The Rapidly Evolving AGB Star, V Hya: ALMA finds a Multi-Ring Circus with High-Velocity Outflows

Abstract

We have observed the mass-losing carbon star V Hya that is apparently transitioning from an AGB star to a bipolar planetary nebula, at an unprecedented angular resolution of ~0".4-0".6 with the Atacama Large Millimeter/submillimeter Wave Array (ALMA). Our 13CO and 12CO (J=3-2 and J=2-1) images have led to the discovery of a remarkable set of six expanding rings within a flared, warped Disk structure Undergoing Dynamical Expansion (DUDE) that lies in the system's equatorial plane. We also find, for the first time, several bipolar, high-velocity outflows, some of which have parabolic morphologies, implying wide opening angles, while one (found previously) is clumpy and highly collimated. The latter is likely associated with the high-velocity bullet-like ejections of ionized gas from V Hya; a possible molecular counterpart to the oldest of the 4 bullets can be seen in the 12CO images. We find a bright, unresolved central source of continuum emission (FWHM size <~165 au); about 40% of this emission can be produced in a standard radio photosphere, while the remaining 60% is likely due to thermal emission from very large (mm-sized) grains, having mass >~10^{-5} Msun. We have used a radiative transfer model to fit the salient characteristics of the DUDE's 13CO and 12CO emission out to a radius of 8" (3200 au) with a flared disk of mass 1.7 x 10^{-3} Msun, whose expansion velocity increases increases very rapidly with radius inside a central region of size ~200 au, and then more slowly outside it, from 9.5 to 11.5 km/s. The DUDE's underlying density decreases radially, interspersed with local increases that represent the observationally well-characterised innermost three rings.

arXiv commentAccepted for ApJ [48 pages total -- 18 pages (text), 3 tables (2 pages), 25 figures]

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