THE THERMAL RADIO JET OF CEPHEUS-A HW2 AND THE WATER MASER DISTRIBUTION AT 0.''08 SCALE (60 AU)

We report observations of the thermal radio jet in Cepheus A HW2, and the associated water masers, carried out with the highest (0 ''.08) an gular resolution available to date at the VLA (A configuration at lamb da = 1.3 cm). To calibrate the 1.3 cm continuum emission, we used the strong (similar to 1000 Jy) H2O maser source as the reference, thus co rrecting the amplitude and phase instabilities introduced by the atmos phere. This powerful technique, first applied here to a star-forming r egion, allowed us to achieve a dynamic range of 15,000 : 1 for the str ongest maser feature, a signal-to-noise ratio (S/N) of 70 : 1 for the radio jet and an accuracy of the order of 1 mas in the relative positi ons between the radio-continuum jet and the H2O masers in the region. We resolved the 1.3 cm jet into two maxima plus a fainter tail to the southwest. The separation between these two maxima (0 ''.14) and the t otal size of the jet (0 ''.39) are both consistent with models for a b iconical ionized jet. The observed flux density (39 mJy) is, however, higher than expected. We detected 39 H2O maser spots toward the Cepheu s A region, 28 of which are associated with the HW2 object, most of th em distributed on either sides of the radio jet. We suggest that these latter maser features might be tracing a circumstellar molecular disk of radius similar to 300 AU, nearly perpendicular to the radio jet. T he velocity gradient of 30 +/- 10 km s(-1) observed in the H2O spots o ver 600 AU along the axis perpendicular to the radio jet could be grav itationally bound by a central mass of 70 +/- 40 M..