OH Maser Observations of Likely Planetary Nebulae Precursors

Abstract

We present OH maser observations at 1612, 1665, 1667, and 1720 MHz for 86 likely post-asymptotic giant branch (post-AGB) stars selected from a survey of 1612 MHz maser sources in the Galactic plane. The observations were taken with the Parkes Telescope and the Australia Telescope Compact Array between 2002 September and 2003 August. Post-AGB stars are the precursors to planetary nebulae, the diverse morphological range of which is unexplained. The maser observations were taken to investigate the onset and incidence of stellar wind asymmetries during the post-AGB phase. We redetected all 86 sources at 1612 MHz, while 27 sources were detected at 1665 MHz and 45 at 1667 MHz. One source was redetected at 1720 MHz. We present a classification scheme for the maser profiles and show that 25% of sources in our sample are likely to have asymmetric or bipolar outflows. From a comparison of the maser and far-infrared properties we find that there is a likely evolutionary trend in the shape of the maser profiles with some sources evolving from double-peaked to irregular to fully bipolar profiles. A subset of higher mass sources stand out as having almost no mainline emission and generally double-peaked profiles. At least 30% of sources in the sample have variable peak flux intensities at one or more of the frequencies observed. We also confirm a previously noted 1667 MHz overshoot phenomenon.