Simulations of metastable decay in two- and three-dimensional models with microscopic dynamics

We present a brief analysis of the crossover phase diagram for the decay of a metastable phase in a simple dynamic lattice-gas model of a two-phase system. We illustrate the nucleation-theoretical analysis with dynamic Monte Carlo simulations of a kinetic Ising lattice gas on square and cubic lattices. We predict several regimes in which the metastable lifetime has different functional forms, and provide estimates for the crossovers between the different regimes. In the multidroplet regime, the Kolmogorov-Johnson-Mehl-Avrami theory for the time dependence of the order-parameter decay and the two-point density correlation function allows extraction of both the order parameter in the metastable phase and the interfacial velocity from the simulation data.Comment: 14 pages, 4 figures, submitted to J. Non-Crystalline Solids, conference proceeding for IXth International Conference on the Physics of Non-Crystalline Solids, October, 199

Heavy metals in a light white dwarf: abundances of the metal-rich, extremely low-mass GALEX J1717+6757

Using the Hubble Space Telescope, we detail the first abundance analysis enabled by farultraviolet spectroscopy of a low-mass (0.19 M) white dwarf (WD), GALEX J1717+6757, which is in a 5.9-h binary with a fainter, more-massive companion. We see absorption from nine metals, including roughly solar abundances of Ca, Fe, Ti, and P. We detect a significantly sub-solar abundance of C, and put upper limits on N and O that are also markedly sub-solar. Updated diffusion calculations indicate that all metals should settle out of the atmosphere of this 14 900 K, log g = 5.67 WD in the absence of radiative forces in less than 20 yr, orders of magnitude faster than the cooling age of hundreds of Myr. We demonstrate that ongoing accretion of rocky material that is often the cause of atmospheric metals in isolated, more massive WDs is unlikely to explain the observed abundances in GALEX J1717+6757. Using new radiative levitation calculations, we determine that radiative forces can counteract diffusion and support many but not all of the elements present in the atmosphere of this WD; radiative levitation cannot, on its own, explain all of the observed abundance patterns, and additional mechanisms such as rotational mixing may be required. Finally, we detect both primary and secondary eclipses using ULTRACAM high-speed photometry, which we use to constrain the low-mass WD radius and rotation rate as well as update the ephemeris from the discovery observations of this WD+WD binary

X-ray Spectroscopy and Variability of AGN Detected in the 2 Ms Chandra Deep Field-North Survey

We investigate the nature of the faint X-ray source population through X-ray spectroscopy and variability analyses of 136 AGN detected in the 2 Ms Chandra Deep Field-North survey with > 200 background-subtracted 0.5-8.0 keV counts [F(0.5-8.0 keV)=(1.4-200)e-15 erg cm^{-2} s^{-1}]. Our preliminary spectral analyses yield median spectral parameters of Gamma=1.61 and intrinsic N_H=6.2e21 cm^{-2} (z=1 assumed when no redshift available) when the AGN spectra are fitted with a simple absorbed power-law model. However, considerable spectral complexity is apparent (e.g., reflection, partial covering) and must be taken into account to model the data accurately. Moreover, the choice of spectral model (i.e., free vs. fixed photon index) has a pronounced effect on the derived N_H distribution and, to a lesser extent, the X-ray luminosity distribution. Ten of the 136 AGN (~7%) show significant Fe Kalpha emission-line features with equivalent widths in the range 0.1-1.3 keV. Two of these emission-line AGN could potentially be Compton thick (i.e., Gamma < 1.0 and large Fe Kalpha equivalent width). Finally, we find that 81 (~60%) of the 136 AGN show signs of variability, and that this fraction increases significantly (~80-90%) when better photon statistics are available.Comment: Submitted to Advances in Space Research for New X-ray Results from Clusters of Galaxies and Black Holes (Oct 2002; Houston, TX), eds. C. Done, E.M. Puchnarewicz, M.J. Ward. Requires cospar.sty (6 pgs, 10 figs

Evidence for mass accretion driven by spiral shocks onto the white dwarf in SDSS J123813.73–033933.0

We present high-time-resolution photometry and phase-resolved spectroscopy of the short-period (⁠Porb=80.52min⁠) cataclysmic variable SDSS J123813.73–033933.0, observed with the Hubble Space Telescope (HST), the Kepler/K2 mission, and the Very Large Telescope (VLT). We also report observations of the first detected superoutburst. SDSS J1238–0339 shows two types of variability: quasi-regular brightenings recurring every ≃8.5  h during which the system increases in brightness by ≃0.5mag, and a double-hump quasi-sinusoidal modulation at the orbital period. The detailed K2 light curve reveals that the amplitude of the double-humps increases during the brightenings and that their phase undergoes a ≃90° phase shift with respect to the quiescent intervals. The HST  data unambiguously demonstrate that these phenomena both arise from the heating and cooling of two relatively large regions on the white dwarf. We suggest that the double-hump modulation is related to spiral shocks in the accretion disc resulting in an enhanced accretion rate heating two localized regions on the white dwarf, with the structure of the shocks fixed in the binary frame explaining the period of the double humps. The physical origin of the 8.5  h brightenings is less clear. However, the correlation between the observed variations of the amplitude and phase of the double-humps with the occurrence of the brightenings is supportive of an origin in thermal instabilities in the accretion disc

The Chandra Deep Field North Survey. XIII. 2 Ms point-source catalogs

We present point-source catalogs for the 2 Ms exposure of the Chandra Deep Field North, currently the deepest X-ray observation of the universe in the 0.58.0 keV band. Five hundred and three (503) X-ray sources are detected over an 448 arcmin2 area in up to seven X-ray bands. Twenty (20) of these X-ray sources lie in the central 5.3 arcmin2 Hubble Deep Field North (13,600 sources deg-2). The on-axis sensitivity limits are 2.5 × 10-17 ergs cm-2 s-1 (0.52.0 keV) and 1.4 × 10-16 ergs cm-2 s-1 (28 keV). Source positions are determined using matched-filter and centroiding techniques; the median positional uncertainty is 03. The X-ray colors of the detected sources indicate a broad variety of source types, although absorbed AGNs (including a small number of possible Compton-thick sources) are clearly the dominant type. We also match lower significance X-ray sources to optical counterparts and provide a list of 79 optically bright (R 23) lower significance Chandra sources. The majority of these sources appear to be starburst and normal galaxies. The average backgrounds in the 0.52.0 keV and 28 keV bands are 0.056 and 0.135 counts Ms-1 pixel-1, respectively. The background count distributions are very similar to Poisson distributions. We show that this 2 Ms exposure is approximately photon limited in all seven X-ray bands for regions close to the aim point, and we predict that exposures up to 25 Ms (0.52.0 keV) and 4 Ms (28 keV) should remain nearly photon limited. We demonstrate that this observation does not suffer from source confusion within 6 of the aim point, and future observations are unlikely to be source-confusion limited within 3 of the aim point even for source densities exceeding 100,000 deg-2. These analyses directly show that Chandra can achieve significantly higher sensitivities in an efficient, nearly photon-limited manner and be largely free of source confusion. To allow consistent comparisons, we have also produced point-source catalogs for the 1 Ms Chandra Deep Field South (CDF-S). Three hundred and twenty-six (326) X-ray sources are included in the main Chandra catalog, and an additional 42 optically bright X-ray sources are included in a lower significance Chandra catalog. We find good agreement with the photometry of the previously published CDF-S catalogs; however, we provide significantly improved positional accuracy

Bildanalytische Bewertung von Oberflaecheneigenschaften ungestrichener und gestrichener Papiere

SIGLEAvailable from TIB Hannover: RO 3209(2000,3) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Ultraviolet studies of interacting binaries

Interacting Binaries consist of a variety of stellar objects in different stages of evolution and those containing accreting compact objects still represent a major challenge to our understanding of not only close binary evolution but also of the chemical evolution of the Galaxy. These end-points of binary star evolution are ideal laboratories for the study of accretion and outflow processes, and provide insight on matter under extreme physical conditions. One of the key-questions of fundamental relevance is the nature of SN Ia progenitors. The study of accreting compact binary systems relies on observations over the entire electromagnetic spectrum and we outline here those unresolved questions for which access to the ultraviolet range is vital, as they cannot be addressed by observations in any other spectral region