SiO high velocity maser emission in O-rich evolved stars

Highly sensitive simultaneous observations of the 2→1 line of the first vibrationally excited state of SiO, and of the 2→1 line of CO have been obtained toward 37 O-rich evolved stars. In ∼ 45% of these objects, the SiO line profiles present unusually broad wings, reaching, and in some cases exceeding, the terminal velocity of the circumstellar envelope. All of the objects exhibiting SiO line wings are Mira or semiregular variables and most of them have terminal velocities for their circumstellar envelopes below 10-12 kms−1 andmoderate mass loss rates. In the opposite, all the OH/IR stars have very narrow SiO lines relative to the CO linewidth. In R Leo, the velocity extent of the SiO emission shows a time variability. Variations of the SiO line wings are also observed in other O-rich stars between 1994 and 1995. The time variations observed in these objects suggest changes in the physical structure and velocity field of the innermost regions of the envelopes over timescales of months/years no necessarily related to the periodic optical pulsation of Mira stars. Our data, which lack of angular resolution to resolve the SiO emission, cannot give an unique answer to the origin of the SiO line wings. However, radiative transfer models indicate that in stars with large terminal velocities for their circumstellar envelopes, the effects of turbulence and rotation are negligible in the emergent line wings. On the contrary, rotating Kleperian disks can produce prominent line wings in envelopes with moderate or low terminal velocities. The effect of asymmetric mass loss processes and of gas infall/outflow on the emergent SiO line profile have been also analyzed and can explain, at least qualitatively, the presence of SiO line wings.