dc.description.abstract | We present two observational results of the molecular envelopes around evolved stars. First, we have imaged at high angular resolution the CO (J=2-1) line of pi Gru, a nearby S star, using the Sub-Millimeter Array. We detected CO emission with the very broad velocity width of ~90 km/s. The high resolution images show that the CO traces a flattened envelope expanding at a velocity of ~15 km/s, and a bipolar high velocity outflow with a velocity of up to ~50 km/s perpendicular to the equatorial plane. The morphology of low-velocity CO emission can be reproduced by model consisting of a slow equatorial wind with expanding velocity of 15 km/s and inclined to the line of sight about 55 degree, together with a central cavity of 200 AU in radius. In contrast to the spherical morphology commonly seen in AGB envelopes, the disk-outflow structure observed in pi Gru adds to the increasing evidence that bipolar structures seen in many planetary nebulae and proto-planetary nebulae already exist in the late stage of the AGB phase. The presence of high-velocity outflow in pi Gru also suggest high-velocity plays an important role in the shaping of circumstellar envelopes around post AGB stars.
Second, using the Very Large Array, we observed the NH3(1,1), (2,2), (3,3) transitions lines from the Egg Nebula, a famous nearby proto planetary nebula. The morphology and kinematics in three different transitions all shows four distinct lobes align with the polar axis and equatorial direction. Combining with resent H2 and CO line observation suggest that the NH3 emission traces the hot molecular gas behind the shock front which is shocked and heated by the interaction between the high velocity outflows and the surrounding AGB envelope. The velocity gradient along N-S direction in the east lobe can be interpreted as the different velocities in different outflows, which suggest high velocity outflow dominate the kinematics in the circumstellar envelope. Besides, from the NH3 line ratio we suppose that the gas temperature is higher in the bipolar direction which might imply the interaction is much stronger in the bipolar than in equatorial direction. | en_US |