Sun Kwok

Circumstellar Arcs in AGB Post-AGB Stars

One of the most unexpected discoveries of the Hubble Space Telescope (HST) observations of planetary nebulae (PN) is the revelation of concentric arcs in the nebulae. As of 2000, circumstellar arcs have been found in three PN (NGC 7027, Bond 2000; NGC 6543, Bond 2000, Terzian & Hajian 2000; Hb 5, Balick & Wilson 2000), and three proto-planetary nebulae (PPN) (AFGL 2688, Sahai et al. 1998; IRAS 17150–3224, Kwok et al. 1998; IRAS 17441–2411, Su et al. 1998). Even though some of these nebulae have bipolar shapes, the arcs are remarkably circular. These arcs are likely to have originated from the remnants of the circumstellar envelopes of the asymptotic giant branch (AGB) progenitors, therefore are manifestations of the mass loss process on the AGB. This hypothesis is confirmed by the ground-based detection of similar arcs in the circumstellar envelopes of the carbon star IRC+10216 Mauron & Huggins 1999. In this paper, we report the detection of circumstellar arcs in the HST image of IRC+10216 and summarize the properties of circumstellar arcs observed in PN and PPN.