DETECTION OF H21-ALPHA MASER EMISSION AT 662 GHZ IN MWC-349

We report the detection of maser emission towards MWC349 in the 662 GH z H21alpha line, by far the highest frequency radio recombination line (RRL) detected to date. The peak flux densities of the two maser spik es are a factor approximately 1.7 stronger than those of the H26alpha transition at 354 GHz observed simultaneously. This increase and the a ccompanying increase of the widths of both spikes indicates that the d egree of maser saturation is still increasing at quantum numbers n app roximately 21. The broad pedestal emission has increased even more, by factors of 2-4 relative to the H26alpha pedestal, probably due to str ong stimulated emission. The relative line enhancement is greatest at the extreme velocities from which we infer a velocity of 42 km s-1 for the ionized wind. The velocity separation of the H21alpha maser spike s is identical to that observed at H26alpha, confirming the apparent f lattening of the disk rotation curve noted before. This may be explain ed by the disk being braked inside the H30alpha emission radius or by a situation where all RRL masers below n approximately 30 are emitted by essentially the same volume of gas.