Long-term polarization observations of Mira variable stars suggest asymmetric structures

Dept. of Physics & Astronomy, East Tennessee State University, PO Box 70300, Johnson City,TN 37614, USAemail: neilsonh@etsu.eduAbstract.Mira and semi-regular variable stars have been studied for centuries but continue tobe enigmatic. Oneunsolved mysteryis the presence of polarization from these stars. In particular,we present 40 years of polarization measurements for the prototype o Ceti and V CVn and findvery different phenomena for each star. The polarization fraction and position angle for Mira isfound to be small and highly variable. On the other hand, the polarization fraction for V CVnis large and variable, from 2 - 7 %, and its position angle is approximately constant, suggestinga long-term asymmetric structure. We suggest a number of potential scenarios to explain theseobservations.Keywords.(stars:) circumstellar matter, stars: variables: other, techniques: polarimetric