VLBI ASTROMETRIC IDENTIFICATION OF THE RADIO EMITTING REGION IN ALGOL AND DETERMINATION OF THE ORIENTATION OF THE CLOSE BINARY

A minute displacement of the radio source in Algol was measured by Very Long Baseline Interferometry (VLBI) during two consecutive orbital revolutions of the close binary. The magnitude of the displacement unambiguously indicates that the less massive star of the close binary, a K subgiant, is the star responsible for the nonthermal radio emission of the system. This is consistent with the idea that the radio emission in Algol is related to the strong magnetic activity of the subgiant. The orientation and sense of the displacement on the sky that are directly deduced from our astrometric VLBI observations imply (1) that the orbital plane of the close binary is at P.A. = + 52-degrees +/- 5-degrees (P.A. = 0-degrees is north and P.A. = 90-degrees is east) and (2) that the sense of circulation of the close binary is clockwise, as seen on the sky. Thus, the long-period and close binary orbital motions are almost orthogonal and counterrevolving and this is relevant for evolution and dynamical studies. The orientation and sense of circulation of the close binary determined by our VLBI observations are identical to the findings of Rudy (1979) and Kemp et al. (1981, 1983), obtained less directly with optical polarization observations.