bands of Interstellar CO between 925 and 1150 A
• The 12C/13C ratio in the Planetary Nebula NGC3242 from Hubble Space Telescope STIS observations
• High-Resolution Measurements of Intersystem Bands of Carbon Monoxide toward X Persei
• The circumstellar shell of the post-AGB star HD 56126: the ${}^{12}$CN/${}^{13}$CN isotope ratio and fractionation
• 13CI in high-mass star-forming clouds

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• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
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• Herschel/HIFI observations of [CII] and [13CII] in PDRs
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• Approximations for modelling CO chemistry in GMCs: a comparison of approaches
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• Discovery of a Perseus-like cloud in the early Universe: HI-to-H2 transition, carbon monoxide and small dust grains at zabs=2.53 towards the quasar J0000+0048
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• Interstellar CN and CH+ in Diffuse Molecular Clouds: 12C/13C Ratios and CN Excitation
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• Herschel observations of Extra-Ordinary Sources: Methanol as a probe of physical conditions in Orion KL
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• Ultraviolet Measurements of Interstellar C2
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• Probing the Inner Regions of Protoplanetary Disks with CO Absorption Line Spectroscopy
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• Disk and outflow signatures in Orion-KL: The power of high-resolution thermal infrared spectroscopy
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• Multireference configuration interaction study of the predissociation of C${}_2$ via its $F{}^1{\Pi }_u$ state
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• The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus
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• Physical Properties and HI-to-H2 Transition Across Taurus Linear Edge
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