Article

The Chandra X-ray Survey of Planetary Nebulae (ChanPlaNS): Probing Binarity, Magnetic Fields, and Wind Collisions

The Astronomical Journal (Impact Factor: 4.97). 04/2012; 144(2). DOI: 10.1088/0004-6256/144/2/58
Source: arXiv

ABSTRACT 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 4 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 ~70%. 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 star photospheres, possibly indicating
a high frequency of binary companions to CSPNe. Other potential explanations
include self-shocking winds 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.

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    ABSTRACT: We present results from the most recent set of observations obtained as part of the Chandra X-ray observatory Planetary Nebula Survey (ChanPlaNS), the first comprehensive X-ray survey of planetary nebulae (PNe) in the solar neighborhood (i.e., within ~1.5 kpc of the Sun). The survey is designed to place constraints on the frequency of appearance and range of X-ray spectral characteristics of X-ray-emitting PN central stars and the evolutionary timescales of wind-shock-heated bubbles within PNe. ChanPlaNS began with a combined Cycle 12 and archive Chandra survey of 35 PNe. ChanPlaNS continued via a Chandra Cycle 14 Large Program which targeted all (24) remaining known compact (R_neb <~ 0.4 pc), young PNe that lie within ~1.5 kpc. Results from these Cycle 14 observations include first-time X-ray detections of hot bubbles within NGC 1501, 3918, 6153, and 6369, and point sources in HbDs 1, NGC 6337, and Sp 1. The addition of the Cycle 14 results brings the overall ChanPlaNS diffuse X-ray detection rate to ~27% and the point source detection rate to ~36%. It has become clearer that diffuse X-ray emission is associated with young (<~5x10^3 yr), and likewise compact (R_neb<~0.15 pc), PNe with closed structures and high central electron densities (n_e>~1000 cm^-3), and rarely associated with PNe that show H_2 emission and/or pronounced butterfly structures. Hb 5 is one such exception of a PN with a butterfly structure that hosts diffuse X-ray emission. Additionally, of the five new diffuse X-ray detections, two host [WR]-type CSPNe, NGC 1501 and NGC 6369, supporting the hypothesis that PNe with central stars of [WR]-type are likely to display diffuse X-ray emission.
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