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R.A. Gruendl, Y.-H. Chu (University of Illinois), M.A. Guerrero (IAA, CSIC), M. Meixner (STScI)
Planetary Nebulae (PNe) consist of material ejected by stars with masses \le8-10 M\odot and form through the interaction between the current fast stellar wind and previous asymptotic giant branch (AGB) wind. PNe exhibit a large variety of shapes but most can be classified as either elliptical or bipolar. Different scenarios have been suggested for these two broad classes of PNe, including non-isotropic mass loss in the AGB phase, early shaping of the nebula through fast collimated outflows, and binarity. The impact of the fast stellar winds or collimated outflows, with velocities often >1000 km~s-1, produces shock-heated gas that fills the interior of the PN and is expected to emit X-rays. Indeed, XMM-Newton and Chandra have detected diffuse X-ray emission from several elliptical PNe, but the large number of Chandra observations of bipolar PNe have yielded positive detections in only two young nebulae - NGC\,7027 and Mz\,3.
We have used XMM-Newton to observe two evolved bipolar PNe: NGC\,2346 and NGC\,7026. These two PNe are selected because of their different bipolar lobe structures, open lobes in NGC\,2346 and closed lobes in NGC\,7026. Diffuse X-ray emission is detected from only NGC\,7026. The X-ray emission fills its bipolar lobes and shows a spectrum consistent with thin plasma emission at a temperature of 1.5\times106 K. Its X-ray luminosity is ~1\times1032 ergs~s-1 for a distance of 1.9 kpc.
We have examined all available Chandra and XMM-Newton observations of bipolar PNe and find that diffuse X-ray emission is detected only from three nebulae with closed lobe structures. No bipolar PNe with open lobes have been detected in X-rays. Evidently, hot gas is dispersed in open lobes but can be confined within closed lobes to produce detectable X-ray emission.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.