X-ray Emission from Planetary Nebulae and their Central Stars: a Status Report

In the era of Chandra and XMM-Newton, the detection (or nondetection) of diffuse and/or point-like X-ray sources within planetary nebulae (PNe) yields important, unique insight into PN shaping processes. Diffuse X-ray sources, whether due to ``hot bubbles'' or to collimated outflows or jets, allow us to probe the energetic shocks within PN wind interaction regions. Meanwhile, X-ray point sources provide potential diagnostics of magnetic fields, accretion disks, and/or binary companions at PN cores. Here, I highlight recent X-ray observational results and trends that have the potential to shed new light on the origin and evolution of the structure of PNe.Comment: 8 pages, 3 figures; to appear in proceedings of "Asymmetrical Planetary Nebulae IV" (ed. R. Corradi et al.

The usefulness of Klett's inversion algorthms to simulated satellite Lidar returns

The lidar equation is a special form of the radiative transport equation in single scattering approximation and describes the return signal of a lidar. Klett's algorithm for retrieving the total extinction coefficient profile was developed for application to ground-based lidar returns by a backward integration from the far end to the near end, the range where the incident and backscattered pulse overlap totally. The applicability of Klett's algorithm to satellite backscatter lidar returns was assessed. The simulated data of a 1 J Alexandrite laser operated at about 0.7 micron and at a satellite flight level of 840 km

The Egg Nebula (AFGL 2688): Deepening Enigma

Recent observations of the Egg Nebula (AFGL 2688), obtained at ever-increasing spatial and spectral resolution, have revealed a perplexing array of phenomena. Many of these phenomena present challenges to our understanding of this object as an emerging, bipolar planetary nebula. Here, we consider two particularly intriguing aspects of the Egg: the peculiar structure and kinematics of its equatorial regions, and the nature of an apparent widely separated companion to the central star. In the first case, we use recently acquired Hubble Space Telescope images to demonstrate that the H2 emission distributed east and west of the central star is spatially coincident with a dusty, equatorial disk or torus. The H2 is thus constrained to lie near the equatorial plane, casting doubt on pure radial outflow models for the equatorial kinematics. In the second case, we show that the apparent companion (``Peak A'') may be an accreting white dwarf that has undergone one or more thermonuclear bursts.Comment: 4 pages, 1 figure; to appear in "Asymmetrical Planetary Nebulae III" editors M. Meixner, J. Kastner, N. Soker, & B. Balick (ASP Conf. Series

Constraining the X-ray Luminosities of Asymptotic Giant Branch Stars: TX Cam and T Cas

To probe the magnetic activity levels of asymptotic giant branch (AGB) stars, we used XMM-Newton to search for X-ray emission from two well-studied objects, TX Cam and T Cas. The former star displays polarized maser emission indicating magnetic field strengths of B ~ 5 G; the latter is one of the nearest known AGB stars. Neither star was detected by XMM-Newton. We use the upper limits on EPIC (CCD detector) count rates to constrain the X-ray luminosities of these stars, and derive L_X < 10^{31} erg s^{-1} (<10^{30} erg s^{-1}) for an assumed X-ray emission temperature T_X = 3x10^6 K (10^7 K). These limits represent <~ 10% (<~ 1%) of the X-ray luminosity expected under models in which AGB magnetic fields are global and potentially play an important role in collimating and/or launching AGB winds. We suggest, instead, that the B field strengths inferred from maser observations are representative of localized, magnetic clouds.Comment: 12 pages, 1 figure; submitted to the Astrophysical Journa

Serendipitous XMM-Newton Detection of X-ray Emission from the Bipolar Planetary Nebula Hb 5

We report the serendipitous detection by the XMM-Newton X-ray Observatory of an X-ray source at the position of the Type I (He- and N-rich) bipolar planetary nebula Hb 5. The Hb 5 X-ray source appears marginally resolved. While the small number of total counts (~170) and significant off-axis angle of the X-ray source (~7.8') precludes a definitive spatial analysis, the morphology of the X-ray emission appears to trace the brightest features seen in optical images of Hb 5. The X-ray spectrum is indicative of a thermal plasma at a temperature between 2.4 and 3.7 MK and appears to display strong Neon emission. The inferred X-ray luminosity is L_X = 1.5 x 10^32 ergs/s. These results suggest that the detected X-ray emission is dominated by shock-heated gas in the bipolar nebula, although we cannot rule out the presence of a point-like component at the position of the central star. The implications for and correspondence with current models of shock-heated gas in planetary nebulae is discussed.Comment: 11 pages, 2 figures, Accepted by The Astrophysical Journa

Comparing shocks in planetary nebulae with the solar wind termination shock

We show that suprathermal particles, termed pick-up ions (PUIs), might reduce the postshock temperature of the fast wind and jets in some planetary nebulae (PNs) and in symbiotic systems. The goal is to explain the finding that the temperature of the hot bubble formed by the post-shock gas in some PNs and symbiotic nebulae is lower, sometimes by more than an order of magnitude, than the value expected from simple hydrodynamical calculations. Although various explanations have been proposed, there is as yet no prefered solution for this low tempeature problem. PUIs have been invoked to explain the low temperature behind the termination shock of the solar wind. While in the case of the solar wind the neutral atoms that turn into PUIs penetrate the pre-shock solar wind region from the interstellar medium (ISM), in PNs the PUI source is more likely slowly moving clumps embedded in the fast wind or jets. These clumps are formed by instabilities or from backflowing cold gas. Our estimates indicate that in young PNs these PUIs will thermalize before leaving the system. Only in older PNs whose sizes exceed ~5000 AU and for which the fast-wind mass loss rate is below ~10^{-7} Mo/yr do we expect the PUIs to be an efficient carrier of energy out of the postshock region (the hot bubble).Comment: Accepted for publication in Ap