HYDROGEN MASERS .2. MWC 349A

The conditions of masing and lasing in the hydrogen recombination line s (HRLs) in the disk and outflow of MWC 349A are studied. Comparison o f the observed integrated fluxes of virtually all the alpha-lines meas ured in this source-from H2 alpha through H92 alpha-with a simple mode l of spontaneous emission shows that observable HRL masing is limited to the interval of the principal quantum numbers n approximate to 10-4 0. We use our analytical and numerical results for the conditions of o ptimum masing and the morphological parameters of the photoevaporated circumstellar disk modeled by Hollenbach and coworkers to obtain analy tical approximations and quantitative estimates for the expected unsat urated maser gain and the degree of saturation in HRL masers in the di sk of MWC 349A. It is shown that the unsaturated gains of the infrared HRL lasers should be very high, but in fact all of these lasers are s trongly saturated. Because of saturation, their relative intensity wit h respect to the spontaneous emission should drop steeply toward small er n's, which explains why optical and near-infrared HRL lasers are no t seen in MWC 349A. This result is quite general and makes the prognos is for observable high-frequency HRL lasers in other sources pessimist ic as well. An exception might arise because of special geometry, if l asing were confined to a very small solid angle. We show that weak mas ing from the outflow of MWC 349A is only possible in low-n alpha-lines (n less than or similar to 20-30), and then only if the outflow begin s much closer to the central star than the Hollenbach model predicts ( approximate to 3 x 10(15) cm). We therefore conclude that maser emissi on from the disk, rather than from the outflow, is responsible for the observed weak amplification of the ''pedestal'' spectral components i n the alpha-lines with n less than or similar to 40. The fluxes in the lower frequency alpha-lines are well explained by spontaneous emissio n from the outflow, with proper corrections for free-free absorption. We review the current state and prospects of observations of masing an d lasing hydrogen beta-lines in MWC 349A. We argue in favor of ''close in n,'' rather than ''close in frequency'' alpha/beta pairs for inten sity analysis, when maser amplification is present.