Spectropolarimetric study of Mira-type variable stars

Abstract: Everything started back in 1978, with the discovery by McLean & Coyne of
hydrogen emission lines, undergoing polarization, on the variable star
Omicron Ceti (a.k.a. Mira). Those emissions seemed likely to be linked with a
shockwave propagating through the stellar atmosphere. Further observations
showed that emission -, and thus polarization -, lasted during 80% of
luminosity variation period. According to shockwave theory, a magnetic field
should appear just behind the shock front, inducing polarization, but this
field has never been conclusively detected so far.

Here, we present a full spectropolarimetric study (on the four Stokes
parameters I,Q,U and V), made with instrument NARVAL at TBL (Telescope
Bernard Lyot) in the French Pyrenees, aimed toward the complete
characterization of the polarization behavior with respect to the shockwave
intensity. The detected signatures are most probably of magnetic origin.