Excited-state OH Mainline Masers in AU Geminorum and NML Cygni

TL;DR

This study investigates excited-state OH maser emissions in evolved stars AU Geminorum and NML Cygni using VLA and EVLA observations, finding such emissions are rare and likely transient or caused by shocks, challenging previous expectations.

Contribution

The paper provides the first detailed observations of excited-state OH masers in these stars, showing such emissions are uncommon and likely transient or shock-induced, refining understanding of maser phenomena in evolved stars.

Findings

Excited-state OH masers detected in AU Gem but not in NML Cyg.

Excited-state OH emission is likely transient or shock-related.

Such emissions are generally not expected in normal evolved star evolution.

Abstract

Excited-state OH maser emission has previously been reported in the circumstellar envelopes of only two evolved stars: the Mira star AU Geminorum and the hypergiant NML Cygni. We present Very Large Array (VLA) observations of the 1665, 1667, and excited-state 4750 MHz mainline OH transitions in AU Gem and Expanded Very Large Array (EVLA) observations of the excited-state 6030 and 6035 MHz OH mainline transitions in NML Cyg. We detect masers in both mainline transitions in AU Gem but no excited-state emission in either star. We conclude that the excited-state OH emission in AU Gem is either a transient phenomenon (such as for NML Cyg outlined below), or possibly an artifact in the data, and that the excited state OH emission in NML Cyg was generated by an episode of enhanced shock between the stellar mass-loss and an outflow of the Cyg OB2 association. With these single exceptions, it therefore appears that excited-state OH emission indeed should not be predicted nor observable in evolved stars as part of their normal structure or evolution.