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.

Automated problem scheduling and reduction of synchronization delay effects

It is anticipated that in order to make effective use of many future high performance architectures, programs will have to exhibit at least a medium grained parallelism. A framework is presented for partitioning very sparse triangular systems of linear equations that is designed to produce favorable preformance results in a wide variety of parallel architectures. Efficient methods for solving these systems are of interest because: (1) they provide a useful model problem for use in exploring heuristics for the aggregation, mapping and scheduling of relatively fine grained computations whose data dependencies are specified by directed acrylic graphs, and (2) because such efficient methods can find direct application in the development of parallel algorithms for scientific computation. Simple expressions are derived that describe how to schedule computational work with varying degrees of granularity. The Encore Multimax was used as a hardware simulator to investigate the performance effects of using the partitioning techniques presented in shared memory architectures with varying relative synchronization costs

Digital Color Imaging

This paper surveys current technology and research in the area of digital color imaging. In order to establish the background and lay down terminology, fundamental concepts of color perception and measurement are first presented us-ing vector-space notation and terminology. Present-day color recording and reproduction systems are reviewed along with the common mathematical models used for representing these devices. Algorithms for processing color images for display and communication are surveyed, and a forecast of research trends is attempted. An extensive bibliography is provided

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