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Abstract:

We have analyzed HST archival and IUE NEWSIPS archival SWP spectra of the dwarf novae CM Del and AH Her obtained when AAVSO light curve data indicated the occurrence of dwarf nova quiescence. HST archival outburst data for AH Her is examined as well. We have computed synthetic, high gravity spectra in LTE with solar composition using TLUSTY195 and SYNSPEC42 and carried out fits of these models to the far UV continuum and narrow absorption line spectra. For CM Del, we find that the far UV spectrum is dominated by a hot (T-eff = 22,000 K, log g = 8) white dwarf with subsolar Si abundance and all other observed metal transitions at essentially their solar values. For AH Her, we find evidence for a hot white dwarf with T-eff = 27,000 K, log g = 8 and subsolar Si abundances. An HST spectrum (GHRS G160M) of AH Her reveals blue-asymmetric SiIV profiles. If due to wind outflow, we derive a single-scattering upper limit mass loss rate of 10(-9) M./yr(-1). Synthetic profile fits to 12 coadded HST medium resolution G160M spectra of the Si IV (1393Angstrom, 1402Angstrom) resonance doublet of CM Del yield a white dwarf rotational velocity estimate of v sin i = 400 +70/-30 km/s(-1), T-wd = 27, 000K +/- 1000K and Si/H = 1 +0.5/-0.3 x solar. This value of v sin i is discussed in the context of angular momentum transfer and rotation rates for other CV primary degenerate stars.

Abstract:

We report optical observations of the VY Scl star BZ Cam during its optical low-state in 1999/2000. We re-interprete its nebula as being photo-ionized by hypothesized, transient, luminous, supersoft X-ray emission of BZ Cam during its optical low-states.

Abstract:

Using a stellar binary population synthesis code, we calculate the production rates and lifetimes of several types of possible gamma-ray burst (GRB) progenitors. We consider mergers of NS-NS, BH-NS, BH-WD systems and helium star mergers (a possible outcome of common envelope evolution of a compact object entering the envelope of a giant with a helium core). We discuss the viability of each GRB model, and alternatively assuming that all bursts are connected with binary mergers we constrain the required collimation of GRBs. Assuming that possible GRB progenitors are born in different types of galaxies we compute their spatial distribution and compare it with the observed locations of GRB afterglows within their hosts. We discuss constraints on the compact object merger model of GRBs imposed by this comparison and find that the observations of afterglows and their host galaxies imply that the helium mergers are the most promising binary progenitors of GRBs.

Abstract:

We report preliminary results from multi-wavelength multiepoch observations of XTEJ1859+226 and XTEJ1118+480. The outburst lightcurves and IR-optical-UV spectra of XTEJ1859+226 suggest an irradiation-prolonged thermal-viscous disc instability model for the outburst. XTEJ1859+226 probably has P-orb less than or similar to 1d. XTEJ1118+480 has P-orb approximate to 4.1hr and was observed in an unusual 'low-hard state' outburst. It exhibits variability on timescales similar to 10s at wavelengths ranging from infrared to X-ray. The accreting material in XTEJ1118+480 has been CNO processed; we discuss implications for the prior evolution of this unusual binary.

Abstract:

We undertake a comparison of observed Algo-type binaries with a library of computed Case A binary evolution tracks. The library consists of 5500 binary tracks with various values of initial primary mass M-10, mass ratio q(0), and period P-0, designed to sample the phase-space of Case A binaries in the range -0.10 less than or equal to log M-10 < 1.7. Each binary is evolved using a standard code with the assumption that both total mass and orbital angular momentum are conserved. This code follows the evolution of both stars until the point where contact or reverse mass transfer occurs. The resulting binary tracks show a rich variety of behaviour which we sort into several subclasses of Case A and Case B. We present the results of this classification, the final mass ratio and the fraction of time spent in Roche Lobe overflow for each binary system. The conservative assumption under which we created this library is expected to hold for a broad range of binaries, where both components have spectra in the range GO to 131 and luminosity class III - V. We gather a list of relatively well-determined observed hot Algol-type binaries meeting this criterion, as well as a list of cooler Algol-type binaries where we expect significant dynamo-driven mass loss and angular momentum loss. We fit each observed binary to our library of tracks using a chi(2)-minimizing procedure. We find that the hot Algols display overall acceptable chi(2) confirming the conservative assumption, while the cool Algols show much less acceptable chi(2) suggesting the need for more free parameters, such as mass and angular momentum loss.