R. J. Pfeiffer

ON THE LINEAR POLARIZATION OF CLOSE BINARIES.

There now exist thousands of filtered and unfiltered linear polarization measures distributed among dozens of close binaries. This assemblage of data is studied for the incidence of polarization variability among the binary systems. With the exception of U Oph, unevolved binaries do not show intrinsic polarization. It is relatively easy to detect polarization variability among evolved pairs whose photospheres are separated by at least 10 solar radii. For evolved pairs closer together than this threshold value, an assortment of factors is described which makes it difficult to detect the systemic polarization unless mass transfer is proceeding vigorously. The cause of the binary polarization is commonly scattering from circumstellar material, but in some systems the same mechanisms which cause polarization variability in single cool giants may be operating.

Binary Activity at the Top of the Main Sequence: Y Cyg and U Oph

The binary stars at the top of the main sequence are, in principle, well-known with regard to interior and envelope structure. By mobilizing all available observational data for two of the best-known binaries — Y Cyg and U Oph — it is possible to place limits on the intrinsic activity of these pairs and, by implication, on others similar to them. Each of these binaries shows intrinsic polarization at the level of about 0.3%, traceable to a systemic envelope which may undergo variations as it is fed by stellar winds. The weak winds from each component star intersect between the stars setting up a standing shock of scatterers. Small-scale intrinsic variability in broad- and medium- bandpass light curves has also been seen. In the case of U Oph, residuals from the best theoretical light curve correlate loosely in phase with the distribution of the scatterers modelled from the polarimetry. Three medium-band light curves of U Oph, biased by HI absorption, show effects of variable circumstellar absorption and possibly scattering. Hot, massive main sequence binaries are indeed active ones.