VLA OBSERVATIONS OF CYG-OB2 NO 5 - DETECTION OF A WEAK RADIO COMPANION AND NEW OBSERVATIONS AND MODELS FOR THE MAIN COMPONENT

We present Very Large Array observations of the contact binary star Cyg OB2 No. 5 obtained over the last four years. A weak (approximately 0.6 mJy at 6 cm) radio companion has been detected approximately 0.8'' to the NE of the known main radio component. However, most of the radio continuum and the variability appear to be associated with the main component (that coincides in position with the optical contact binary). The centimeter continuum emission of the main component increased in 1989-1992 to levels similar to those observed in 1983-1986. The radio emission seems to alternate between a ''low'' and a ''high'' state with a period of about seven years. We derived its brightness temperature from 3.6 cm observations during ''high'' state, obtaining T(B) = 100 000 +/- 30 000 K. This large value rules out a possible thermal origin for the radio continuum emission. Several models are discussed to account for the characteristics of the radio emission from the main component. The most satisfactory consists of a synchrotron-emitting envelope produced near the outer region of the free-free radio photosphere (approximately 10(15) cm at cm wavelengths).