The Structure of Objects in Transition from the AGB to PN

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Session 36 -- Giant and Supergiant Stars
Oral presentation, Monday, 9, 1995, 2:00pm - 3:30pm

[36.04D] The Structure of Objects in Transition from the AGB to PN

S. R. Trammell (U. Chicago)

Using several different observing techniques, I have investigated the evolutionary process by which stars of one to eight solar masses expel their outer envelopes, create planetary nebulae (PNe), and subsequently cool as white dwarfs. I present spectropolarimetry of 31 AGB/post-AGB stars, and find that 17 of these objects have intrinsic polarizations that are indicative of large scale asymmetries. This result indicates that aspherical structure is frequently present early in PNe formation process.

Spectropolarimetry also provides a tool to separate the reflected and direct spectra, which are superimposed to produce the total spectra of some young planetary nebulae. For GL 618, M1-92, and M2-56, I find that the bipolar lobes are shock heated ($V_{s}$ = 30 - 100 km $s^{-1}$), not photoionized. Near-IR spectra of M1-92 and M2-56 indicate that the observed $H_{2}$ emission is also produced in a shock.

In collaboration with Gary Hill, I have developed a new dual beam imaging polarimeter for use on the McDonald Observatory 2.1m telescope. Using this instrument and a direct CCD imaging system, I have obtained imaging polarimetry and imaging of four objects. With this data I have studied the distribution of shock heated gas in GL 618 and M1-92, and find that the shock emission appears to be concentrated in regions whose brightness peaks are exterior to the peak in the continuum emission. In M1-91 and M2-9, I find bar-like structures extending from the knots across the lobes, perpendicular to the bipolar axis. At approximately the same positions as the bar extensions, the level of polarization increases, indicating a higher density of scatterers in this region. There seem be rings of higher density material in the lobes of these objects, possibly resulting from episodic mass loss events.

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