J. H. Kastner, R. Montez Jr., B. Balick, D. J. Frew, B. Miszalski, R. Sahai, E. Blackman, Y. -H. Chu, O. De Marco, A. Frank, M. A. Guerrero, J. A. Lopez, V. Rapson, A. Zijlstra, E. Behar, V. Bujarrabal, R. L. M. Corradi, J. Nordhaus, Q. Parker, C. Sandin, D. Schönberner, N. Soker, J. L. Sokoloski, M. Steffen, T. Ueta, E. Villaver
We present an overview of the initial results from the Chandra Planetary Nebula Survey (ChanPlaNS), the first systematic (volume-limited) Chandra X-ray Observatory survey of planetary nebulae (PNe) in the solar neighborhood. The first phase of ChanPlaNS targeted 21 mostly high-excitation PNe within ~1.5 kpc of Earth, yielding 3 detections of diffuse X-ray emission and 9 detections of X-ray-luminous point sources at the central stars (CSPNe) of these objects. Combining these results with those obtained from Chandra archival data for all (14) other PNe within ~1.5 kpc that have been observed to date, we find an overall X-ray detection rate of 68%. Roughly 50% of the PNe observed by Chandra harbor X-ray-luminous CSPNe, while soft, diffuse X-ray emission tracing shocks formed by energetic wind collisions is detected in ~30%; five objects display both diffuse and point-like emission components. The presence of X-ray sources appears correlated with PN density structure, in that molecule-poor, elliptical nebulae are more likely to display X-ray emission (either point-like or diffuse) than molecule-rich, bipolar or Ring-like nebulae. All but one of the X-ray point sources detected at CSPNe display X-ray spectra that are harder than expected from hot (~100 kK) central stars emitting as simple blackbodies, possibly indicating a high frequency of binary companions to CSPNe. Other potential explanations include self-shocking winds, non-LTE photospheric emission from the CSPN, or PN mass fallback. Most PNe detected as diffuse X-ray sources are elliptical nebulae that display a nested shell/halo structure and bright ansae; the diffuse X-ray emission regions are confined within inner, sharp-rimmed shells. All sample PNe that display diffuse X-ray emission have inner shell dynamical ages <~5x10^3 yr, placing firm constraints on the timescale for strong shocks due to wind interactions in PNe.
View original:
http://arxiv.org/abs/1204.6055
No comments:
Post a Comment