Interferometric confirmation of ‘water fountain’ candidates

Abstract

Water fountain stars (WFs) are evolved objects with water masers tracing highvelocity jets (up to several hundreds of km s −1 ). They could represent one of the first manifestations of collimated mass-loss in evolved objects and thus, be a key to understanding the shaping mechanisms of planetary nebulae. Only 13 objects had been confirmed so far as WFs with interferometer observations. We present new observations with the Australia Telescope Compact Array and archival observations with the Very Large Array of four objects that are considered to be WF candidates, mainly based on single-dish observations. We confirm IRAS 17291-2147 and IRAS 18596+0315 (OH 37.1-0.8) as bona fide members of the WF class, with high-velocity water maser emission consistent with tracing bipolar jets. We argue that IRAS 15544-5332 has been wrongly considered as a WF in previous works, since we see no evidence in our data nor in the literature that this object harbours high-velocity water maser emission. In the case of IRAS 19067+0811, we did not detect any water maser emission, so its confirmation as a WF is still pending. With the result of this work, there are 15 objects that can be considered confirmed WFs. We speculate that there is no significant physical difference between WFs and obscured post-AGB stars in general. The absence of high-velocity water maser emission in some obscured post-AGB stars could be attributed to a variability or orientation effect.

Searching for nascent planetary nebulae: OHPNe candidates in the SPLASH survey

“…However, special physical conditions in the CSE can temporarily shield these molecules from dissociation (Miranda et al 2001) and maser-pumping can also be produced in collimated mass ejections with lower mass-loss rates than in the strong AGB winds (Suárez et al 2009). In particular, OH masers can trace bipolar outflows in the AGB and post-AGB phases (Chapman 1988;Zijlstra et al 2001), and so far, there are fifteen known cases where H 2 O masers trace high-velocity collimated jets ( 50 − 300 km s −1 ; Imai 2007; Gómez et al 2017). These objects are called water fountains (WFs).…”

FLASHING: New high-velocity H2O masers in IRAS 18286−0959

“…The WF candidates were first confirmed in the maser spectra with extreme velocity widths much larger than those of typical 1612 MHz OH maser emission exhibiting clear double-horned spectral profiles (10-20 km s −1 , e.g., Likkel et al 1992, c.f., Engels & Bunzel 2015. There exist only 15 WFs confirmed to date (Gómez et al 2017), implying an extremely short timescale of the phase when such high velocity circumstellar H2O masers are visible (<100 yr, Imai 2007) . Interferometric observations of these H2O masers show a wide variety of the spatio-kinematical structures of the masers, reflecting different types of the central stellar systems driving these jets and/or different stages of the jet evolution.…”

Rapidly evolving episodic outflow in IRAS 18113−2503: clues to the ejection mechanism of the fastest water fountain

“…Unraveling the origin and launching mechanisms of high-velocity collimated outflows in evolved stars requires studying them at an early stage after the launching events (e.g., Bujarrabal et al 2001;Vlemmings et al 2014). A group of sources in such an early stage are water fountain stars (WFs), evolved objects in transition between the late AGB and early PNe that show fast 22 GHz H 2 O maser emission tracing post-shock gas at the interfaces between jet-driven outflows and their surrounding CSEs (Imai 2007;Desmurs 2012;Gómez et al 2017). Outflow speeds are usually a few hundred km s −1 , about an order of magnitude faster than the slowly expanding CSE formed in the AGB phase.…”