Imaging MWC 349 from 7 Millimeters to 90 Centimeters

Abstract

MWC 349A is the brightest radio continuum star in the centimeter domain. The thermal radio continuum emission is believed to originate in an ionized bipolar flow that photoevaporates from the surfaces of a neutral Keplerian disk. In this work we present high angular resolution observations taken with the Very Large Array (VLA) at wavelengths from 7 mm to 90 cm that allow the study of structures over 2 orders of magnitude in size in this object. The 7 mm image shows an intermediate equatorial region ∼0.″04 wide, with no free-free emission, that could be the neutral photoevaporating disk around MWC 349A. Combining these data with archival observations at 1.3, 2, 3.6, 6, and 20 cm, we estimate that the flux increases with frequency as ν 0.67±0.03 and the angular size decreases with frequency as ν-0.74±0.03 confirming the presence of a biconical thermal wind that expands at constant velocity. We also report the marginal detection of MWC 349A at 90 cm. At the wavelengths of 3.6, 6, and 20 cm, we image and model the interaction zone between the winds of MWC 349A and MWC 349B, supporting the physical association of these components. Finally, by comparing 6 cm data taken in 1982 and 1996 we find evidence of variability in MWC 349A that is interpreted as a decrease of ∼2% in the mass-loss rate over the time interval of the observations.