A Survey for Water Maser Emission toward Planetary Nebulae: New Detection in IRAS 17347−3139

We report on a water maser survey towards a sample of 27 planetary nebulae(PNe) using the Robledo de Chavela and Medicina single-dish antennas, as wellas the Very Large Array (VLA). Two detections have been obtained: the alreadyknown water maser emission in K 3-35, and a new cluster of masers in IRAS17347-3139. This low rate of detections is compatible with the short life-timeof water molecules in PNe (~100 yr). The water maser cluster at IRAS 17347-3139are distributed on a ellipse of size ~ 0.2" x 0.1", spatially associated withcompact 1.3 cm continuum emission (simultaneously observed with the VLA). Fromarchive VLA continuum data at 4.9, 8.4, and 14.9 GHz, a spectral index alpha =0.76 +- 0.03 is derived for this radio source, which is consistent with eithera partially optically thick ionized region or with an ionized wind. However,the latter scenario can be ruled out on mass-loss considerations, thusindicating that this source is probably a young PN. The spatial distributionand the radial velocities of the water masers are suggestive of a rotating andexpanding maser ring, tracing the innermost regions of a torus formed at theend of the AGB phase. Given that the 1.3 cm continuum emission peak is locatednear one of the tips of the major axis of the ellipse of masers, we speculateon a possible binary nature of IRAS 17347-3139, where the radio continuumemission could belong to one of the components and the water masers would beassociated with a companion.Comment: Accepted by The Astrophysical Journal. 25 pages, 6 figure