Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...476..685w&link_type=abstract
Astrophysical Journal v.476, p.685
Astronomy and Astrophysics
Astronomy
15
Ism: Dust, Extinction, Galaxies: Active, Galaxies: Nuclei, Masers
Scientific paper
Recent calculations have demonstrated that the absorption of infrared, spectral line radiation by dust grains can increase significantly the pumping of the 22 GHz water masers under conditions that may be appropriate for the circumnuclear environment of active galaxies. As a result, improved agreement with the observed maser luminosities is achieved. A key ingredient is a significant difference between the temperatures of the gas and the grains, a difference that is expected in the presence of irradiation by X-rays. In our own galaxy, strong maser emission is observed also for a number of other transitions of the water molecule at higher frequencies. Calculations are presented here to delineate in some detail the maser luminosities that can be expected for both these and the 22 GHz transition when the influence of dust grains is included. Dust grains play a major role when the column density of hydrogen divided by the dispersion velocity for the masing gas exceeds about 1022 cm-2 (km s-1)-1. Beyond this depth, the pumping (and hence the emissivity for saturated masing) is independent of the optical depth to the surface of the cloud. Without dust grains, the pumping is quenched at high optical depths due to trapping of the infrared, spectral line radiation of the water molecule. Calculated luminosities for the water masers at higher frequencies can be comparable with the calculated luminosity of the 22 GHz masers. The calculations here are restricted to the ortho-water configuration.
Wallin Bradley K.
Watson William D.
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