DETECTION OF RADIO-CONTINUUM EMISSION FROM PROCYON

We have detected the F5 IV-V star Procyon as a weak and variable 3.6 cm radio continuum source using the Very Large Array.2 The inferred radio luminosity is 11.7 less-than-or-equal-to log L(nu) less-than-or-equal-to 12.2 which is similar to, though somewhat higher than, the X-band luminosity of the active and flaring Sun. The 33 muJy flux density level at which we detected Procyon on four of five occasions is close to the 36 muJy radio flux density expected from a model in which the radio emission consists of two components: optically thick '' stellar disk '' emission with a 3.6 cm brightness temperature of 2 x 10(4) K that is 50% larger than the solar value, and optically thin coronal emission with an emission measure the same as that indicated by Einstein and EXOSAT X-ray flux measurements in 1981 and 1983. The maximum mass-loss rate of a warm (T(e) approximately 10(4)-10(5) K) stellar wind is constrained by our radio observations to be less than 2 x 10(-11) M. yr-1. An elevated flux density of 115 muJy observed on a single occasion may have been associated with a radio outburst or flare, or, alternatively, been caused by the rotation onto the visible hemisphere of a large, intense active region. In either case, this observation provides circumstantial evidence for the existence of highly localized magnetic fields on the surface of Procyon.