Hydrogen Masers. I. Theory and Prospects

Abstract

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 α-and β-lines whose principal quantum numbers n are between 5 and 100, for gas whose electron number density 3 ≤ log N e (cm −3 ) ≤ 11, at two electron temperatures, T e = 5, 000 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 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 N e < ∼ 10 5 cm −3 . 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.

Low-frequency Carbon Radio Recombination Lines. I. Calculations of Departure Coefficients

“…As a result, the influence of the l sublevel population on the final b n is larger for lower densities than for higher densities at a given T e . The interplay of the rates produce the "bump" which is apparent in the b n distribution (Figure 3) The influence of l-changing collisions on the level populations and the resulting increase in the b n values was already presented by Hummer & Storey (1987) and analyzed in detail by Strelnitski et al (1996) in the context of hydrogen masers. The results of our level population models are in good agreement with those provided by Hummer & Storey (1987) as we show in Section 3.2.…”

Astrophysics in 1997

“…We should have noted in Ap96 ( § 8.1) that the possibility of population inversions in astrophysical molecules and "negative absorption" as a result had been suggested by Cillie (1936) and by Baker & Menzel (1938), but we had momentarily forgotten it, until reminded by Strelnitski et al (1996).…”

Near‐Infrared Interferometric Images of the Hot Inner Disk surrounding the Massive Young Star MWC 349A

“…1989 ;Gordon 1992 ;(Mart• n-Pintado Planesas, & Serabyn 1992). In recent Mart• n-Pintado, years, maser emission was observed at ever shorter wavelengthsÈfrom 1.3 mm to as short as 19 km (see Planesas et al 1992 ;Strelnitski, Ponomarev, & Smith 1996 ;Thum et al 1998). These observations are consistent with the masersÏ originating in a dense plasma created by photoionization of material in a massive Keplerian disk (Hollenbach et al 1994 ;Yorke & Welz 1996).…”

“…Although it is similar to the Herbig Ae/Be stars (the intermediate-or high-mass counterparts to T Tauri stars), it has not been formally included in the recent list of de Winter, & (1994) because its spectral type is The , P e rez uncertain. MWC 349A is unusual among this class of stars because it exhibits masing hydrogen radio recombination line emission, which is interpreted as coming from a region having a large electron density, cm~3, in a n e B 107È108 Keplerian disk at a distance of approximately 20È30 AU from the star et al.…”