1995A&A...300..843T

We have detected four β-lines out of six observed in the frequency range from 100 to 400GHz toward the radio star MWC 349A. The detected β-lines have similar parameters as the pedestal features of the short mm α-lines and probably have the same origin in the ionized wind. We find that the flux ratios of β- and α-line pedestals of nearly the same frequency are much smaller than expected from an optically thin plasma in LTE in all cases. We show with the help of simplified NLTE models of the recombination line emission that low β/α-ratios are due to enhancement of the α-lines by stimulated emission. This is a natural consequence of the high free-free continuum opacity in the ionized wind of MWC 349A. The models also predict that the β/α-ratio is a sensitive probe of the electron density. For the 33β line emitting region the density thus derived is compatible with previous estimates, n_e∼3.10⁷cm^–3, based on a model of the wind's continuum emission. The four newly detected β-lines provide, together with the high frequency α-line pedestals, the most reliable value of the center velocity of the ionized outflow, v_LSR=8±3km/s. This value agrees well with the centroid velocity of the disk, v_LSR=8.2±.3km/s, as derived from the velocities of the blue and red maser spikes, and thus represents the best estimate of the stellar velocity. H32β, at 366.6GHz the highest frequency β-line observed to date, displays on top of the pedestal two weak narrow emission features whose velocities and widths are in qualitative agreement with those of the α-line maser spikes. We tentatively interpret these H32β spikes as a weak disk maser whose estimated optical depth, |τ|=<1, confirms earlier estimates of the α-line maser gain (|τ|≃6).