Fading of light maximum and linear polarization variation in the carbon Mira R Leporis

Abstract

Polarimetry of R Lep obtained over the years 1991-2002 is presented. During this period the star underwent an episode of fading of the brightness at light maximum, after an interval of about 35 yr. An analysis of the data shows that the percentage linear polarization increased as the fading progressed, attained a maximum of slightly over 3 per cent in the V band close to the epoch of minimum, and remained more or less at the same level during and well after the recovery to normal brightness. The polarization, apparently, originated from the circumstellar envelope above the region where the dust that caused the fading in the star condensed. The physical mechanism that causes the rather large polarization during fadings is perhaps selective extinction by aligned foreground grains produced by the passage of shocks through the circumstellar envelope. The small-amplitude, short-term fluctuations in polarization observed in R Lep, which appear to be superposed on the fading-related large-amplitude variation, are probably pulsation-related and arise from the inner zones of the circumstellar envelope.