Further VLBA observations of SiO masers toward Mira variable stars

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA e-mail: bcotton@nrao.edu
² Jodrell Bank Observatory, University of Manchester, Macclesfield Cheshire, SK11 9DL, UK
³ Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
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 Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ NOAO, 950 N. Cherry Ave., PO Box 26732, Tucson, AZ. 85726, USA
⁸ Georgia State University
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ W. M. Keck Observatory, 65-1120 Mamalahoa Hwy, Kamuela, HI 96743, USA

Received: 3 March 2006
Accepted: 7 June 2006

Abstract

We present the results of continued monitoring of the SiO masers at 7 mm wavelength in several of the Mira variable stars reported in Cotton et al. (2004, A&A, 414, 275) (o Ceti = Mira, U Orionis = U Ori, R Aquarii = R Aqr) over a period of 16 months, extending the observations to several pulsation cycles. The observed size of the maser rings varied by 3–14% with time but show no clear correlation with pulsation phase. In all cases, the SiO masers appear just outside the dense molecular layer indicated by near-IR observations. Rotation (or large scale motion) is possibly detected in o Ceti with a period of  years. We find linear polarization up to and at several epochs predominantly tangentially ordered polarization vectors indicate a radial magnetic field direction. Jet-like features are examined in o Ceti and R Aqr and in both cases, the magnetic field appears elongated with the masing structure. This suggests that the dynamic feature in the envelope is dragging the magnetic field or that the gas is constrained to follow magnetic field.