THE NUCLEAR JET IN M81

In this Letter we apply the jet-disk symbiosis model developed for Sag ittarius A to M81*-the nucleus of the nearby galaxy M81. The model ac curately predicts the radio flux and size of M81 for the observed bol ometric luminosity of the nuclear source, with no major free parameter except for the inclination angle. We point out that the usually appli ed, free conical jet emission model implies a longitudinal pressure gr adient that must lead to a moderate acceleration of the jet along its flow direction. This usually neglected, gradual acceleration naturally accounts for the inverted spectrum and the size-frequency relation of M81, and may be a general feature of radio cores. So far, M81* is th e best case for a radio-loud jet nature of the compact radio core in t he nucleus of a spiral galaxy. The fact that one can account for Sgr A and M81* with the same model by simply changing the accretion rate s trongly supports the jet-disk symbiosis model as an explanation for th e compact radio cores of galaxies in general.