A Shock-excited OH Maser in a Post–Asymptotic Giant Branch Envelope?

Abstract

We have observed a sample of OH 1612 MHz masing objects in all four OH ground-state transitions with the Australia Telescope Compact Array. One likely post-asymptotic giant branch (AGB) object is found to emit in the 1612, 1665, and 1720 MHz transitions. We discuss the evidence that this object may be an early post-AGB object and the possibility for such a circumstellar envelope to harbor a 1720 MHz maser. We argue that during a very brief period, just after the star has left the thermally pulsing phase of the AGB and the wind velocity starts to increase, post-AGB objects might show 1720 MHz emission. The best objects to search for such emission would be those that are masing at 1612 and 1665 MHz, but not at 1667 MHz nor in the 22 GHz H2O transition.