HYDROGEN RECOMBINATION BETA-LINES IN MWC-349

We have detected four beta-lines out of six observed in the frequency range from 100 to 400 GHz toward the radio star MWC 349A. The detected beta-lines have similar parameters as the pedestal features of the sh ort mm alpha-lines and probably have the same origin in the ionized wi nd. We find that the flux ratios of beta- and alpha-line pedestals of nearly the same frequency are much smaller than expected from an optic ally thin plasma in LTE in all cases. We show with the help of simplif ied NLTE models of the recombination line emission that low beta/alpha -ratios are due to enhancement of the alpha-lines by stimulated emissi on. This is a natural consequence of the high free-free continuum opac ity in the ionized wind ofMWC 349A. The models also predict that the b eta/alpha-ratio is a sensitive probe of the electron density. For the 33 beta line emitting region the density thus derived is compatible wi th previous estimates, n(e) similar to 3 . 10(7) cm(-3), based on a mo del of the wind's continuum emission. The four newly detected beta-lin es provide, together with the high frequency alpha-line pedestals, the most reliable value of the center velocity of the ionized outflow, up silon(LSR) = 8 +/- 3 km s(-1) This value agrees well with the centroid velocity of the disk, upsilon(LSR) = 8.2 +/- .3 km s(-1), as derived from the velocities of the blue and red maser spikes, and thus represe nts the best estimate of the stellar velocity. H32 beta, at 366.6 GHz the highest frequency beta-line observed to date, displays on top of t he pedestal two weak narrow emission features whose velocities and wid ths are in qualitative agreement with those of the alpha-line maser sp ikes. We tentatively interpret these H32 beta spikes as a weak disk ma ser whose estimated optical depth, \tau\ less than or equal to 1, conf irms earlier estimates of the alpha-line maser gain (\tau\ similar or equal to 6).