EXPLORATION OF A RELIC CIRCUMSTELLAR ENVELOPE IN THE ''WATER FOUNTAIN'' SOURCE IRAS 18286–0959

The water fountain source IRAS 18286–0959 (I18286) was reported as an object exhibiting highly collimated, double-helix stellar jets traced by the H2O maser emission. Using the European VLBI Network, we measured the absolute coordinates of the 1612 MHz OH maser emission in I18286, which is very likely associated with a relic of a circumstellar envelope (CSE) developed in the asymptotic giant branch stage of the central star. The location of the OH maser is near the center of the H2O maser feature cluster, where one of the originating points of the two jets is located. We also mapped 22.2 GHz H2O maser emission in this object using the Very Long Baseline Array in the seasons of 2006-2007 as well as 2008-2009. In the first three epochs in 2006-2007, we detected at least 90 maser features per epoch. In six epochs in 2008-2009, when the observation data had already been published and analyzed to identify the double-helix jets, we newly identified 14 proper motions of H2O maser features which could not be measured in the previous analysis due to their short lifetimes. In this paper, together with the OH maser emission, we focus on ''outlier'' H2O maser features, which exhibit slow expansion velocities (Vexp ∼< 30 km s–1) and are likely associated with either the relic CSE or an equatorial flow of I18286. They were marginally distinguishable from the maser features associated with the jets in I18286.