Issue |
A&A
Volume 414, Number 1, January IV 2004
|
|
---|---|---|
Page(s) | 275 - 288 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20031597 | |
Published online | 12 January 2004 |
VLBA observations of SiO masers towards Mira variable stars
1
National Radio Astronomy Observatory (The National Radio Astronomy Observatory (NRAO) is operated by Associated Universities Inc., under cooperative agreement with the National Science Foundation.) , 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
2
Jet Propulsion Lab., Interferometry Systems and Technology Section, California Institute of Technology, 480 Oak Grove Drive, Pasadena, CA 91109, USA
3
Jodrell Bank Observatory, University of Manchester, Macclesfield Cheshire, SK11 9DL, UK
4
DESPA, Observatoire de Paris, section de Meudon, 5 place Jules Janssen, 92190 Meudon, France
5
Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
6
NOAO, 950 N. Cherry Ave., PO Box 26732, Tucson, AZ. 85726, USA
7
Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
8
Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands (: Wouter Vlemmings, 524 Space Sciences Building, Cornell University, Ithaca, NY 14853-6801, USA.)
9
Harvard–Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
Corresponding author: W. D. Cotton, bcotton@nrao.edu
Received:
2
April
2003
Accepted:
10
October
2003
We present new total intensity and linear polarization VLBA
observations of the and
maser
transitions of SiO at 42.8 and 43.1 GHz in a number of Mira variable stars over
a substantial fraction of their pulsation periods.
These observations were part of an observing program that also
includes interferometric measurements at 2.2 and 3.6 micron
[CITE]; comparison of the results from different
wavelengths allows studying the envelope independently of the poorly
known distances to these stars.
Nine stars were observed at from one to four epochs during 2001.
The SiO emission is largely confined to rings which are
smaller than the inner radius of the dust shells reported by
[CITE].
Two stars (U Orionis, R Aquarii) have maser rings with diameters
corresponding to the size of the hot molecular layer as measured at
3.6 micron; in the other cases, the SiO rings are substantially larger.
Variations of ring diameter for most, but not all stars, had an rms
amplitude in agreement with the models of [CITE]
although the expected relationship between the diameter and pulsation
phase was not seen.
The ring diameter in U Orionis shows remarkably small variation.
A correlation between the 2.2/3.6 μm diameter ratio with that of
the SiO/3.6 μm diameter ratio is likely due to differences in the
opacities at 2.2 and 3.6 μm in a molecular layer.
A further correlation with the inner size of the dust shell reported
by [CITE] suggest some differences in the temperature
structure.
Clear evidence is seen in R Aquarii for an equatorial disk similar to
that reported by [CITE]; rotation is possibly also
detected in S Coronae Boralis.
Key words: stars: atmospheres / stars: AGB and post-AGB / stars: variables / radio lines: stars, Masers
© ESO, 2004
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