Astron. Astrophys. 355, 699-712 (2000)

ISO-SWS spectroscopy of gas-phase C₂H₂ and HCN toward massive young stellar objects ^*

F. Lahuis ^1,2 and E.F. van Dishoeck <sup>3

1</sup> Space Research Organisation Netherlands, P.O.Box 800, 9700 AV Groningen, The Netherlands
² ISO Data Centre, Astrophysics Division, Space Science Department of ESA, Villafranca, P.O.Box 50727, 28080 Madrid, Spain
³ Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands

Received 16 August 1999 / Accepted 11 January 2000

Abstract

Observations of gas-phase C₂H₂ and HCN along the line of sight toward a large sample of deeply embedded massive young stellar objects (YSOs) have been performed using the Short Wavelength Spectrometer on board the Infrared Space Observatory. The vibration-rotation band of C₂H₂ around m and the band of HCN around m have been detected for most lines of sight. These wavelength regions are heavily affected by instrumental fringing and a detailed discussion of the data reduction techniques is given. Comparison with model spectra allows the excitation temperatures and the abundances of the molecules to be determined. The inferred excitation temperatures range from  to 1000 K, and correlate well with each other, indicating that the two molecules probe the same warm gas component. The C₂H₂ and HCN column densities increase by more than an order of magnitude with increasing excitation temperature, and with the amount of heating of the ices. The corresponding abundances of C₂H₂ and HCN in the warm gas increase from  to  with increasing temperatures. The enhanced abundances are compared with a variety of chemical models. The observed gas-phase C₂H₂ most likely results from direct evaporation of interstellar ices, where C₂H₂ must be present at an abundance of with respect to H₂O ice. This abundance is consistent with the measured amount of C₂H₂ in cometary ices. The observed gas-phase HCN abundance shows a stronger increase with temperature and results from a combination of evaporation of ices and high-temperature gas-phase chemistry in the hot core.

Key words: ISM: molecules ISM: clouds stars: formation molecular processes methods: data analysis

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom), with participation of ISAS and NASA.

Send offprint requests to: F. Lahuis

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Contents

• 1. Introduction
• 2. Observations and data reduction
  • 2.1. Observations and sources
  • 2.2. Data reduction
    • 2.2.1. Fringe removal
• 3. Model spectra and fits
  • 3.1. Synthetic spectra
  • 3.2. Hot bands
  • 3.3. Model fits
• 4. Results
  • 4.1. AFGL 2591
  • 4.2. AFGL 2136
  • 4.3. AFGL 4176
  • 4.4. W 33 A
  • 4.5. W 3 IRS 5
  • 4.6. NGC 7538 IRS 1 and IRS 9
  • 4.7. Other sources
• 5. Discussion
  • 5.1. Excitation temperature
  • 5.2. Column densities and abundances
  • 5.3. Chemistry
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000
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