Evolution of 1612 MHz Maser Emission in Expanding Circumstellar Shells

Details Evolution of 1612 MHz Maser Emission in Expanding Circumstellar Shells Evolution of 1612 MHz Maser Emission in Expanding Circumstellar Shells

2005-09-08

arxiv.org/abs/astro-ph/0509218v1

Mon.Not.Roy.Astron.Soc.364:783-795,2005

Astronomy and Astrophysics

Astrophysics

Scientific paper

10.1111/j.1365-2966.2005.09591.x

Observations show that 1612 MHz masers of OH/IR stars can fade on a timescale of a decade. This fading is probably associated with the switch from rapid mass loss, which is ultimately linked with an internal He-shell flash, to the much slower mass loss supported by more quiescent conditions. We study the observed maser decay with a composite computational model, comprising a time-dependent chemical model of the envelope, and a radiation transfer model which provides the maser pumping. Our combined model is able to reproduce the rapid decay of maser intensity, following a sudden drop in the stellar mass-loss rate. The explanation for the rapid fall in maser emission is not a fall in the OH number density, or the kinetic temperature in the inverted layers, but the loss of a radiative pump route which carries population from level 1 to level 4 via levels 16, 15 and 11. The loss of these pump routes is a result, in turn, of a greatly reduced energy density of 53 micron radiation.

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