Broad Recombination Line Objects in W49N on 600 AU Scales

Abstract

High resolution 7 mm observations of the W49N massive star forming region have detected recombination line emission from the individual ultracompact (UC) HII regions on 50 milliarcsecond (600 AU) scales. These line observations, combined with multifrequency, high-resolution continuum imaging of the region at 7 mm (VLA) and at 3 mm and 1 mm (BIMA), indicate that five to seven of the eighteen ultracompact sources in W49N are broad recombination line objects (BRLOs) as described by Jaffe & Martin-Pintado (1999). BRLOs have both broad radio recombination lines ($Δ$V$>$60 \kms) and rising spectra (S$_ν\simν^α$), with $α$ values greater than 0.4. The broad line widths of the H52$α$ line are probably related to motions in the ionized gas rather than pressure broadening. A number of models have been proposed to explain the long lifetime of UC HII regions, including the photoevaporated disk model proposed by Hollenbach et al. (1994). This model can also explain the broad lines, rising spectra and bipolar morphologies of some sources. We suggest$-$based on line and continuum observations as well as source morphology$-$that in a subset of the W49N ultracompact sources we may be observing ionized winds that arise from circumstellar disks.