RESOLVED RADIO EMISSION FROM MODELS OF PHOTOEVAPORATED DISKS AROUND MASSIVE YOUNG STARS

摘要

We study the radio continuum and thermal hydrogen radio recombination line (RRL) emission from photoevaporated disk wind models around massive young stars. We applied the models of Lugo and coworkers to the source MWC 349A. The resolved synthetic radio continuum maps reproduce the observed hourglass morphology at low frequency but are more flattened than the observations at high frequency because the density in the model decreases too fast. These photoevaporated wind models naturally produce RRLs with FWHM Δv ~ 60 km s–1. Nevertheless, recent H66α line observations of MWC 349A by Loinard & Rodríguez have an FWHM Δv ~ 89 km s–1. We propose that such wide lines could be produced by an extra magnetocentrifugal acceleration of the flow due to a poloidal magnetic field anchored in the disk. Such fields could also prevent the flow divergence and the fast density drop of the photoevaporated disk wind model. To mimic this effect we include in this model a large non-thermal velocity dispersion σnt ~ 70 km s–1. The width of the RRLs of this modified model increases with quantum number. This is in contrast with the observed H76α and H92α lines which are narrower than the H66α line. We argue that the low-frequency observations could have suffered from insufficient bandwidth and that new measurements of these lines would be very valuable to constrain the models. Finally, the resolved H66α and H53α line emission maps show the velocity asymmetry expected from flow rotation.