Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.1706w&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #17.06; Bulletin of the American Astronomical Society, Vol. 24, p.1148
Astronomy and Astrophysics
Astronomy
Scientific paper
An understanding of the circularly polarized emission from SiO masers offers a means of estimating the magnetic field in regions where the mass loss from late-type stars is established. Observations have indicated fractional circular polarizations as high as 8.7% from masers in these regions (Barvainis, McIntosh, & Predmore 1987). Given the small magnetic moment of SiO, a fractional circular polarization of this magnitude which relies upon the Zeeman splitting of magnetic substates implies a magnetic field ~ 10-100 {G}. The water maser observations of Fiebig & Gusten, using methods similar to those of Barvainis et al. to estimate the magnetic field, suggest an upper limit of ~ 0.3 {G} for the field in the circumstellar {H}_2{O} maser around VY CMa. The Zeeman splitting estimate of the magnetic field from SiO circular polarization around VY CMa suggests a field of ~ 65 {G}. While the SiO masers may be somewhat closer to the star than the {H}_2{O} masers, this large difference suggests that the circularly polarized emission from the SiO masers may result from a mechanism different from that which causes circularly polarized emission in the {H}_2{O} masers. More detailed radiative transfer calculations show that, if a maser is saturated, an alternative mechanism exists which can produce fractional circular polarization of the magnitude observed with fields as small as ~ 10 {mG}. The large difference in observed circular polarization between the two species of masers may therefore be due to a difference in saturation. As pointed out by Barvainis et al., the asymmetric nature of the circular polarization of the circular polarization probably indicates a velocity gradient along the masers. This asymmetry can be reproduced in these calculations with a gradient of ~ 1.5 thermal breadths or larger. Given the proximity of the SiO masers to the star, such velocity gradients seem reasonable.
Nedoluha Gerald E.
Watson William D.
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