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

Abstract

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.