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

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.

arXiv commentApJ Letter, accepted, 4 pages, 2 figures

References in corpus (6)

• The Circumstellar Environments of NML Cyg and the Cool Hypergiants
• ISO observations of far-infrared rotational emission lines of water vapor toward the supergiant star VY Canis Majoris
• Evolution of 1612 MHz Maser Emission in Expanding Circumstellar Shells
• Search for radiative pumping lines of OH masers: I. The 34.6um absorption line towards 1612 MHz OH maser sources
• A high-sensitivity OH 5-cm line survey in late-type stars
• Observations of the 6 Centimeter Lines of OH in Evolved (OH/IR) Stars

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• Effelsberg Observations of Excited-State (6.0 GHz) OH in Supernova Remnants and W3(OH)
• EVLA Observations of OH Masers in ON 1
• Detection of 4765 MHz OH Emission in a Pre-Planetary Nebula -- CRL 618