Linear polarization in the RV Tauri star AR Puppis

Abstract

The UBVR polarimetry of AR Pup obtained on 14 nights during 1990-93 shows a large variation in both the amount (~ 6 per cent in the U band) and the wavelength dependence of polarization. The simultaneous photometry (20 nights) and polarimetry (30 nights) in the V band during 1997 December-1998 April, which cover about one and a half pulsational cycles, clearly establish the polarization-light curve connection in AR Pup: both go through the maximum and minimum at about the same time. We interpret the observed near-straight line locus of polarization between any two consecutive minima in the Q, U plane as resulting from the simultaneous presence of two independently varying polarization components, one slowly and the other relatively rapidly. We suggest that the mechanism that produces the exceptionally large polarization in AR Pup is, most likely, selective absorption by non-spherical grains, which condense with a certain amount of alignment after each light minimum during a pulsational cycle. BV photometry was also obtained on 44 nights about the times of UBVR polarimetry for information on the contemporaneous light variation. The position angle of polarization in AR Pup does not show any correlation with its mean light level which is found to vary nearly sinusoidally with a 1165 +/- 4 d period.