K. Wilson, A. Speck (University of Missouri - Columbia), D. Lis (Caltech), M. Meixner (Space Telescope Science Institute)
Thermally pulsing asymptotic giant branch (AGB) stars suffer mass loss which leads to the formation of a circumstellar shell of gas and dust. At the end of the AGB phase, mass-loss stops and the circumstellar shell begins to drift away from the star. If the velocity of the AGB wind has been relatively constant, then dust furthest from the star represents the oldest mass loss, while material closer to the star represents more recent mass loss. Hence, the history of mass loss during the AGB phase is imprinted on the dust shell of the post-AGB envelope. By studying the distribution of matter in these circumstellar shells we can gain a better understanding of the mass-loss processes involved in the evolution of these stars. Using far-infrared (IR) ISOPHOT images of the circumstellar shells of 2 post-AGB stars (the Egg nebula and AFGL 618; Speck, Meixner & Knapp 2001) it has been shown that dust shells can be imaged out to a radius of 2-3 pc, which represents a fossil record of the mass loss for the last ~105 years. Furthermore, these dust shells show evidence for episodic changes in the mass loss with a period of ~104 years.
We present new, ground-based, sub-mm observations of these large dust shells. Using SHARCII on the Caltech Submillimeter Observatory, we have imaged the area around AFGL 618 at 350 microns in order to provide further constraints on these dust shells. The sub-mm images have higher spatial resolution than the ISO images, providing a better constraint on the spatial extent of the increase dust density regions and therefore on the duration of the increased mass-loss episodes. Furthermore, the ISO data are linear scans across the source. The new sub-mm observations present a 2-d image of the dust shell, providing a check on the assumption of a spherical dust shell. Finally, observing these dust shells at another wavelength allows a better determination of the temperature profile of the dust shell. This project has been funded through a NASA ADP grant.
Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.