Planetary nebulae (PNe), the near-endpoints of stellar evolution for intermediate-mass stars, have served as astrophysical laboratories for more than a century. With their relatively large numbers in close proximity, PNe serve as textbook examples of plasma and shock processes and provide essential tests of theories of stellar evolution and the origin and enrichment of the heavy elements in the universe. Though PNe are best known as optical emission line sources, many harbor pockets of cold (< 100 K), dense molecular gas, and some of these same PNe also display emission from rarefied, hot (T > 106 K), X-ray-emitting plasma. PNe also exhibit a dizzying variety of optical and near-infrared morphologies: round; elliptical; bipolar; highly point- symmetric; chaotic and clumpy. The physical mechanisms responsible for this PN morphological menagerie — and, in particular, for the evident transformation from a quasi-spherical wind during the progenitor star asymptotic giant branch (AGB) phase to nonspherical or even highly collimated outflow during the PN phase — have been the subject of intense interest and hot debate among PN researchers over the past two decades. This PN shaping problem is multifaceted, with connections to (and implications for) a wide variety of astrophysical processes and systems. In particular, it is now widely believed — though it is also exceedingly difficult to demonstrate! — that many (if not most) PNe are the results of interacting binary systems, and that all PNe likely undergo a (potentially complex) sequence of wind interactions.
Because X-rays are produced both by interacting binary stars and via wind interactions, X-ray observations are key to progress in our understanding of the shaping of planetary nebulae. The community of planetary nebulae (PN) astronomers is therefore undertaking the Chandra Planetary Nebula Survey (ChanPlaNS) — the first systematic X-ray survey of PNe in the solar neighborhood. Among many other key questions, ChanPlaNS will address the following:
- Under what circumstances (physical conditions) do wind-wind shocks lead to X-ray-luminous "hot bubbles" within PNe, and how do these hot bubbles evolve with time?
- How are the kinematics of PNe and the wind properties of their central stars related to the luminosity and morphology of PN X-ray emission?
- What heating and cooling mechanisms govern the temperature of the X-ray-emitting plasma within PNe?
- What can X-ray point sources tell us about the frequency and characteristics of binary systems within PNe, and the relationships of such binaries to (potentially) related systems such as symbiotic stars and SN Ia progenitor binaries?
The team carrying out the ChanPlaNS survey represents a broad swath of the PN astronomical community: X-ray observers (PI J. Kastner, R. Montez, Y.-H. Chu, M. Guerrero); theorists modeling PN shaping processes (N. Soker, A. Frank, E. Blackman, E. Villaver, J. Nordhaus, W. Steffen) and PN X-ray production (M. Steffen, C. Sandin, D. Schoenberner, E. Behar, R. Sahai); observers who have compiled extensive image/spectral libraries (D. Frew, Q. Parker, R. Corradi, B. Miszalski, B. Balick, Sahai, O. De Marco); leaders of recent/ongoing HST imaging studies of PNe and pre-PNe (Sahai, Balick, S. Kwok); experts in far-IR/radio observations of PNe (V. Bujarrabal, T. Ueta, A. Zijlstra, S. Kwok); and experts on the connections between PNe, symbiotic binaries, and novae (J. Sokoloski, Corradi, Soker).