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T. Do, M. Morris (UCLA), R. Sahai, K. R. Stapelfeldt (JPL)
We report far infrared imaging of extended regions around three pre-planetary nebulae, AFGL 2688, OH 231.8+4.2, and IRAS 16342-3814, at 70 and 160 microns with the MIPS instrument on the Spitzer Space Telescope. After a careful subtraction of the point spread function of the central star from these images, we place constraints on the existence of large extended shells and thus on the mass loss rates as a function of radial distance from these stars. Surprisingly, we find no apparent extended emission in AFGL 2688 and OH 231.8+4.2 beyond 100 arcseconds from the central source, down to the surface brightness of the PSF subtraction residual of about 20 MJy/steradian in the region between 100 arcseconds and 200 arcseconds. Beyond 200 arcseconds from the source, we detect no extended emission down to a sensitivity of 0.6 MJy/steradian. In the case of AFGL2688, this result is inconsistent with previous claims of large enhanced mass-loss shells made on the basis of ISO observations. In contrast to the absence of extended dust emission in AFGL 2688 and OH 231.8+4.2, IRAS 16342-3814 does show some evidence for a large shell with a radius of approximately 400 arcseconds in both wavelengths (although an unfortunate placement of interstellar cirrus cannot be entirely ruled out). This corresponds to a shell radius of 4 pc assuming a distance to IRAS 16342 of 2 kpc, and the shell can be modeled as being superimposed on a constant mass loss outflow. Assuming that the emission is circumstellar, the mass loss rate history of this object will be described. We find a constant color temperature throughout the circumstellar envelope, which suggests heating due to the interstellar radiation field. The implications of these observations for enhanced mass loss resulting from events such as thermal pulses will be described.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.