Astron. Astrophys. 362, 723-729 (2000)

On the occurrence of the 6.7 GHz CH₃OH maser emission in UCH II regions

C. Codella ^1,2 and L. Moscadelli <sup>3

1</sup> Istituto di Fisica dello Spazio Interplanetario, CNR, Area di Ricerca Tor Vergata, Via Fosso del Cavaliere 100, 00133 Roma, Italy
² Observatorio Astronómico Nacional (IGN), Apartado 1143, 28800 Alcalá de Henares (Madrid), Spain
³ Stazione Astronomica di Cagliari, Loc. Poggio dei Pini, 09012 Capoterra (Cagliari), Italy

Received 21 April 2000 / Accepted 30 August 2000

Abstract

We present the results of a statistical survey of the 6.7 GHz CH₃OH and 22.2 GHz H₂O emissions towards a large homogeneous sample of ultracompact (UC) H II regions, performed to investigate the nature of the methanol emission: does it point out ionized regions or does it trace earlier stages of the star forming process?

The present detection rates are almost identical: 23% for CH₃OH and 22% for H₂O. Even considered the uncertainties of the survey, the large number of non-detections is consistent with the scenario where methanol masers disappear during the UC phase, like water masers do. Moreover, the probability to have the CH₃OH counterpart of a H₂O source is at least 40%, indicating that these maser emissions are emphasizing two at least partly overlapping evolutionary phases. The comparison between the velocity ranges suggests that methanol and water masers form in different gas components related to the star forming process.

Key words: ISM: clouds ISM: H ii regions ISM: molecules radio lines: ISM

Send offprint requests to: C. Codella (codella@ifsi.rm.cnr.it)

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Contents

• 1. Introduction
• 2. The sample
• 3. Observations
• 4. Results and discussion
  • 4.1. Detection rates
  • 4.2. CH₃OH and H₂O maser properties
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000
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