A multitransitional study of linear polarization in SiO maser emission

Abstract

Linear polarization has been measured in up to four SiO maser transitions for seven sources, including the Orion maser source associated with IRc2. Polarization variability characteristics are reported for some sources which were observed several times over a 6 month period in the v = l, J = 2-1 transition. Stars with low envelope expansion velocities appear to have more highly polarized masers than those with high expansion velocities. Comparison of polarization of individual maser features in different transitions indicates that emission from the same rotational transition in different vibrational states often arises in the same volume of gas, but that masers of different rotational transitions within the same vibrational state arise in different regions. For a given source, the profile-averaged fractional polarizations show a uniformity between different transitions , while the profile-averaged position angles do not in general appear to be correlated between transitions. Subject headings: masers-polarization I. INTRODUCTION Linear polarization appears to be a common characteristic of SiO maser radiation. Troland et al. (1979) found eight of nine sources to be linearly polarized in the v = 1, J = 2-1 transition of SiO at 86.2 GHz, with typical fractional polarizations of 10%-30%. Higher polarizations, near 100%, are also seen on occasion (Clark, Troland, and Johnson 1982). In this paper we present measurements of the linear polarization properties in the i; = 1, J = 2-1 transition for several SiO maser sources over a 6 month period in 1981. We also present near-simultaneous measurements of total flux density and polarization in four different SiO maser transitions (i; = 1 and v = 2, J = 1-0 and J = 2-1) for the S-type long-period variable star x Cygni, and similar measurements in two and three transitions for six other sources.