Astron. Astrophys. 337, 275-286 (1998)

Abundances of ethylene oxide and acetaldehyde in hot molecular cloud cores

A. Nummelin ¹, J.E. Dickens ², P. Bergman ¹, Å. Hjalmarson ¹, W. M. Irvine ², M. Ikeda ³ and M. Ohishi <sup>4

1</sup> Onsala Space Observatory, S-439 92 Onsala, Sweden
² FCRAO, 619 Lederle GRC, University of Massachusetts, Amherst, MA 01003, USA
³ Nobeyama Radio Observatory, Nobeyama, Minamimaki, Minamisaku, Nagano 384-13, Japan
⁴ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan

Received 28 May 1998 / Accepted 12 June 1998

Abstract

We have searched for millimetre-wave line emission from ethylene oxide (c-C₂H₄O) and its structural isomer acetaldehyde (CH₃CHO) in 11 molecular clouds using SEST. Ethylene oxide and acetaldehyde were detected through multiple lines in the hot cores NGC 6334F, G327.3-0.6, G31.41+0.31, and G34.3+0.2. Acetaldehyde was also detected towards G10.47+0.03, G322.2+0.6, and Orion 3´N, and one ethylene oxide line was tentatively detected in G10.47+0.03. Column densities and rotational excitation temperatures were derived using a procedure which fits the observed line intensities by finding the minimum -value. The resulting rotational excitation temperatures of ethylene oxide and acetaldehyde are in the range 16 - 38 K, indicating that these species are excited in the outer, cooler parts of the hot cores or that the excitation is significantly subthermal. For an assumed source size of 20", the deduced column densities are (0.6 - 1) cm^-2 for ethylene oxide and (2 - 5) cm^-2 for acetaldehyde. The fractional abundances with respect to H₂ are X[c-C₂H₄O]=(2 - 6), and X[CH₃CHO]=(0.8 - 3). The ratio X[CH₃CHO]/X[c-C₂H₄O] varies between 2.6 (NGC 6334F) and 8.5 (G327.3-0.6). We also detected and analysed multiple transitions of CH₃OH, CH₃OCH₃, C₂H₅OH, and HCOOH. The chemical, and possibly evolutionary, states of NGC 6334F, G327.3-0.6, G31.41+0.31, and G34.3+0.2 seem to be very similar.

Key words: ISM: molecules ISM: abundances ISM: H ii-regions radio lines: ISM

Send offprint requests to: A. Nummelin

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Analysis
• 4. Results
  • 4.1. General
  • 4.2. NGC 6334F
  • 4.3. G327.3-0.6
  • 4.4. G31.41+0.31
  • 4.5. G34.3+0.2
  • 4.6. Other sources
  • 4.7. Other molecular species detected
    • 4.7.1. CH₃OH
    • 4.7.2. CH₃OCH₃
    • 4.7.3. C₂H₅OH
    • 4.7.4. HCOOH
• 5. Discussion
  • 5.1. Molecular excitation and abundances
  • 5.2. Comparison with chemical models
• 6. Conclusions
• Acknowledgements
• Appendix A: partition functions and statistical weights
• References

© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998
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