We present the results of far-infrared imaging of extended regions around three bipolar pre-planetary nebulae , AFGL 2688 , OH 231.8+4.2 , and IRAS 16342 - 3814 , at 70 and 160 \mu m 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 extended shells and thus on the mass outflow rates as a function of radial distance from these stars .
We find no apparent extended emission in AFGL 2688 and OH 231.8+4.2 beyond 100 arcseconds from the central source .
In the case of AFGL 2688 , this result is inconsistent with a previous report of two extended dust shells made on the basis of ISO observations .
We derive an upper limit of 2.1 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } and 1.0 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } for the dust mass loss rate of AFGL 2688 and OH 231.8 , respectively , at 200 arcseconds from each source .
In contrast to these two sources , IRAS 16342 - 3814 does show extended emission at both wavelengths , which can be interpreted as a very large dust shell with a radius of \sim 400 arcseconds and a thickness of \sim 100 arcseconds , corresponding to 4 pc and 1 pc , respectively , at a distance of 2 kpc .
However , this enhanced emission may also be galactic cirrus ; better azimuthal coverage is necessary for confirmation of a shell .
If the extended emission is a shell , it can be modeled , with some assumptions about its dust properties , as enhanced mass outflow at a dust mass outflow rate of 1.5 \times 10 ^ { -6 } M _ { \odot } yr ^ { -1 } superimposed on a steady outflow with a dust mass outflow rate of 1.5 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } .
Because of the size of the possible shell , it is likely that this shell has swept up a substantial mass of interstellar gas during its expansion , so these estimates are upper limits to the stellar mass loss rate .
We find a constant color temperature of 32 K throughout the circumstellar envelope of IRAS 16342 - 3814 , which is consistent with heating by the interstellar radiation field .