V-HYDRAE - THE MISSING LINK BETWEEN SPHERICAL RED GIANTS AND BIPOLAR PLANETARY-NEBULAE

We have performed fully sampled mapping of CO (J=1-0) and (J=2-1) emis sion around the red giant carbon star V Hya, with the IRAM 30m telesco pe. The velocity structure of the lines reveals two symmetric high vel ocity wings that we interpret as arising from a bipolar flow. Exactly between the red and the blue cones lies a low velocity component showi ng the same symmetry axis. We suggest that this component is a moderat ely oblate spheroid with biconical holes centered on the minor axis, o r a thick torus with the same symmetry axis as the bipolar flow. The h igh 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 paramete rs of the bipolar flow : it appears to have a wide opening angle (65 d egrees) and shows a radially decreasing velocity law, starting at a ve locity of at least 50 km s(-1) at the flow inner radius. We suggest th at this behaviour is due to an increase with time of the flow ejection velocity close to the star. In contrast, the low-velocity component e xpands at a constant velocity of 7.5 km s(-1). From our model we also derive the total mass loss rate of V Hya (similar to 1.5 x 10(-6) M(.) yr(-1)), with about 90% of the molecular gas expelled in the high-vel ocity jet. The circumstellar envelope around V Hya contains similar to 2.1 x 10(-3) M(.), with about four times more gas in the bipolar flow than the low-velocity component. We compare our observations with oth er evidence for asymmetric mass loss from V Hya. Considering also the star's fast rotation revealed by the photospheric lines, we conclude t hat V Hya is probably experiencing the short binary common envelope ev olution phase between the AGB and the planetary nebula stage, where hi ghly asymmetric mass loss develops.