HYDROGEN MASERS .1. THEORY AND PROSPECTS

The discovery of the first high-gain hydrogen recombination line (HRL) maser in the millimeter/submillimeter spectrum of the emission-line s tar MWC 349A requires an expansion of current paradigms about HRLs. In this paper we reexamine the problem of non-LTE populations in recombi ning hydrogen and specify the conditions necessary for high-gain masin g and lasing in HRLs. To do so, we use the extensive new results on hy drogen level populations produced by Storey & Hummer and our calculati ons for the net (that is, line plus continuum) absorption coefficient for HRLs. We present results for the alpha- and beta-lines whose princ ipal quantum numbers n are between five and 100, for gas whose electro n number density 3 less than or equal to log N-e(cm(-3))less than or e qual to 11, at two electron temperatures, T-e = 5000 and 10,000 K. We show that the unsaturated maser gain in an HRL is a sharp function of N-e, and thus to achieve high-gain masing, each line requires a suffic iently extended region over which the density is rather closely specif ied. Saturation of masing recombination lines is a critical considerat ion. We derive a simple equation for estimating the degree of saturati on from the observed flux density and the interferometric and/or model information about the amplification path length, avoiding the vague i ssue of the solid angle of masing. We also present a qualitative way t o approach the effects of saturation on adjacent emission lines, altho ugh the detailed modeling is highly case-specific. We draw attention t o another non-LTE phenomenon active in hydrogen: the overcooling of po pulations. This occurs for HRLs with n greater than or similar to 20, in gas where N-e less than or similar to 10(5) cm(-3). Observationally , the HRL overcooling might manifest itself as an anomalously weak emi ssion recombination line, or as a ''dasar,'' that is, an anomalously s trong absorption line. In the simplest case of a homogeneous H rr regi on, the absorption can be observed on the proper free-free continuum o f the region, if some conditions for the line or/and continuum optical depths are satisfied. We briefly discuss the prospects of detecting h ydrogen masers, lasers, and dasars in several classes of Galactic and extragalactic objects, including compact H II regions, Be or Wolf-Raye t stars, starburst galaxies, and active galactic nuclei.