H₂O Maser Observations of Candidate Post‐AGB Stars and Discovery of Three High‐Velocity Water Sources

Abstract

We present the results of 22 GHz H 2 O maser observations of a sample of 85 postYasymptotic giant branch ( post-AGB) candidate stars, selected on the basis of their OH 1612 MHz maser and far-infrared properties. All sources were observed with the Tidbinbilla 70 m radio telescope, and 21 detections were made; 86 GHz SiO Mopra observations of a subset of the sample are also presented. Of the 21 H 2 O detections, 15 are from sources that are likely to be massive AGB stars and most of these show typical, regular H 2 O maser profiles. In contrast, nearly all the detections of more evolved stars exhibited high-velocity H 2 O maser emission. Of the five sources seen, v223 (W43A, IRAS 18450À0148) is a well-known ''water-fountain'' source that belongs to a small group of post-AGB stars with highly collimated, high-velocity H 2 O maser emission. A second source in our sample, v270 (IRAS 18596+0315), is also known to have high-velocity emission. We report the discovery of similar emission from a further three sources, d46 (IRAS 15445À5449), d62 (IRAS 15544À5332), and b292 (IRAS 18043À2116). The source d46 is an evolved post-AGB star with highly unusual maser properties. The H 2 O maser emission from d62 is probably associated with a massive star. The source b292 is a young post-AGB star that is highly likely to be a water-fountain source, with masers detected over a velocity range of 210 km s À1 .

Deep into the Water Fountains

“…Besides the detection of 321 GHz H 2 O maser emission (Tafoya et al 2014), Pérez-Sánchez et al (2011), Walsh et al (2009 also reported the detection of the 22 GHz H 2 O maser transition. In most cases the bandwidth used for the observations led to the detection of maser spectral features over velocity ranges > 300 km s −1 , a fact that confirmed the results reported by Deacon et al (2007).…”

“…Therefore, from the velocity of the 1665 MHz OH maser spectral features, the authors suggested a radial velocity for the stellar source of 87.5 km s −1 . Deacon et al (2007) reported the detection of high-velocity 22 GHz H 2 O maser emission, with spectral features having velocities outside the velocity range defined by the double-peaked 1665 MHz OH maser spectrum (∆v= v red −v blue ≈ 33 km s −1 , Deacon et al 2004). This led the authors to classify the source as a Water Fountain nebulae.…”

Submillimeter H2O masers in water-fountain nebulae

“…It did not exhibit significant variability between the two epochs of our observations. In this source, the 22 GHz maser emission reported by Deacon et al (2007) and Pérez-Sánchez et al (2011) appears over the velocity range from ∼−150 km s −1 to ∼−75 km s −1 . In addition, the OH maser emission consists of a broad spectral feature that covers the velocity range from ∼−200 km s −1 to ∼−110 km s −1 (Deacon et al 2007).…”

“…In this source, the 22 GHz maser emission reported by Deacon et al (2007) and Pérez-Sánchez et al (2011) appears over the velocity range from ∼−150 km s −1 to ∼−75 km s −1 . In addition, the OH maser emission consists of a broad spectral feature that covers the velocity range from ∼−200 km s −1 to ∼−110 km s −1 (Deacon et al 2007). Thus, the 321 GHz H 2 O maser emission has a velocity range in better agreement with the range of the OH maser emission do the 22 GHz water masers.…”

“…Deacon et al (2007) adopted a systemic velocity equal to the midpoint of the OH profiles, near a velocity of v LSR = −150 km s −1 (see Fig. 1a).…”