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We present Chandra high resolution imaging of the circumnuclear regions of two nearby active galaxies, namely the starburst/AGN composite Seyfert 1.8 NGC 1365 and the archetypal Seyfert 1 NGC 4151. In NGC 1365, the X-ray morphology shows a biconical soft X-ray-emission region extending ∼5 kpc in projection from the nucleus, coincident with the optical high-excitation outflows. Chandra HRC imaging of the NGC 4151 nucleus resolves X-ray emission from the 4 arcsec radio jet and the narrow line region (NLR) clouds. Our results demonstrate the unique power of spatially resolved spectroscopy with Chandra, and support previous claims that frequent jet-ISM interaction may explain why jets in Seyfert galaxies appear small, slow, and thermally dominated. [ABSTRACT FROM AUTHOR]

I describe examples of long-term studies of interacting binary systems which have detected dramatic changes in the spatial and spectral distribution of the emission related to changes in accretion processes. These studies demonstrate that there are many aspects of accretion processes in general, and of wind accretion processes in particular, that are not yet understood. The key to further accretion studies is resolving a wide range of interacting binaries and studying their components and mass flows. A sub-milliarcsecond resolution is required to carry out detailed studies of nearby interacting binaries. I discuss future prospects for resolving accreting binary systems using the Stellar Imager — a many-element interferometer in space. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

A recurrent topic in very high energy γ-ray astronomy is the morphological characterization of extended sources, which are lacking a clear counterpart in other wavelengths. Whithout a counterpart, the shape of such a source is not known a priori. Therefore blind/unbiased approaches are required to describe such a newly detected source in terms of detached emission regions ('features') or eventually even separate objects. Ideally, the search takes into account the observation statistic and the instrument response (e.g. the exposure and the point spread function). And finally, a detection significance for the features needs to be provided. [ABSTRACT FROM AUTHOR]

Exo-planets are typically thought to form in protoplanetary disks left over from protostellar disk of their newly formed host star. However, additional planetary formation and evolution routes may exist in old evolved binary systems. Here we discuss the implications of binary stellar evolution on planetary systems in such environments. In these binary systems stellar evolution could lead to the formation of symbiotic stars, where mass is lost from one star and could be transferred to its binary companion, and may form an accretion disk around it. This raises the possibility that such a disk could provide the necessary environment for the formation of a new, second generation of planets in both circumstellar or circumbinary configurations. Pre-existing first generation planets surviving the post-MS evolution of such systems would be dynamically effected by the mass loss in the systems and may also interact with the newly formed disk. Such planets and/or planetesimals may also serve as seeds for the formation of the second generation planets, and/or interact with them, possibly forming atypical planetary systems. Second generation planetary systems should be typically found in white dwarf binary systems, and may show various observational signatures. Most notably, second generation planets could form in environment which are inaccessible, or less favorable, for first generation planets. The orbital phase space available for the second generation planets could be forbidden (in terms of the system stability) to first generation planets in the pre-evolved progenitor binaries. In addition planets could form in metal poor environments such as globular clusters and/or in double compact object binaries. Observations of exo-planets in such forbidden or unfavorable regions could possibly serve to uniquely identify their second generation character. Finally, we point out a few observed candidate second generation planetary systems, including Gl 86, HD 27442 and all of the currently observed circumbinary planet candidates. A second generation origin for these systems could naturally explain their unique configurations. [ABSTRACT FROM AUTHOR]

The nature of type Ia supernovae (SNe Ia) is still unclear. Employing Eggleton's stellar evolution code with the optically thick wind assumption, we systematically studied the He star donor channel of SNe Ia, in which a carbon-oxygen white dwarf accretes material from a He main-sequence star or a He subgiant to increase its mass to the Chandrasekhar mass. We mapped out the initial parameters for producing SNe Ia in the orbital period-secondary mass plane for various WD masses from this channel. According to a detailed binary population synthesis approach, we find that this channel can produce SNe Ia with short delay times (∼100 Myr) implied by recent observations. We obtained many properties of the surviving companions of this channel after SN explosion, which can be verified by future observations. We also find that the surviving companions from the SN explosion scenario have a high spatial velocity (>400 km/s), which could be an alternative origin for hypervelocity stars (HVSs), especially for HVSs such as US 708. [ABSTRACT FROM AUTHOR]

AM CVn stars are the final outcome of a fine tuned binary star evolution. They are hydrogen deficient and have orbital periods less than 65 minutes, which make them possible detectable sources for Gravitational Wave (GW) radiation. Mass is transferred between a low mass star with a variable degree of degeneracy to a heavier white dwarf. The Sloan Digital Sky Survey (SDSS) and other surveys have increased the number of such binaries to 25, including two with orbital periods of less than 10 minutes. For these, four different models have been proposed, including one without mass transfer, driven by electricity generated by the secondary star moving in the magnetic field of the primary. AM CVn stars are also possible progenitors of SN Ia or weaker explosive events. There are three birth channels for AM CVn stars: Either a low mass white dwarf donor, a low mass helium star donor, or a strongly evolved hydrogen CV. In this contribution I will discuss observational constraints that may give us possibilities to determine birth channels for AM CVn stars based on mass-radius relations and chemical composition. [ABSTRACT FROM AUTHOR]

Two distinct scenarios have been proposed to explain the origin of dust observed in the high-redshift (z = 6.4) quasar J1148+5251. The first scenario suggests that because of the relatively young age of the universe (∼890 Myr), only massive stars could have produced the large amount of dust observed in this object. The second scenario assumes a significantly older galactic age, so that most of the dust could have formed in lower-mass AGB stars that would have sufficient time to evolve off the main sequence. In this contribution we offer a critical analysis of both scenarios, and briefly discuss alternative suggestions for the origin of dust in this object. [ABSTRACT FROM AUTHOR]

Masses determined from classical Cepheids in binary systems are a primary test of both pulsation and evolutionary calculations. The first step is to determine the orbit from ground-based radial velocities. Complementary satellite data from Hubble, FUSE, IUE, and Chandra provide full information about the system. A summary of recent results on masses is given. Cepheids have also provided copious information about the multiplicity of massive stars, as well as the distribution of mass ratios and separations. This provides some important constraints for star formation scenarios including differences between high and low mass results and differences between close and wide binaries. [ABSTRACT FROM AUTHOR]

The Sun is the largest reservoir representing the matter of the early solar nebula. Its isotopic composition for the elements N, Ne, Mg, Si, At, and Fe has been determined by measuring solar wind abundances with the CELIAS experiment on board SOHO and other spacecraft instruments. These measurements and theoretical considerations indicate that the solar wind, in particular the coronal-hole type solar wind, is much less isotopically fractionated than solar energetic particles. The data give evidence that the isotopic abundance ratios typically vary by only ∼1 - 2% per amu in different solar wind regimes such as coronal-hole type and streamer-belt associated solar wind, ejecta of coronal mass ejections (CMEs), or so-called 'blobs'. [ABSTRACT FROM AUTHOR]

We present a study on the abundance of oxygen relative to hydrogen in the solar minimum corona and for the first time we measure this quantity in polar coronal holes. The results are derived from the observations of the extended corona obtained with the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO. The diagnostic method used to obtain the oxygen abundance is based on the resonant components of the O VI 1032 Å and HI 1216 Å emission lines. This method fully accounts for the effects of the outflow velocity of the solar wind, which can be determined through Doppler dimming, and of the width of the absorbing profiles of the coronal ions or neutral atoms involved in resonant scattering. The oxygen abundance is higher in the polar coronal hole regions, where the fast wind is accelerated, than in the streamer belt. In the polar regions the observed oxygen abundance is consistent with the photospheric value and with the composition results obtained with Ulysses for the fast wind. The oxygen abundance values derived with UVCS suggest that the plasma remains substantially contained in quiescent streamers, that therefore do not contribute significantly to the solar wind. [ABSTRACT FROM AUTHOR]

Core collapse supernovae (SN) are the final stages of stellar evolution in massive stars during which the central region collapses, forms a neutron star (NS), and the outer layers are ejected. Recent explosion scenarios assumed that the ejection is due to energy deposition by neutrinos into the envelope but detailed models do not produce powerful explosions. There is mounting evidence for an asphericity in the SN which is difficult to explain within this picture. This evidence includes the observed high polarization, pulsar kicks, high velocity iron-group and intermediate-mass elements material observed in remnants, etc. The discovery of highly magnetars revived the idea that the basic mechanism for the ejection of the envelope is related to a highly focused MHD-jet formed at the NS. Our 3-D hydro simulations of the jet propagation through the star confirmed that the mechanism can explain the asphericities. In this paper, detailed 3-D models for jet induced explosions of “classical” core collapse supernovae are presented. We demonstrate the influence of the jet properties and of the underlaying progenitor structure on the final density and chemical structure. Finally, we discuss the observational consequences, predictions and tests of this scenario. [ABSTRACT FROM AUTHOR]

We describe our study of long-term variability of o Ceti (Mira A), the prototype of the Mira-type pulsating stars. Our study was originally undertaken to search for coherent long-period variability, but the results of our analysis didn’t uncover this. However, we detected a low-frequency “red noise” in the Fourier spectrum of the o Ceti century-long light curve. We have since found similar behavior in other Miras and pulsating giant stars and have begun a study of a large sample of Mira variables. Similar red noise has been previously detected in red supergiants and attributed to supergranular convection. Its presence in Miras suggests the phenomenon may be ubiquitous in cool giant pulsators. These results support high-angular resolution observations of Miras and supergiants showing asymmetries in their surface brightness distributions, which may be due to large supergranular convection cells. Theoretical modeling, and numerical simulations of pulsation processes in late-type giants and supergiants should therefore take into account the effects of deep convection and large supergranular structures, which in turn may provide important insights into the behavior of Miras and other giant and supergiant pulsators. In this work, we summarize our results for o Ceti, present preliminary results of our broader study of Mira variables, and discuss how the results of this study may be used by future studies of AGB variables. [ABSTRACT FROM AUTHOR]