Dichroic Masers Due to Radiation Anisotropy and the Influence of the Hanle Effect on the Circumstellar SiO Polarization

Asensio Ramos, A.; Landi Degl'Innocenti, E.; Trujillo Bueno, J.
Referencia bibliográfica

The Astrophysical Journal, Volume 625, Issue 2, pp. 985-995.

Fecha de publicación:
6
2005
Número de autores
3
Número de autores del IAC
1
Número de citas
23
Número de citas referidas
17
Descripción
The theory of the generation and transfer of polarized radiation, mainly developed for interpreting solar spectropolarimetric observations, allows us to reconsider, in a more rigorous and elegant way, a physical mechanism that has been suggested some years ago to interpret the high degree of polarization often observed in astronomical masers. This mechanism, for which the name of ``dichroic maser'' is proposed, can operate when a low-density molecular cloud is illuminated by an anisotropic source of radiation (e.g., a nearby star). Here we investigate completely unsaturated masers and show that selective stimulated emission processes are capable of producing highly polarized maser radiation in a nonmagnetic environment. The polarization of the maser radiation is linear and is directed tangentially to a ring equidistant to the central star. We show that the Hanle effect due to the presence of a magnetic field can produce a rotation (from the tangential direction) of the polarization by more that 45° for some selected combinations of the strength, inclination, and azimuth of the magnetic field vector. However, these very same conditions produce a drastic inhibition of the maser effect. The rotations of about 90° observed in SiO masers in the evolved stars TX Cam by Kemball & Diamond and IRC +10011 by Desmurs and coworkers may then be explained by a local modification of the anisotropy of the radiation field, being transformed from mainly radial to mainly tangential.