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Astron. Astrophys. 347, 194-202 (1999)

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5. Conclusions

The most reasonable explanation to our results is that HD101584 has recently left the AGB during which it experienced a final intense ([FORMULA][FORMULA]), and highly (possibly axi-) symmetric mass loss (as inferred from the very symmetric line profiles). There remains [FORMULA]0.1[FORMULA] of molecular gas around the star from this period. A very efficient acceleration to [FORMULA]50[FORMULA] of a substantial fraction of this gas has occurred. About 25% of the gas has been accelerated to very high velocities ([FORMULA]50[FORMULA]) in a bipolar outflow with maximum velocities in excess of 130[FORMULA] (and possibly as high as [FORMULA]500[FORMULA] depending on the uncertain inclination angle), causing the original AGB-envelope to be stretched out in the polar regions. The linear increase in outflow velocity with distance from the source centre suggests either a short period of intense acceleration a few hundred years ago (as estimated from the dynamical age), or a fast wind blowing through the slower AGB-envelope. In this scenario the bulk of the CO emission and the OH emission originate from the higher-density equatorial region (taking the form of a disk or a toroid). The data suggest that a substantial velocity gradient has been set up also in this gas, and the OH emission originates from regions closer to the star than does the CO emission. The plane of the disk/toroid lies almost exactly in the N-S direction, and the bipolar outflow points away from us to the E and towards us to the W. The inclination of the disk/toroid with respect to the line-of-sight is not easily estimated. There are arguments for both a small inclination angle (the high expansion velocities, the star is clearly visible in the UV/optical region), and a large inclination angle (the structure of the OH and CO emission, the optical absorption lines). We favour the latter and estimate a crude upper limit to the inclination angle of [FORMULA]75o. Therefore, the dynamical age of the outflow may be as low as [FORMULA]50 yr, in which case time variations in both the stellar and circumstellar spectra should be observable. However, much higher spatial resolution observations are required to resolve the true configuration, and we refrain from more speculation on this issue. It is also possible that the dynamical age only provides a lower limit to the outflow time scale, since the escaping material may, for some reason, be observable in CO emission only within a certain distance of the star. The momentum of the high-velocity gas is so high that it precludes a radiation-driven wind. A narrow, central feature, prominent in the 13CO lines, suggests the existence of high-density gas, presumably close to the star. This could be the circumbinary disk inferred from optical and infrared data.

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999