Other
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.3907l&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #39.07; Bulletin of the American Astronomical Society, Vol. 24, p.1180
Other
Scientific paper
The bipolar nebula AFGL 2688 (the ``Egg Nebula'') is a post-asymptotic giant branch (AGB) star in the relatively rapid transition to a planetary nebula (PN). We present new near-infrared images in standard I-, J-, H-, and K-bandpasses. These images, taken with a variety of telescopes and spatial resolutions, reveal a wealth of structure in the nebula not previously seen. We find that the north lobe is not smooth, but clumpy. There is evidence for periodic variations in mass loss. In addition, we present narrowband (Delta V ~ 2000 km s(-1) ) images centered on the v=1 -> 0 S(1) line of molecular hydrogen (2.121 microns) and nearby continuum. The narrowband imaging reveals that emission in the equatorial region is a result of line emission from shocked H_2. Such emission is also found in the lobe regions. Our high quality K-band images show the equatorial emission to be in the form of a torus extending around the object at a distance R ~ 7 times 10(16) ({D/ 1\ kpc}) cm. We have analyzed the broadband images using models which assume single scattering of photons from the central star. We find the bipolar axis is inclined to the plane of the sky by i ~ 13(deg) . The distribution of scattering dust which best reproduces the images is one in which the dust density decreases with increasing stellar latitude, with a region near the poles that is free of dust. These results are similar to those of previous studies of AFGL 2688 (i.e., Yasef-Zadeh, Morris, & White 1984, ApJ, 278, 186). This dust distribution is strikingly similar to that found for AFGL 618, another bipolar proto-PN (Latter et al. 1992, ApJ, 389, 347). Futhermore, the optical depth in AFGL 2688 is found to have very little wavelength dependence, which implies that the dust content is different than that of the general interstellar medium. This result is consistent with polarimetric studies (i.e., Jones & Dyck 1978, ApJ, 220, 159), which find that the scattering grains are likely to be large graphite particles.
Deutsch Lynne K.
Hora Joseph L.
Kelly Douglas M.
Latter William B.
Maloney Philip R.
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