1996A&A...314..871K

We have performed fully sampled mapping of CO (J=1-0) and (J=2-1) emission around the red giant carbon star V Hya, with the IRAM 30m telescope. The velocity structure of the lines reveals two symmetric high velocity wings that we interpret as arising from a bipolar flow. Exactly between the red and the blue cones lies a low velocity component showing the same symmetry axis. We suggest that this component is a moderately oblate spheroid with biconical holes centered on the minor axis, or a thick torus with the same symmetry axis as the bipolar flow. The high signal-to-noise ratio and spectral resolution of the data allow a detailed comparison of the observed lines with a model of the envelope. Using this model, we derive the geometrical and kinematical parameters of the bipolar flow: it appears to have a wide opening angle (65deg) and shows a radially decreasing velocity law, starting at a velocity of at least 50km/s at the flow inner radius. We suggest that this behaviour is due to an increase with time of the flow ejection velocity close to the star. In contrast, the low-velocity component expands at a constant velocity of 7.5km/s. From our model we also derive the total mass loss rate of V Hya (∼1.5x10^–6M_☉/yr), with about 90% of the molecular gas expelled in the high-velocity jet. The circumstellar envelope around V Hya contains ∼2.1x10^–3M_☉, with about four times more gas in the bipolar flow than the low-velocity component. We compare our observations with other evidence for asymmetric mass loss from V Hya. Considering also the star's fast rotation revealed by the photospheric lines, we conclude that V Hya is probably experiencing the short binary common envelope evolution phase between the AGB and the planetary nebula stage, where highly asymmetric mass loss develops.