Astronomical masers and Dicke’s superradiance

ABSTRACT We consider the radiation properties and processes of a gas with a population inversion using the formalism based on the Maxwell–Bloch equations. We focus on the maser action and Dicke’s superradiance to establish their relationship in the overall radiation process during the temporal evolution of the system as a function of position. We show that the maser action and superradiance are not competing phenomena but are rather complementary and define two distinct limits for the intensity of radiation. Masers characterize the quasi-steady state limit, when the population inversion density and the polarization amplitude vary on time-scales longer than those of non-coherent processes affecting their evolution (e.g. collisions), while superradiance defines the fast transient regime taking place when these conditions are reversed. We show how a transition from a maser regime to superradiance will take place whenever a critical threshold for the column density of the population inversion is reached, at which point a strong level of coherence is established in the system and a powerful burst of radiation can ensue during the transient regime. This critical level also determines the spatial region where a transition from the unsaturated to the saturated maser regimes will take place; superradiance can thus be seen as the intermediary between the two. We also quantify the gain in radiation intensity attained during the superradiance phase relative to the two maser regimes and show how the strong coherence level during superradiance is well suited to explain observations that reveal intense and fast radiation flares in maser-hosting regions.

Widespread HCN maser emission in carbon-rich evolved stars

Context. HCN is a major constituent of the circumstellar envelopes of carbon-rich evolved stars, and rotational lines from within its vibrationally excited states probe parts of these regions closest to the stellar surface. A number of such lines are known to show maser action. Historically, in one of them, the 177 GHz J = 2 → 1 line in the l-doubled bending mode has been found to show relatively strong maser action, with results only published for a single object, the archetypical high-mass loss asymptotic giant branch (AGB) star IRC+10216. Aims. To examine how common 177 GHz HCN maser emission is, we conducted an exploratory survey for this line toward a select sample of carbon-rich asymptotic giant branch stars that are observable from the southern hemisphere. Methods. We used the Atacama Pathfinder Experiment 12 meter submillimeter Telescope (APEX) equipped with a new receiver to simultaneously observe three J = 2 → 1 HCN rotational transitions, the (0, 11c, 0) and (0, 11d, 0) l-doublet components, and the line from the (0,0,0) ground state. Results. The (0, 11c, 0) maser line is detected toward 11 of 13 observed sources, which all show emission in the (0,0,0) transition. In most of the sources, the peak intensity of the (0, 11c, 0) line rivals that of the (0,0,0) line; in two sources, it is even stronger. Except for the object with the highest mass-loss rate, IRC+10216, the (0, 11c, 0) line covers a smaller velocity range than the (0,0,0) line. The (0, 11d, 0) line, which is detected in four of the sources, is much weaker than the other two lines and covers a velocity range that is smaller yet, again except for IRC+10216. Compared to its first detection in 1989, the profile of the (0, 11c, 0) line observed toward IRC+10216 looks very different, and we also appear to see variability in the (0,0,0) line profile (at a much lower degree). Our limited information on temporal variabilitydisfavors a strong correlation of maser and stellar continuum flux. Conclusions. Maser emission in the 177 GHz J = 2 → 1 (0, 11c, 0) line of HCN appears to be common in carbon-rich AGB stars. Like for other vibrationally excited HCN lines, our observations indicate that the origin of these lines is in the acceleration zone of the stellar outflow in which dust is forming. For all the stars toward which we detect the maser line, the number of photons available at 7 and 14 μm, corresponding to transitions to vibrationally excited states possibly involved in its pumping, is found to be far greater than that of the maser photons, which makes radiative pumping feasible. Other findings point to a collisional pumping scheme, however.