Talk abstract details

Maser emission occurs in different regions of the circumstellar envelopes (CSEs) of evolved stars and can be studied at high angular resolution using radio interferometers. These masers are useful probes of the dynamics and kinematics of the outflow from AGB stars. Moreover, masers can be important tracers of the magnetic field strength and morphology at various distances from the central stars. It is expected that the magnetic field plays an important role in transforming spherically symmetric asymptotic giant branch (AGB) stars into a-spherical planetary nebulae (PNe). Theoretical modeling indicated that magnetically collimated jets may be responsible for the formation of the a-spherical PNe. Water fountain sources are a class of post-AGB objects in which H$_2$O masers indicate high velocity collimated jets. Our radio interferometric observations indicate that asymmetries are also present in the OH maser region of the envelope. We performed kinematical reconstruction in order to understand the distribution of OH masers in the CSEs of these stars. Our results show that the OH masers could have either equatorial or bi-conical distribution. Additionally, the observations reveal significant field strength for the OH maser region of these objects, which show the possible role of the magnetic field in collimating the CSEs.
At distances close to the central stars, between the photosphere and the dust formation zone, SiO masers occur. SiO maser polarimetry has been performed for Mira variables and supergiants and seems to indicate dynamically significant and ordered magnetic fields. We extended these studies and performed VLBA SiO maser polarization observations of objects with more extreme mass-loss, in order to understand the origin of the transition between the AGB and PNe. These observations will enable us to understand the SiO emission mechanisms and possibly distinguish between competing models on the origin of the SiO maser polarization.