Hydrogen Masers. I: Theory and Prospects

The discovery of the first naturally occurring high gain hydrogen recombination line (HRL) maser, in the millimeter and submillimeter spectrum of the emission line star MWC349, requires an expansion of current paradigms about HRLs. In this paper we re-examine in general the physics of non-LTE populations in recombining hydrogen and specify the conditions necessary for high-gain masing and lasing in HRLs. To do so we use the extensive new results on hydrogen level populations produced by Storey and Hummer (1995), and our calculations for the net (that is, line plus continuum) absorption coefficient for the hydrogen, and we present results for the αand β-lines whose principal quantum numbers n are between 5 and 100, for gas whose electron number density 3 ≤ logNe (cm ) ≤ 11, at two electron temperatures, Te = 5, 000 and 10,000 K. We show that the unsaturated maser gain in an HRL is a sharp function of Ne, and thus to achieve high-gain masing each line requires a sufficiently extended region over which the density is rather closely specified. Saturation of masing recombination lines is a critical consideration. We derive a simple equation for estimating the degree of saturation from the observed flux density and the interferometric and/or model information about the amplification path length, avoiding the vague issue of the solid angle of masing. We also present a qualitative way to approach the effects of saturation on adjacent emission lines, although the detailed modeling is highly case-specific. We draw attention to another non-LTE phenomenon active in hydrogen: the overcooling of populations. This occurs for HRLs with n ∼ 20, in gas where Ne <∼ 10 5 cm. Observationally the HRL overcooling might manifest itself as an anomalously weak emission recombination line, or as a “dasar,” that is, an anomalously strong absorption line. In the simplest case of a homogeneous HII region the absorption can be observed on the proper free-free continuum of the region, if some conditions for the line or/and continuum optical depths are satisfied. We briefly discuss the prospects of detecting hydrogen masers, lasers and dasars in several classes of galactic and extragalactic objects, including compact HII regions, Be or Wolf-Rayet stars, and AGNs. Subject headings: masers — hydrogen: recombination lines — stars: WR stars — stars: Be stars — galaxies: AGN —stars:particular:MWC 349.