Photographic Measures of the Polarization of Starlight in h and Persei

Abstract

A new photographic method is described for measuring the polarization of starlight using a polaroid filter which allows exposures to be taken at three position angles and simultaneously records brightness differences not due to polarization effects through three control areas. Overlapping of star images limits the use of the method to areas of sky where the star density is approximately |$\leqslant\,6\,\times\,{10}^{3}$| stars per square degree, on average to V ~16 at the galactic equator. About 50 000 individual measures of 527 stars to V ~13 in hand χ Persei were made on 16 fine grain Schmidt plates with a semi-automatic digital iris photometer. The computer programmes used to reduce the data are described. Using the Stokes parameters, comparison with published photoelectric measures of 54 stars gives an r.m.s. deviation of |$\pm\,{0}^\text{m}. 029$| per star. For 53 stars, the mean photoelectric value is |$p\,=\,{0}^\text{m}.083$| at 115 ° and the corresponding photographic mean is |$p\,=\,{0}^\text{m}.080$| at 114 ° , suggesting that any instrumental polarization is less than the accuracy achieved, which is confirmed by some photographic measures at the north galactic pole. Stars with large proper motions have a much smaller amount of polarization at 112 ° , and stars fainter than V = 12 have a rather larger amount of polarization at 114 ° Three new variable stars have been found from the magnitude measures.