THE THERMAL RADIO JET OF CEPHEUS A HW2 AND THE WATER MASER DISTRIBUTION AT 0.08″ SCALE (60 AU)

摘要

We report observations of the thermal radio jet in Cepheus A HW2, and the associated water masers, carried out with the highest (0.08″) angular resolution available to date at the VLA (A configuration at λ = 1.3 cm). To calibrate the 1.3 cm continuum emission, we used the strong (～1000 Jy) H_2O maser source as the reference, thus correcting the amplitude and phase instabilities introduced by the atmosphere. This powerful technique, first applied here to a star-forming region, allowed us to achieve a dynamic range of 15,000 : 1 for the strongest 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 positions between the radio-continuum jet and the H_2O 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 total size of the jet (0.39″) are both consistent with models for a biconical ionized jet. The observed flux density (39 mJy) is, however, higher than expected. We detected 39 H_2O maser spots toward the Cepheus A region, 28 of which are associated with the HW2 object, most of them distributed on either sides of the radio jet. We suggest that these latter maser features might be tracing a circumstellar molecular disk of radius ～300 AU, nearly perpendicular to the radio jet. The velocity gradient of 30 ± 10 km s~(-1) observed in the H_2O spots over 600 AU along the axis perpendicular to the radio jet could be gravitationally bound by a central mass of 70 ± 40 solar mass.