2015MNRAS.451.3161S

A method for measuring the difference between centroids of polarized flux and total flux of an astronomical object - polaroastrometry - is proposed. The deviation of the centroid of flux corresponding to Stokes parameter Q or U from the centroid of total flux multiplied by dimensionless Stokes parameter q or u, respectively, was used as a signal. The efficiency of the method is demonstrated on the basis of observations made in the V band by using an instrument combining features of a two-beam polarimeter with a rotating half-wave plate and a speckle interferometer. The polaroastrometric signal noise is 60-70 µas rms for a total number of accumulated photoelectrons Ne of 10⁹ from a 70-cm telescope; this corresponds to a total integration time of 500 s and an object magnitude V = 6 mag. At smaller Ne, the noise increases as ~ 1.7 arcsec/sqrt(N_e), while at larger Ne, it remains the same owing to imperfection of the half-wave plate. For main-sequence stars that are unpolarized and polarized by interstellar dust and the Mira-type variable R Tri the signal was undetectable. For the Mira-type variable χ Cyg the polaroastrometric signal is found to be 310±70 and 300±70 µas for Stokes Q and U, respectively; for o Cet these values are 490±100 and 1160±100 µas. The significant value of the polaroastrometric signal provides evidence of the asymmetry of the polarized flux distribution.