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[en] The observations of WZ Sge provide us with the best test for computational codes trying to calculate the accretion process onto a white dwarf. We find that the accreted material on WZ Sge cannot spread over the whole star at least on a time scale of ten years. Numerical models which include the angular momentum of the accreted material provide the best fit to the observations
[en] We show that current ideas for forming the period gap in cataclysmic variables (CVs) give a simple explanation for the spike in the observed distribution of magnetic CVs, provided that they have a bimodal distribution of white dwarf masses M1 at the start of their secular evolution. The majority should have M1 very close to a value of ∼ 0.6-0.7 Msolarmasses and form at periods of ∼ 4-6 hr, with a much smaller group having larger M1 and forming at periods up to 10 hr. Nova explosions remove almost exactly the accreted mass for the low-mass group, so that M1 remains at the initial value of ∼ 0.6-0.7 Msolarmasses while the high-mass group may lose small amounts of mass. These conclusions are supported by nova theory and offer the possibility of an observational test of gap forming hypotheses. (author)
[en] Over the past few years, we have been investigating the effects of accretion onto massive white dwarfs and its implications for their growth in mass toward the Chandrasekhar limit, in attempts to identify a possible relationship between SN I and novae. In our studies we have considered accretion at various mass accretion rates onto a variety of different white dwarf masses. We have found that there is a critical white dwarf mass above which a significant fraction of the accreted mass can remain on the white dwarf after the outburst. Below this value of the white dwarf mass, all of the accreted mass, plus core material dredged up into the envelope, is ejected as a result of the explosion. Our latest results include accretion and boundary layer heating produced by the infalling material. From these studies, we have identified some members of the class of recurrent novae, those involving a thermonuclear runaway, as the novae that are occurring on very massive white dwarfs and evolving toward an SN I explosion. One of the outgrowths of our uv studies of novae in outburst has been the identification of a class of novae which eject material that is very rich in the elements from oxygen to aluminum. We have shown that these outbursts occur on ONeMg white dwarfs, which are necessarily very massive white dwarfs. 11 refs
[en] The role of differential and solid body rotation in the evolution of massive stars undergoing mass loss is discussed. The implications for Of, WR, β Cephei stars and shell stars are brought out. (Auth.)
[en] We present a new photometric study of the 71 s oscillation in the old nova DQ Herculis. All timings of the oscillation maxima may be phased with a simple ephemeris; the most likely choice has a large P term, which will, if continued, halt the speedup of the oscillation in approx.40 years.The oscillation shows a wealth of fascinating detail in its phase and amplitude variations around the binary orbit. We confirm the oscillation phase shift in eclipse found by Warner et al., and provide a more accurate description. We find that the pulses arrive systematically late by approx.6 s for a 1 hour interval around orbital phase 0.7. Multicolor observations establish that the visible oscillation arises from the reprocessing of the white dwarf's pulsed ultraviolet flux by the accretion disk.We also report new eclipse timings, and show that a sinusoidal modulation with a period of approx.14 years is present in all the timings sine 1954. This effect cannot be explained by the presence of a distant companion or by classical apsidal motion
[en] We discuss the emerging trend that super-Chandrasekhar Type Ia Supernovae (SCSNe) with progenitor mass estimates significantly exceeding ∼1.4 Msun tend to explode in metal-poor environments. While Taubenberger et al. noted that some of the SCSNe host galaxies are relatively metal-poor, we focus quantitatively on their locations in the hosts to point out that in three out of four cases, the SCSNe explosions occurred in the outer edge of the disks of their hosts. It is therefore very likely that their progenitors had far lower metallicities than those implied by the metallicity of their hosts' central regions. In two cases (SN 2003fg and SN 2009dc) the explosion sites were outside ∼99% of the host's light, and in one case (SN 2006gz) the host's radial metallicity slope indicates that the explosion site is in a metal-poor region. The fourth case (SN 2007if) has the lowest spectroscopically measured SN Ia host metallicity (Childress et al. 2011). It may be possible to explain each of these unusually bright events through some progenitor scenario specific to that case, but a much simpler and straightforward conclusion would be to ascribe the controlling factor to the only physical aspect they have in common-metal-poor environments.
[en] We report the discovery of 1RXS J173006.4+033813, a polar cataclysmic variable with a period of 120.21 minutes. The white dwarf primary has a magnetic field of B = 42+6-5 MG and the secondary is an M3 dwarf. The system shows highly symmetric double-peaked photometric modulation in the active state as well as in quiescence. These arise from a combination of cyclotron beaming and ellipsoidal modulation. The projected orbital velocity of the secondary is K2 = 390 ± 4 km s-1. We place an upper limit of 830 ± 65 pc on the distance.
[en] The final fate of accreting C + O white dwarfs is either thermonuclear explosion or collapse, if the white dwarf mass grows to the Chandrasekhar mass. We discuss how the fate depends on the initial mass, age, composition of the white dwarf and the mass accretion rate. Relatively fast accretion leads to a carbon deflagration at low central density that gives rise to a Type Ia supernova. Slower accretion induces a helium detonation that could be observed as a Type Ib supernova. If the initial mass of the C + O white dwarf is larger than 1.2 Msub solar, a carbon deflagration starts at high central density and induces a collapse of the white dwarf to form a neutron star. We examine the critical condition for which a carbon deflagration leads to collapse, not explosion. For the case of explosion, we discuss to what extent the nucleosynthesis models are consistent with spectra of Type Ia and Ib supernovae. 61 refs., 18 figs
[en] Understanding boundary layer heating is crucial in determining the thermal structure of the accreted envelope of a prenova white dwarf. The matched asymptotic expansion method was used to solve consistently for the structure of accretion disks transferring matter onto rotating white dwarfs. The fraction of accretion energy transported into prenova white dwarf envelopes was calculated. These results should be used by modelers of nova eruptions; they will produce significantly lower degeneracies and weaker explosions than expected until now. Detailed models of accretion disks and boundary layers can also be used to calculate the amount of white dwarf heating during a dwarf nova outburst. In general, such models can serve as input to model atmosphere codes to predict more realistic spectra of disk-accreting objects. 29 refs
[en] Photometry and spectroscopy results for the SN 1981b in NGC 4536 are examined and compared with SN I models. Three recent models for such SNs are discussed, two of which arise from the accreting white dwarf scenario and the third of which could be the result of single-star evolution. The observational results for SN 1981b are generally consistent with the carbon deflagration of an accreting white dwarf. The assumption of backbody emissivity at maximum light leads to a high luminosity, as expected for any model that synthesizes on the order of a solar mass of Ni-56. The spectra establish that intermediate mass elements were present with roughly solar relative abundances in the outer parts of the ejecta, and suggest that freshly synthesized iron was present in the inner parts. 33 references