SpitzerMIPS Imaging of NGC 650: Probing the History of Mass Loss on the Asymptotic Giant Branch

We present the far-infrared (IR) maps of a bipolar planetary nebula (PN), NGC650, at 24, 70, and 160 micron taken with the Multiband Imaging Photometer forSpitzer (MIPS) on-board the Spitzer Space Telescope. While the two-peakemission structure seen in all MIPS bands suggests the presence of a nearedge-on dusty torus, the distinct emission structure between the 24 micron mapand the 70/160 micron maps indicates the presence of two distinct emissioncomponents in the central torus. Based on the spatial correlation of these twofar-IR emission components with respect to various optical line emission, weconclude that the 24 micron emission is largely due to the [O IV] line at 25.9micron arising from highly ionized regions behind the ionization front, whereasthe 70 and 160 micron emission is due to dust continuum arising fromlow-temperature dust in the remnant asymptotic giant branch (AGB) wind shell.The far-IR nebula structure also suggests that the enhancement of mass loss atthe end of the AGB phase has occurred isotropically, but has ensued only in theequatorial directions while ceasing in the polar directions. The present dataalso show evidence for the prolate spheroidal distribution of matter in thisbipolar PN. The AGB mass loss history reconstructed in this PN is thusconsistent with what has been previously proposed based on the past optical andmid-IR imaging surveys of the post-AGB shells.Comment: 9 pages in the emulated ApJ format with 6 figures, to appear in Ap