Astron. Astrophys. 322, 962-974 (1997)

Molecular emission from the diffuse gas around Ophiuchi

H.S. Liszt

National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA

Received 7 October 1996 / Accepted 15 November 1996

Abstract

We have mapped 18cm OH and 9cm CH emission from the diffuse gas around Oph, and find no trace of emission from the 4.5 km s^-1 gas whose OH line was previously believed to represent pre-shock gas. The pattern of OH emission is strikingly dissimilar to those of CO, HCO and CH in that the strong km s^-1 component seen to the South is largely absent in OH. CH more closely resembles CO, peaking on either side of the star, and comparison of CO and CH shows that the CO line `turns on' over a very narrow range of CH line strength. Alternatively, OH emission actually weakens as CO brightens in the km s^-1 line, but this may be only an excitation effect, and not the result of a declining OH abundance.

We mapped 3mm HCO emission over the inner portion of the gas distribution occulting Oph, finding that its behaviour roughly parallels that of CO. We searched for emission from HCN, CS (J=2-1), CN, and H at the CO and HCO emission peak South of the star. Of these, only HCN was found and it is quite weak compared to HCO ( K vs. 0.09 K). Emission from the other mm-wave species is absent in spectra with rms noise 0.01 K; CS J=2-1 emission was sought also unsuccessfully toward the star. The column densities inferred from these intensities are uncertain owing to unknown physical conditions along the line of sight, chiefly the kinetic temperature, but they are not necessarily much larger than would be found toward the star. Relative abundances of the high-dipole moment species are better determined; N(HCN)/N(HCO ) 1, N(CS)/N(HCO ) 1.

Key words: ISM: molecules; abundances radio lines: ISM

Send offprint requests to: H. Liszt

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Contents

• 1. Introduction
• 2. Observations
  • 2.1. 18cm OH emission measurements
  • 2.2. 9cm CH emission measurements
  • 2.3. 3mm observations of HCO , HCN, H, CS, and CN
  • 2.4. New ¹² CO, ¹³ CO, and C¹⁸ O J=1-0 profiles
  • 2.5. Conventions
• 3. Large-scale emission patterns of CH, OH, and CO
  • 3.1. CH emission and excitation
  • 3.2. OH emission and excitation
  • 3.3. Comparison of OH, CH and CO emission
    • 3.3.1. CH and CO
    • 3.3.2. CO and OH
    • 3.3.3. CH and OH
• 4. Chemical abundances in the gas around Oph
  • 4.1. Emission and column densities at the HCO emission peak
• 5. CO isotope ratio, excitation, and conversion factor
  • 5.1. CO excitation and the CO- conversion factor in diffuse gas
• 6. Summary
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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