On-sky demonstration of optical polaroastrometry

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 mu as 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 N-e, the noise increases as approximate to 1.7 arcsec/root N-e, while at larger N-e, 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 chi Cyg the polaroastrometric signal is found to be 310 +/- 70 and 300 +/- 70 mu as for Stokes Q and U, respectively; for o Cet these values are 490 +/- 100 and 1160 +/- 100 mu as. The significant value of the polaroastrometric signal provides evidence of the asymmetry of the polarized flux distribution.