Astron. Astrophys. 355, 979-993 (2000)

The spatial and velocity structure of circumstellar water masers

F. Colomer ¹, M.J. Reid ², K.M. Menten ³ and V. Bujarrabal <sup>1

1</sup> Observatorio Astronómico Nacional (IGN), Apartado 1143, 28800 Alcalá de Henares, Spain
² Harvard-Smithsonian Center for Astrophysics, MS-42, 60 Garden Street, Cambridge, MA 02138, USA
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received 23 April 1999 / Accepted 13 December 1999

Abstract

We report observations with the VLA of 22 GHz H₂O maser emission in the circumstellar envelopes of 11 late-type stars. We model the maser emission towards six of them (IRC +60169, R Crt, U Her, RX Boo, R LMi, and R Cas) with a computer program we have developed, that fits three-dimensional Gaussian sources (in position and velocity) to the measured intensity distribution. This approach to the problem is superior to the standard procedure of fitting two-dimensional Gaussian models to images for each frequency channel, and it is shown to be a powerful method of identifying blended and/or weak maser features.

As a result, we find that the H₂O maser emission is located in a thin shell expanding from the star in four cases, despite of the fact that the emission is clumpy and the shells appear incomplete. Moreover, our maps indicate that the H₂O maser emission comes from inner parts of the circumstellar envelope, which are comparable in extent to the regions in which dust grains form and in which the expanding envelope has not reached its terminal velocity.

Our analysis also provides the spectral linewidth of each maser feature, which contains information about the physical conditions in the emitting region. In particular, it provides a tool to discriminate whether or not these masers are saturated as, for unsaturated masers, theory predicts that a correlation between the strongest features and the narrowest linewidths should exist. This behaviour is found in at least U Her, RX Boo, and R Cas.

Key words: masers techniques: interferometric stars: circumstellar matter stars: late-type

Send offprint requests to: Francisco Colomer (colomer@oan.es; http://www.oan.es/ )

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Contents

• 1. Introduction
• 2. Observations
• 3. Data analysis
• 4. Results for the individual sources
  • 4.1. R Crt
  • 4.2. U Her
  • 4.3. IRC +60169
  • 4.4. RX Boo
  • 4.5. R Cas
  • 4.6. R LMi
  • 4.7. Other sources
• 5. Discussion
  • 5.1. Morphology of the maser regions
  • 5.2. Maser linewidths and saturation
  • 5.3. The location of the H₂O masers in circumstellar envelopes around red giant stars
  • 5.4. Other applications of the 3D spectral line emission fitting code
• 6. Conclusions
• Appendices
  • A: the 3D Gaussian fitting program "FG3"
    • A.1. The Levenberg-Marquardt method
    • A.2. Implementation
    • A.3. Using constraints on the parameters
    • A.4. Notes on the reliability of the solutions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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