Identification of oxygen-rich late/post-asymptotic giant branch stars and water fountains via maser and infrared criteria

Hong Kiu Bosco Yung, Jun-ichi Nakashima Christian Henkel & Chih-Hao Hsia

Nicolaus Copernicus Astronomical Center, Poland

The transitional phase between the asymptotic giant branch (AGB) and post-AGB phases holds the key to the understanding of the late-stage metamorphosis of intermediate-mass stars. Nonetheless, this critical phase has not been well-studied due to the shortage of good samples. In this project, we explored an efficient method to identify evolved stars with oxygen-rich envelopes in this transitional phase, which include a rare class of objects - the "water fountains (WF)." Our method considers the OH and H2O maser spectra, the near-infrared Q-parameters (these are colour indices accounting for the effect of extinction), the far-infrared AKARI colours, and the spectral energy distributions (SEDs). Here we first present the results of a new survey on OH and H2O masers. There were 108 colour-selected objects: 53 of them were observed in the three OH maser lines (1612, 1665, and 1667 MHz), with 24 detections (16 new for 1612 MHz); and 106 of them were observed in the H2O maser line (22 GHz), with 24 detections (12 new). We identify a new potential WF source, IRAS 19356+0754, with large velocity coverages of both OH and H2O maser emission. In addition, several objects with high-velocity OH maser emission are reported for the first time. High-velocity maser emission is a sign of jet, a common phenomenon happens during the late-stage metamorphosis. The Q-parameters as well as the infrared [09]-[18] and [18]-[65] AKARI colours of the surveyed objects are then calculated. The SEDs of several key objects are also modelled with a radiative-transfer code (DUSTY). We suggest that the infrared properties, which have been the most popular tools in judging stellar evolutionary status, may not be sufficient to identify objects in the transitional phase. Nonetheless, by considering altogether the maser and infrared properties, the efficiency of identifying such important samples could be improved.

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