Physics
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7120s&link_type=abstract
HST Proposal ID #7120
Physics
Hst Proposal Id #7120
Scientific paper
IRC+10216 is the second brightest non-Solar System object in the IR sky, second only to eta Carinae. At 5 microns {its peak flux} it is so bright that most ground-based IR detectors are unable to image it. We {Skinner, Meixner & Bobrowksy, In Preparation} have just obtained WFPC2 snapshot images of it. Briefly, we find that at 5000A it is so faint that we see only a faint blob, but at 814W we see a beautiful bipolar nebula {image attached}, strongly reminiscent of the Cygnus Egg Nebula {Sahai & Trauger images in HST Greatest Hits library}. We just barely see diffraction spikes, indicating that at 8140A we are just able to see the central star. Nebulosity has been known to exist around IRC+10216 for a decade or more, but this is the first time it has been shown to be a classic bipolar nebula. We have generated an axially symmetric full radiative transfer model of the toroidal dust shell, which nicely reproduces the dimensions of the visible reflection nebula and the large contrast in brightness between the two lobes. # The Cygnus Egg Nebula is a C-rich post-AGB star, and as such is the evolutionary product of a C-star such as IRC+10216. Much debate has been conducted over the past few years concerning when and how bipolarity arises in post-AGB nebulae such as the Cygnus Egg. It is generally thought that it must arise on the AGB, yet most mm-wave and radio interferometer images of AGB star shells appear to show spherical symmetry. Thus in the past few years the suggestion has arisen that AGB stars may undergo a `superwind' phase at the end of their lives, during which mass-loss accelarates to very large rates {possibly as high as 1E-3 solar masses per year}, and becomes equatorially concentrated. The equatorial dust torus allows visible light to escape along the polar axis where the density of dust grains is lowest, forming bipolar reflection nebulae. The WFPC2 images of IRC+10216 are the first to confirm that this is probably the case. # The Cygnus Egg Nebula exhibits a variety of concentric rings of reflection nebulosity, which probably reflect separate episodes of high mass-loss. We may see such rings in IRC+10216, in a deep image. At the longer NICMOS wavelengths, we may see the central star very strongly, because of the lower dust optical depth. Polarization images would show the location of the geometric center of the dust shell, which is the star. Note that Kastner & Weintraub {ApJ 434 719} show a significant offset between the peak J-band surface brightness and the polarization center. If real thi simplies that even at 1.25um we are really only seeing the reflection nebula.
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