Lifetime of OH masers at the tip of the asymptotic giant branch

Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: dengels@hs.uni-hamburg.de

Received: 10 July 2007
Accepted: 20 September 2007

Abstract

Context.A large fraction of otherwise similar asymptotic giant branch stars (AGB) do not show OH maser emission. As shown recently, a restricted lifetime may give a natural explanation as to why only part of any sample emits maser emission at a given epoch.

Aims.We wish to probe the lifetime of 1612 MHz OH masers in circumstellar shells of AGB stars.

Methods.We reobserved a sample of OH/IR stars discovered more than 28 years ago to determine the number of stars that may have since lost their masers.

Results.We redetected all 114 OH masers. The minimum lifetime inferred is 2800 years (1σ). This maser lifetime applies to AGB stars with strong mass loss leading to very red infrared colors. The velocities and mean flux density levels have not changed since their discovery. As the minimum lifetime is of the same order as the wind crossing time, strong variations in the mass-loss process affecting the excitation conditions on timescales of ≈3000 years or less are unlikely.