Abstract: Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system1, 2, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. There are two classes of models1, 3: double-degenerate (involving two white dwarfs in a close binary system2, 4) and single-degenerate models5, 6. In the latter, the primary white dwarf accretes material from a secondary companion until conditions are such that carbon ignites, at a mass of 1.38 times the mass of the Sun. The type Ia supernova SN 2011fe was recently detected in a nearby galaxy7. Here we report an analysis of archival images of the location of SN 2011fe. The luminosity of the progenitor system (especially the companion star) is 10–100 times fainter than previous limits on other type Ia supernova progenitor systems8, 9, 10, allowing us to rule out luminous red giants and almost all helium stars as the mass-donating companion to the exploding white dwarf.

Abstract: We have used multi-epoch images from the Infrared Array Camera on board the Spitzer Space Telescope to search for substellar companions to stars in the solar neighborhood based on common proper motions. Through this work, we have discovered a faint companion to the white dwarf WD 0806-661. The comoving source has a projected separation of 130'', corresponding to 2500 AU at the distance of the primary (19.2 pc). If it is physically associated, then its absolute magnitude at 4.5 μm is ~1 mag fainter than the faintest known T dwarfs, making it a strong candidate for the coolest known brown dwarf. The combination of M 4.5 and the age of the primary (1.5 Gyr) implies an effective temperature of ~300 K and a mass of ~7 M Jup according to theoretical evolutionary models. The white dwarf's progenitor likely had a mass of ~2 M ☉, and thus could have been born with a circumstellar disk that was sufficiently massive to produce a companion with this mass. Therefore, the companion could be either a brown dwarf that formed like a binary star or a giant planet that was born within a disk and has been dynamically scattered to a larger orbit.

Abstract: SDSSJ125733.63+542850.5 (hereafter SDSSJ1257+5428) is a compact white dwarf binary from the Sloan Digital Sky Survey that exhibits high-amplitude radial velocity variations on a period of 4.56 hr. While an initial analysis suggested the presence of a neutron star or black hole binary companion, a follow-up study concluded that the spectrum was better understood as a combination of two white dwarfs. Here we present optical spectroscopy and ultraviolet fluxes which directly reveal the presence of the second white dwarf in the system. SDSSJ1257+5428’s spectrum is a composite, dominated by the narrow-lined spectrum from a cool, low-gravity white dwarf (Teff � 6300 K, log g = 5–6.6) with broad wings from a hotter, high-mass white dwarf companion (11,000–14,000 K; ∼1 M� ). The high-mass white dwarf has unusual line profiles which lack the narrow central core to Hα that is usually seen in white dwarfs. This is consistent with rapid rotation with v sin i = 500–1750 km s −1 , although other broadening mechanisms such as magnetic fields, pulsations, or a helium-rich atmosphere could also be contributory factors. The cool component is a puzzle since no evolutionary model matches its combination of low gravity and temperature. Within the constraints set by our data, SDSSJ1257+5428 could have a total mass greater than the Chandrasekhar limit and thus be a potential Type Ia supernova progenitor. However, SDSSJ1257+5428’s unusually low-mass ratio q ≈ 0.2 suggests that it is more likely that it will evolve into an accreting double white dwarf (AM CVn star).

Abstract: The asteroseismic analysis of white dwarfs allows us to peer below their photospheres and determine their internal structure. At ∼28 000 K EC20058−5234 is the hottest known pulsating helium atmosphere white dwarf. As such, it constitutes an important link in the evolution of white dwarfs down the cooling track. It is also astrophysically interesting because it is at a temperature where white dwarfs are expected to cool mainly through the emission of plasmon neutrinos. In the present work, we perform an asteroseismic analysis of EC20058−5234 and place the results in the context of stellar evolution and time-dependent diffusion calculations. We use a parallel genetic algorithm complemented with targeted grid searches to find the models that fit the observed periods best. Comparing our results with similar modelling of EC20058−5234’s cooler cousin CBS114, we find a helium envelope thickness consistent with time-dependent diffusion calculations and obtain a precise mode identification for EC20058−5234.

Abstract: Using high-resolution Ultraviolet and Visual Echelle Spectrograph (UVES) spectra of the eclipsing post-common envelope binary QS Vir, we detect material along the line of sight to the white dwarf at orbital phase ϕ= 0.16. We ascribe this to a stellar prominence originating from the M dwarf secondary star which passes in front of the white dwarf at this phase. This creates sharp absorption features in the hydrogen Balmer series and Ca ii H&K lines. The small size of the white dwarf allows us to place tight constraints on the column density of hydrogen in the n= 2 level of log 10 N2= 14.10 ± 0.03 cm−2 and, assuming local thermodynamical equilibrium, the temperature of the prominence material of ∼9000 K. The prominence material is at least 1.5 stellar radii from the surface of the M dwarf. The location of the prominence is consistent with emission features previously interpreted as evidence for Roche lobe overflow in the system. We also detect Mg ii 4481 A absorption from the white dwarf. The width of the Mg ii line indicates that the white dwarf is not rapidly rotating, in contrast to previous work, hence our data indicate that QS Vir is a pre-cataclysmic binary, yet to initiate mass transfer, rather than a hibernating cataclysmic variable as has been suggested.

Abstract: Within the theoretical framework of some modern unification theories the constants of nature are functions of cosmological time. White dwarfs offer the possibility of testing a possible variation of G and, thus, to place constraints to these theories. We present full white dwarf evolutionary calculations in the case that G decreases with time. White dwarf evolution is computed in a self-consistent way, including the most up-to-date physical inputs, non-gray model atmospheres and a detailed core chemical composition that results from the calculation of the full evolution of progenitor stars. We find that the mechanical structure and the energy balance of white dwarfs are strongly modified by the presence of a varying G. In particular, for certain values of the rate of change of G, the evolution of cool white dwarfs is markedly affected. The impact of a varying G is more notorious in the case of more massive white dwarfs. In view of the recent results reporting that a very accurate white dwarf cooling age can be derived for the old and metal-rich open cluster NGC 6791, our study suggests that this cluster could be a potential target to constrain or detect a ypothetical secular variation of G.

Abstract: SUMMARY: We recalculated the maximum white dwarf mass in ultra-compact X-ray binaries obtained in an earlier paper (Arbutina 2011), by taking the effects of super-Eddington accretion rate on the stability of mass transfer into account. It is found that, although the value formally remains the same (under the assumed approximations), for white dwarf masses M2 0.1MCh mass ratios are extremely low, implying that the result for Mmax is likely to have little if any practical relevance.

Abstract: We have obtained near‐IR photometry for the 11 Praesepe white dwarfs, to search for an excess indicative of a dusty debris disk. All the white dwarfs are in the DAZ temperature regime, however we find no indications of a disk around any white dwarf. We have, however determined that the radial velocity variable white dwarf WD0837+185 could have an unresolved T8 dwarf companion that would not be seen as a near‐IR excess.

Abstract: We present new ultraviolet spectra of the hottest known, peculiar white dwarf H1504+65, obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. H1504+65 is the hottest known white dwarf (Teff=200 000 K) and has an atmosphere mainly composed by carbon and oxygen, augmented with high amounts of neon and magnesium. This object is unique and the origin of its surface chemistry is completely unclear. We probably see the naked core of either a C–O white dwarf or even a O–Ne–Mg white dwarf. In the latter case, this would be the first proof that such white dwarfs can be the outcome of single-star evolution. The new observations were performed to shed light on the origin of this mysterious object.

Abstract: Abstract White dwarfs grow as the cores of red giants and, in particular, carbon-oxygen white dwarfs grow in asymptotic giant branch (AGB) stars. The evolution of an AGB star is a competition between growth of the core and loss of the stellar envelope, typically in a wind. It is complicated by thermal pulses driven periodically by unstable helium shell burning. Dredge up following such pulses delays core growth. The compression at the center of a cold carbon-oxygen core means that carbon ignites when it reaches a mass of 1.38 M⊙. This begins the familiar thermonuclear runaway of the Type Ia supernova (SN Ia). At higher temperatures carbon can ignite more gently and burn mostly to neon to leave a core rich in oxygen, neon and magnesium. Such cores can go on to collapse to neutron stars with a release of only neutrinos. Accepted mass-loss prescriptions for giants mean that the range of masses of single stars that leave carbon-oxygen white dwarfs is somewhere from around 1 to 8 M⊙. We investigate how unusual mass loss, perhaps brought about by interaction with a binary companion, can radically alter the single star picture. Though population syntheses treat some possibilities with various prescriptions, there is sufficient doubt over the physics, the observations, and the implementation of mass loss and binary interaction that there is scope for several more unusual progenitors of carbon-oxygen white dwarfs and hence SNe Ia.

Abstract: Accretion disks in white dwarf systems are believed to be tilted. In a recent publication, the lift force has been suggested to be a source to disk tilt, a source that is likely relevant to all accretion disk systems. Lift is generated by slightly different supersonic gas stream speeds flowing over and under the disk at the bright spot. In this conference proceeding, we focus on whether a brown dwarf donor star accreting onto a white dwarf primary has enough mass to contribute to disk tilt. We also would like to obtain whether a white dwarf - brown dwarf close binary system has enough mass to induce and maintain a disk tilt of four degrees. We adopt SDSS 103533.03+055158.4 as our model system which has a mass transfer rate of (10 \pm 2) x 10-12 M* yr-1. We find that the brown dwarf in SDSS 1035 does not have enough mass to contribute to disk tilt. We find a gross magnitude of the minimum mass transfer rate to be - 10-10 M* yr-1 . We conclude that SDSS 1035 does not seem to have a high enough mass transfer rate to induce and maintain an observable disk tilt. Hence one reason why brown dwarf donor systems may be so difficult to find could be due to their low mass transfer rates which do not induce observable dynamical effects that is typical in white dwarf-red dwarf CVs.

Abstract: 1. Hot White Dwarfs 2. Cool White Dwarfs 3. Stars with Unusual Compositions: Carbon and Oxygen in Cool White Dwarfs 4. Planets orbiting White Dwarfs 5. White Dwarf Circumstellar Disks: Observations 6. The Origin and Evolution of White Dwarf Dust Disks 7. Planetary Nebulae around White Dwarfs: Revelations from the Infrared

Abstract: The faint (g = 16.9) hot white dwarf BOKS 53856 was observed by the Kepler Mission in short cadence mode during mid-2009. Analysis of these observations reveals a highly stable modulation with a period of 6.1375 hr and a 2.46% half-amplitude. The folded light curve has an unusual shape that is difficult to explain in terms of a binary system containing an unseen companion more luminous than an L0 brown dwarf. Optical spectra of BOKS 53856 show a T{sub eff} = 34,000 K, log g = 8.0 DA white dwarf. There are few, if any, known white dwarfs in this temperature range exhibiting photometric variations similar to those we describe. A magnetic spin-modulated white dwarf model can in principle explain the light curve, an interpretation supported by spectral observations of the H{alpha} line showing evidence of Zeeman splitting.

Abstract: The discovery of a limiting mass for white dwarf stars is today usually attributed to Subramanian Chandrasekhar. However, it would seem that an article by Edmund Stoner, which appeared in the Philosophical Magazine in 1930, was the first publication to give a convincing demonstration of the existence of a limiting mass for white dwarfs. We examine here why it is that the contributions of Stoner and others towards this discovery have been largely forgotten.

Abstract: The primary white dwarf of the cataclysmic variable SDSS J074531.92+453829.6 was discovered to exhibit non-radial pulsations in 2006 January. This accreting white dwarf underwent its first recorded dwarf nova outburst in 2006 October, during which its brightness increased by more than 5 mag. A Hubble Space Telescope (HST) ultraviolet spectrum, obtained one year after the outburst, revealed a white dwarf temperature of 16,500 K, hotter than all other known accreting white dwarf pulsators. This implies that the accreting primary white dwarf of SDSS J074531.92+453829.6 was heated to temperatures beyond the instability strip during the outburst. Optical observations acquired a year after the outburst did not reveal any evidence of pulsations, suggesting that the white dwarf had not cooled to quiescence by then. We recently acquired optical high-speed time-series photometry on this cataclysmic variable SDSS J074531.92+453829.6 more than three years after its outburst to find that pulsations have now returned to the primary white dwarf. Moreover, the observed pulsation periods agree with pre-outburst periods within the uncertainties of a few seconds. This discovery is significant because it indicates that the outburst did not affect the interior stellar structure, which governs the observed pulsation frequencies. It also suggests that the surface of the white dwarf has now cooled to quiescence. Using this discovery in addition to the prior HST temperature measurement of 16,500 K, we have been able to constrain the matter accreted during the 2006 outburst. This is the first time an accreting white dwarf was unambiguously observed to resume pulsating after an outburst.

Abstract: We present the first white dwarf mass distributions of a large and homogeneous sample of post-common envelope binaries (PCEBs) and wide white dwarf main-sequence (WDMS) binaries directly obtained from observations. Both distributions are statistically independent, with PCEBs showing a clear concentration of systems towards the low-mass end of the distribution and the white dwarf mass distribution of wide WDMS binaries being similar to that of single white dwarfs. Our results provide evidence that the majority of low-mass (Mwd≲ 0.5 M⊙) white dwarfs are formed in close binaries.