# Posts Tagged white dwarf

## Recent Postings from white dwarf

### The ELM Survey. VII. Orbital Properties of Low Mass White Dwarf Binaries

We present the discovery of 15 extremely low mass (5 < log{g} < 7) white dwarf candidates, 9 of which are in ultra-compact double-degenerate binaries. Our targeted ELM Survey sample now includes 76 binaries. The sample has a lognormal distribution of orbital periods with a median period of 5.4 hr. The velocity amplitudes imply that the binary companions have a normal distribution of mass with 0.76 Msun mean and 0.25 Msun dispersion. Thus extremely low mass white dwarfs are found in binaries with a typical mass ratio of 1:4. Statistically speaking, 95% of the white dwarf binaries have a total mass below the Chandrasekhar mass and thus are not Type Ia supernova progenitors. Yet half of the observed binaries will merge in less than 6 Gyr due to gravitational wave radiation; probable outcomes include single massive white dwarfs and stable mass transfer AM CVn binaries.

### Identifying IGR J14091-6108 as a magnetic CV with a massive white dwarf using X-ray and optical observations

IGR J14091-6108 is a Galactic X-ray source known to have an iron emission line, a hard X-ray spectrum, and an optical counterpart. Here, we report on X-ray observations of the source with XMM-Newton and NuSTAR as well as optical spectroscopy with ESO/VLT and NOAO/SOAR. In the X-rays, this provides data with much better statistical quality than the previous observations, and this is the first report of the optical spectrum. Timing analysis of the XMM data shows a very significant detection of 576.3+/-0.6 s period. The signal has a pulsed fraction of 30%+/-3% in the 0.3-12 keV range and shows a strong drop with energy. The optical spectra show strong emission lines with significant variability in the lines and continuum, indicating that they come from an irradiated accretion disk. Based on these measurements, we identify the source as a magnetic Cataclysmic Variable of Intermediate Polar (IP) type where the white dwarf spin period is 576.3 s. The X-ray spectrum is consistent with the continuum emission mechanism being due to thermal Bremsstrahlung, but partial covering absorption and reflection are also required. In addition, we use the IP mass (IPM) model, which suggests that the white dwarf in this system has a high mass, possibly approaching the Chandrasekhar limit.

### Circumstellar Debris and Pollution at White Dwarf Stars

Circumstellar disks of planetary debris are now known or suspected to closely orbit hundreds of white dwarf stars. To date, both data and theory support disks that are entirely contained within the preceding giant stellar radii, and hence must have been produced during the white dwarf phase. This picture is strengthened by the signature of material falling onto the pristine stellar surfaces; disks are always detected together with atmospheric heavy elements. The physical link between this debris and the white dwarf host abundances enables unique insight into the bulk chemistry of extrasolar planetary systems via their remnants. This review summarizes the body of evidence supporting dynamically active planetary systems at a large fraction of all white dwarfs, the remnants of first generation, main-sequence planetary systems, and hence provide insight into initial conditions as well as long-term dynamics and evolution.

### GW Librae: A unique laboratory for pulsations in an accreting white dwarf

Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of HST ultraviolet spectroscopy taken in 2002, 2010 and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in May~2013, we obtained new HST/COS ultraviolet observations that displayed unexpected behaviour: besides showing variability at ~275s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits high-amplitude variability on a ~4.4h time-scale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to ~30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star

### The White Dwarf Binary Pathways Survey I: A sample of FGK stars with white dwarf companions

The number of white dwarf plus main-sequence star binaries has increased rapidly in the last decade, jumping from only ~30 in 2003 to over 3000. However, in the majority of known systems the companion to the white dwarf is a low mass M dwarf, since these are relatively easy to identify from optical colours and spectra. White dwarfs with more massive FGK type companions have remained elusive due to the large difference in optical brightness between the two stars. In this paper we identify 934 main-sequence FGK stars from the Radial Velocity Experiment (RAVE) survey in the southern hemisphere and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey in the northern hemisphere, that show excess flux at ultraviolet wavelengths which we interpret as the likely presence of a white dwarf companion. We obtained Hubble Space Telescope ultraviolet spectra for nine systems which confirmed that the excess is indeed caused, in all cases, by a hot compact companion, eight being white dwarfs and one a hot subdwarf or pre-helium white dwarf, demonstrating that this sample is very clean. We also address the potential of this sample to test binary evolution models and type Ia supernovae formation channels.

### GK Per and EX Hya: Intermediate polars with small magnetospheres

Observed hard X-ray spectra of intermediate polars are determined mainly by the accretion flow velocity at the white dwarf surface, which is normally close to the free-fall velocity. This allows to estimate the white dwarf masses as the white dwarf mass-radius relation M-R and the expected free-fall velocities at the surface are well known. This method is widely used, however, derived white dwarf masses M can be systematically underestimated because the accretion flow is stopped at and re-accelerates from the magnetospheric boundary R_m, and therefore, its velocity at the surface will be lower than free-fall.To avoid this problem we computed a two-parameter set of model hard X-ray spectra, which allows to constrain a degenerate M - R_m dependence. On the other hand, previous works showed that power spectra of accreting X-ray pulsars and intermediate polars exhibit breaks at the frequencies corresponding to the Keplerian frequencies at the magnetospheric boundary. Therefore, the break frequency \nu_b in an intermediate polar power spectrum gives another relation in the M - R_m plane. The intersection of the two dependences allows, therefore, to determine simultaneously the white dwarf mass and the magnetospheric radius. To verify the method we analyzed the archival Suzaku observation of EX Hya obtaining M /M_sun= 0.73 \pm 0.06 and R_ m / R = 2.6 \pm 0.4 consistent with the values determined by other authors. Subsequently, we applied the same method to a recent NuSTAR observation of another intermediate polar GK~Per performed during an outburst and found M/M_sun = 0.86 \pm 0.02 and R_ m / R = 2.8 \pm 0.2. The long duration observations of GK Per in quiescence performed by Swift/BAT and INTEGRAL observatories indicate increase of magnetosphere radius R_m at lower accretion rates.

### The type Iax supernova, SN 2015H: a white dwarf deflagration candidate

We present results based on observations of SN 2015H which belongs to the small group of objects similar to SN 2002cx, otherwise known as type Iax supernovae. The availability of deep pre-explosion imaging allowed us to place tight constraints on the explosion epoch. Our observational campaign began approximately one day post-explosion, and extended over a period of about 150 days post maximum light, making it one of the best observed objects of this class to date. We find a peak magnitude of M$_r$ = -17.27 $\pm$ 0.07, and a ($\Delta m_{15})_r$ = 0.69 $\pm$ 0.04. Comparing our observations to synthetic spectra generated from simulations of deflagrations of Chandrasekhar mass carbon-oxygen white dwarfs, we find reasonable agreement with models of weak deflagrations that result in the ejection of ~0.2 M$_{\odot}$ of material containing ~0.07 M$_{\odot}$ of 56Ni. The model light curve however, evolves more rapidly than observations, suggesting that a higher ejecta mass is to be favoured. Nevertheless, empirical modelling of the pseudo-bolometric light curve suggests that $\lesssim$0.6 M_sun of material was ejected, implying that the white dwarf is not completely disrupted, and that a bound remnant is a likely outcome.

### The post-outburst pulsations of the accreting white dwarf in the cataclysmic variable GW Librae

We present new time-series photometry of the accreting pulsating white dwarf system GW Librae obtained in 2012 and 2013 at the University of Canterbury Mt John Observatory in New Zealand. Our 2012 data show the return of a $\sim$19 minute periodicity that was previously detected in 2008. This pulsation mode was a dominant feature of our quality May 2012 data set, which consisted of six contiguous nights; a detailed analysis indicated a degree of frequency variability. We show by comparison with the previously identified pulsation modes that this periodicity is best explained as a new mode, and that the quasi-stability of the periods appears to be a general feature of the pulsations in these systems. We also find a previously unreported 3-hour modulation period, which we believe to be related to the known two and four hour periods of so far unknown origin.

### Discovery of spin-up in the X-ray pulsar companion of the hot subdwarf HD 49798

The hot subdwarf HD 49798 has an X-ray emitting compact companion with a spin-period of 13.2 s and a dynamically measured mass of 1.28+/-0.05 M_sun, consistent with either a neutron star or a white dwarf. Using all the available XMM-Newton and Swift observations of this source, we could perform a phase-connected timing analysis extending back to the ROSAT data obtained in 1992. We found that the pulsar is spinning up at a rate of (2.15+/-0.05)x10^{-15} s/s. This result is best interpreted in terms of a neutron star accreting from the wind of its subdwarf companion, although the remarkably steady period derivative over more than 20 years is unusual in wind-accreting neutron stars. The possibility that the compact object is a massive white dwarf accreting through a disk cannot be excluded, but it requires a larger distance and/or properties of the stellar wind of HD 49798 different from those derived from the modelling of its optical/UV spectra.

### The SDSS spectroscopic catalogue of white dwarf-main sequence binaries: new identifications from DR9-12

We present an updated version of the spectroscopic catalogue of white dwarf-main sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS). We identify 939 WDMS binaries within the data releases (DR) 9-12 of SDSS plus 40 objects from DR 1-8 that we missed in our previous works, 646 of which are new. The total number of spectroscopic SDSS WDMS binaries increases to 3294. This is by far the largest and most homogeneous sample of compact binaries currently available. We use a decomposition/fitting routine to derive the stellar parameters of all systems identified here (white dwarf effective temperatures, surface gravities and masses, and secondary star spectral types). The analysis of the corresponding stellar parameter distributions shows that the SDSS WDMS binary population is seriously affected by selection effects. We also measure the NaI 8183.27, 8194.81 absorption doublet and Halpha emission radial velocities (RV) from all SDSS WDMS binary spectra identified in this work. 98 objects are found to display RV variations, 62 of which are new. The RV data are sufficient enough to estimate the orbital periods of three close binaries.

### The crowded magnetosphere of the post common envelope binary QS Virginis

We present high speed photometry and high resolution spectroscopy of the eclipsing post common envelope binary QS Virginis (QS Vir). Our UVES spectra span multiple orbits over more than a year and reveal the presence of several large prominences passing in front of both the M star and its white dwarf companion, allowing us to triangulate their positions. Despite showing small variations on a timescale of days, they persist for more than a year and may last decades. One large prominence extends almost three stellar radii from the M star. Roche tomography reveals that the M star is heavily spotted and that these spots are long-lived and in relatively fixed locations, preferentially found on the hemisphere facing the white dwarf. We also determine precise binary and physical parameters for the system. We find that the 14,220 +/- 350K white dwarf is relatively massive, 0.782 +/- 0.013Ms, and has a radius of 0.01068 +/- 0.00007Rs, consistent with evolutionary models. The tidally distorted M star has a mass of 0.382 +/- 0.006Ms and a radius of 0.381 +/- 0.003Rs, also consistent with evolutionary models. We find that the magnesium absorption line from the white dwarf is broader than expected. This could be due to rotation (implying a spin period of only ~700 seconds), or due to a weak (~100kG) magnetic field, we favour the latter interpretation. Since the M star's radius is still within its Roche lobe and there is no evidence that its over-inflated we conclude that QS Vir is most likely a pre-cataclysmic binary just about to become semi-detached.

### Stellar laboratories. VII. New Kr IV - VII oscillator strengths and an improved spectral analysis of the hot, hydrogen-deficient DO-type white dwarf RE0503-289

For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. New of Kr IV - VII oscillator strengths for a large number of lines allow to construct more detailed model atoms for our NLTE model-atmosphere calculations. This enables us to search for additional Kr lines in observed spectra and to improve Kr abundance determinations. We calculated Kr IV - VII oscillator strengths to consider radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Kr lines exhibited in high-resolution and high-S/N ultraviolet (UV) observations of the hot white dwarf RE 0503-289. We reanalyzed the effective temperature and surface gravity and determined Teff = 70 000 +/- 2000 K and log (g / cm/s**2) = 7.5 +/- 0.1. We newly identified ten Kr V lines and one Kr VI line in the spectrum of RE 0503-289. We measured a Kr abundance of -3.3 +/- 0.3 (logarithmic mass fraction). We discovered that the interstellar absorption toward RE 0503-289 has a multi-velocity structure within a radial-velocity interval of -40 km/s < vrad < +18 km/s. Reliable measurements and calculations of atomic data are a prerequisite for state-of-the-art NLTE stellar-atmosphere modeling. Observed Kr V - VII line profiles in the UV spectrum of the white dwarf RE 0503-289 were simultaneously well reproduced with our newly calculated oscillator strengths.

### An Ultramassive 1.28 M$_\odot$ White Dwarf in NGC 2099

With the Keck I Low-Resolution Imaging Spectrometer we have observed nine white dwarf candidates in the very rich open cluster NGC 2099 (M37). The spectroscopy shows seven to be DA white dwarfs, one to be a DB white dwarf, and one to be a DZ white dwarf. Three of these DA white dwarfs are consistent with singly evolved cluster membership: an ultramassive (1.28$^{+0.05}_{-0.08}$ M$_\odot$) and two intermediate-mass (0.70 and 0.75 M$_\odot$) white dwarfs. Analysis of their cooling ages allows us to calculate their progenitor masses and establish new constraints on the initial-final mass relation. The intermediate-mass white dwarfs are in strong agreement with previous work over this mass regime. The ultramassive white dwarf has $V$ = 24.5, $\sim$2 mag fainter than the other two remnants. The spectrum of this star has lower quality, so the derived stellar properties (e.g., T$_{\rm eff}$, log g) have uncertainties that are several times higher than the brighter counterparts. We measure these uncertainties and establish the star's final mass as the highest-mass white dwarf discovered thus far in a cluster, but we are unable to calculate its progenitor mass because at this high mass and cooler T$_{\rm eff}$ its inferred cooling age is highly sensitive to its mass. At the highest temperatures, however, this sensitivity of cooling age to an ultramassive white dwarf's mass is only moderate. This demonstrates that future investigations of the upper-mass end of the initial-final mass relation must identify massive, newly formed white dwarfs (i.e., in young clusters with ages 50-150 Myr).

### Constraining white dwarf structure and neutrino physics in 47 Tucanae

We present a robust statistical analysis of the white dwarf cooling sequence in 47 Tucanae. We combine HST UV and optical data in the core of the cluster, Modules for Experiments in Stellar Evolution (MESA) white dwarf cooling models, white dwarf atmosphere models, artificial star tests, and a Markov Chain Monte Carlo (MCMC) sampling method to fit white dwarf cooling models to our data directly. We use a technique known as the unbinned maximum likelihood to fit these models to our data without binning. We use these data to constrain neutrino production and the thickness of the hydrogen layer in these white dwarfs. The data prefer thicker hydrogen layers $(q_\mathrm{H}=3.2\e{-5})$ and we can strongly rule out thin layers $(q_\mathrm{H}=10^{-6})$. The neutrino rates currently in the models are consistent with the data. This analysis does not provide a constraint on the number of neutrino species.

### Extreme abundance ratios in the polluted atmosphere of the cool white dwarf NLTT19868

We present an analysis of intermediate-dispersion spectra and photometric data of the newly identified cool, polluted white dwarf NLTT19868. The spectra obtained with X-shooter on the Very Large Telescope (VLT)-Melipal show strong lines of calcium, and several lines of magnesium, aluminium and iron. We use these spectra and the optical-to-near infrared spectral energy distribution to constrain the atmospheric parameters of NLTT19868. Our analysis shows that NLTT19868 is iron poor with respect to aluminium and calcium. A comparison with other cool, polluted white dwarfs shows that the Fe to Ca abundance ratio (Fe/Ca) varies by up to approximately two orders of magnitudes over a narrow temperature range with NLTT19868 at one extremum in the Fe/Ca ratio and, in contrast, NLTT888 at the other extremum. The sample shows evidence of extreme diversity in the composition of the accreted material: In the case of NLTT888, the inferred composition of the accreted matter is akin to iron-rich planetary core composition, while in the case of NLTT19868 it is close to mantle or bulk-Earth composition depleted by subsequent chemical separation at the bottom of the convection zone.

### The white dwarf population within 40 pc of the Sun

The white dwarf luminosity function is an important tool to understand the properties of the Solar neighborhood, like its star formation history, and its age. Here we present a population synthesis study of the white dwarf population within 40~pc from the Sun, and compare the results of this study with the properties of the observed sample. We use a state-of-the-art population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable white dwarf cooling sequences, an accurate description of the Galactic neighborhood, and a realistic treatment of all the known observational biases and selection procedures. We find a good agreement between our theoretical models and the observed data. In particular, our simulations reproduce a previously unexplained feature of the bright branch of the white dwarf luminosity function, which we argue is due to a recent episode of star formation. We also derive the age of the Solar neighborhood employing the position of the observed cut-off of the white dwarf luminosity function, obtaining ~8.9+-0.2 Gyr. We conclude that a detailed description of the ensemble properties of the population of white dwarfs within 40pc of the Sun allows us to obtain interesting constraints on the history of the Solar neighborhood.

### A Numerical Model for Accretion in Intermediate Polars with Dipolar Magnetic Fields

A three-dimensional numerical model for an accretion process investigation in the magnetosphere of a white dwarf in magnetic cataclysmic variables is developed. The model assumes that the white dwarf has a dipole magnetic field with its symmetry axis inclined to the rotation axis. The model is based on the equations of modified MHD, that describe the mean flow parameters in the wave MHD turbulence. Diffusion of the magnetic field and radiative heating and cooling are taken into account. The suitability of the model is confirmed by modeling the accretion in a typical intermediate polar. The computations show that a magnetosphere forms around the accretor, with the accretion occurring via columns. The accretion columns have a curtain-like shape, and arc-shaped zones of energy release form on the surface of the white dwarf in the magnetic poles area as a result of the matter infall.

### Stellar laboratories. VI. New Mo IV - VII oscillator strengths and the molybdenum abundance in the hot white dwarfs G191-B2B and RE0503-289

For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. To identify molybdenum lines in the ultraviolet (UV) spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE0503-289 and to determine their photospheric Mo abundances, newly calculated Mo IV - VII oscillator strengths are used. We identified twelve Mo V and nine Mo VI lines in the UV spectrum of RE0503-289 and measured a photospheric Mo abundance of 1.2 - 3.0 x 10**-4 (mass fraction, 22500 - 56400 times the solar abundance). In addition, from the As V and Sn IV resonance lines, we measured mass fractions of arsenic (0.5 - 1.3 x 10**-5, about 300 - 1200 times solar) and tin (1.3 - 3.2 x 10**-4, about 14300 35200 times solar). For G191-B2B, upper limits were determined for the abundances of Mo (5.3 x 10**-7, 100 times solar) and, in addition, for Kr (1.1 x 10**-6, 10 times solar) and Xe (1.7 x 10**-7, 10 times solar). The arsenic abundance was determined (2.3 - 5.9 x 10**-7, about 21 - 53 times solar). A new, registered German Astrophysical Virtual Observatory (GAVO) service, TOSS, has been constructed to provide weighted oscillator strengths and transition probabilities. Reliable measurements and calculations of atomic data are a prerequisite for stellar-atmosphere modeling. Observed Mo V - VI line profiles in the UV spectrum of the white dwarf RE0503-289 were well reproduced with our newly calculated oscillator strengths. For the first time, this allowed to determine the photospheric Mo abundance in a white dwarf.

### White Dwarf Mergers on Adaptive Meshes I. Methodology and Code Verification

The Type Ia supernova progenitor problem is one of the most perplexing and exciting problems in astrophysics, requiring detailed numerical modeling to complement observations of these explosions. One possible progenitor that has merited recent theoretical attention is the white dwarf merger scenario, which has the potential to naturally explain many of the observed characteristics of Type Ia supernovae. To date there have been relatively few self-consistent simulations of merging white dwarf systems using mesh-based hydrodynamics. This is the first paper in a series describing simulations of these systems using a hydrodynamics code with adaptive mesh refinement. In this paper we describe our numerical methodology and discuss our implementation in the compressible hydrodynamics code CASTRO, which solves the Euler equations, and the Poisson equation for self-gravity, and couples the gravitational and rotation forces to the hydrodynamics. Standard techniques for coupling gravitation and rotation forces to the hydrodynamics do not adequately conserve the total energy of the system for our problem, but recent advances in the literature allow progress and we discuss our implementation here. We present a set of test problems demonstrating the extent to which our software sufficiently models a system where large amounts of mass are advected on the computational domain over long timescales. Future papers in this series will describe our treatment of the initial conditions of these systems and will examine the early phases of the merger to determine its viability for triggering a thermonuclear detonation.

### White Dwarf Mergers on Adaptive Meshes I. Methodology and Code Verification [Replacement]

The Type Ia supernova progenitor problem is one of the most perplexing and exciting problems in astrophysics, requiring detailed numerical modeling to complement observations of these explosions. One possible progenitor that has merited recent theoretical attention is the white dwarf merger scenario, which has the potential to naturally explain many of the observed characteristics of Type Ia supernovae. To date there have been relatively few self-consistent simulations of merging white dwarf systems using mesh-based hydrodynamics. This is the first paper in a series describing simulations of these systems using a hydrodynamics code with adaptive mesh refinement. In this paper we describe our numerical methodology and discuss our implementation in the compressible hydrodynamics code CASTRO, which solves the Euler equations, and the Poisson equation for self-gravity, and couples the gravitational and rotation forces to the hydrodynamics. Standard techniques for coupling gravitation and rotation forces to the hydrodynamics do not adequately conserve the total energy of the system for our problem, but recent advances in the literature allow progress and we discuss our implementation here. We present a set of test problems demonstrating the extent to which our software sufficiently models a system where large amounts of mass are advected on the computational domain over long timescales. Future papers in this series will describe our treatment of the initial conditions of these systems and will examine the early phases of the merger to determine its viability for triggering a thermonuclear detonation.

### Empirical consequential angular momentum loss can solve long standing problems of CV evolution

The observed orbital period distribution of cataclysmic variables (CVs), the space density derived from observations, and the observed orbital period minimum are known to disagree with theoretical predictions since decades. More recently, the white dwarf (WD) masses in CVs have been found to significantly exceed those of single WDs, which is in contrast to theoretical expectations as well. We here claim that all these problems are related and can be solved if CVs with low-mass white dwarfs are driven into dynamically unstable mass transfer due to consequential angular momentum loss (CAML). Indeed, assuming CAML increases as a function of decreasing white dwarf mass can bring into agreement the predictions of binary population models and the observed properties of the CV population. We speculate that a common envelope like evolution of CVs with low-mass WDs following a nova eruption might be the physical process behind our empirical prescription of CAML.

### A comparative analysis of the observed white dwarf cooling sequence from globular clusters

We report our study of features at the observed red end of the white dwarf cooling sequences for three Galactic globular clusters: NGC\,6397, 47\,Tucanae and M\,4. We use deep colour-magnitude diagrams constructed from archival Hubble Space Telescope (ACS) to systematically investigate the blue turn at faint magnitudes and the age determinations for each cluster. We find that the age difference between NGC\,6397 and 47\,Tuc is 1.98$^{+0.44}_{-0.26}$\,Gyr, consistent with the picture that metal-rich halo clusters were formed later than metal-poor halo clusters. We self-consistently include the effect of metallicity on the progenitor age and the initial-to-final mass relation. In contrast with previous investigations that invoked a single white dwarf mass for each cluster, the data shows a spread of white dwarf masses that better reproduce the shape and location of the blue turn. This effect alone, however, does not completely reproduce the observational data - the blue turn retains some mystery. In this context, we discuss several other potential problems in the models. These include possible partial mixing of H and He in the atmosphere of white dwarf stars, the lack of a good physical description of the collision-induced absorption process and uncertainties in the opacities at low temperatures. The latter are already known to be significant in the description of the cool main sequence. Additionally, we find that the present day local mass function of NGC\,6397 is consistent with a top-heavy type, while 47\,Tuc presents a bottom-heavy profile.

### A large, long-lived structure near the trojan L5 point in the post common-envelope binary SDSS J1021+1744

SDSS J1021+1744 is a detached, eclipsing white dwarf / M dwarf binary discovered in the Sloan Digital Sky Survey. Outside the primary eclipse, the light curves of such systems are usually smooth and characterised by low-level variations caused by tidal distortion and heating of the M star component. Early data on SDSS J1021+1744 obtained in June 2012 was unusual in showing a dip in flux of uncertain origin shortly after the white dwarf's eclipse. Here we present high-time resolution, multi-wavelength observations of 35 more eclipses over 1.3 years, showing that the dip has a lifetime extending over many orbits. Moreover the "dip" is in fact a series of dips that vary in depth, number and position, although they are always placed in the phase interval 1.06 to 1.26 after the white dwarf's eclipse, near the L5 point in this system. Since SDSS J1021+1744 is a detached binary, it follows that the dips are caused by the transit of the white dwarf by material around the Lagrangian L5 point. A possible interpretation is that they are the signatures of prominences, a phenomenon already known from H-alpha observations of rapidly rotating single stars as well as binaries. What makes SDSS J1021+1744 peculiar is that the material is dense enough to block continuum light. The dips appear to have finally faded out around 2015 May after the first detection by Parsons et al. in 2012, suggesting a lifetime of years.

### A Dark Spot on a Massive White Dwarf

We present the serendipitous discovery of eclipse-like events around the massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We selected J1529+2928 for time-series photometry based on its spectroscopic temperature and surface gravity, which place it near the ZZ Ceti instability strip. Instead of pulsations, we detect photometric dips from this white dwarf every 38 minutes. Follow-up optical spectroscopy observations with Gemini reveal no significant radial velocity variations, ruling out stellar and brown dwarf companions. A disintegrating planet around this white dwarf cannot explain the observed light curves in different filters. Given the short period, the source of the photometric dips must be a dark spot that comes into view every 38 min due to the rotation of the white dwarf. Our optical spectroscopy does not show any evidence of Zeeman splitting of the Balmer lines, limiting the magnetic field strength to B<70 kG. Since up to 15% of white dwarfs display kG magnetic fields, such eclipse-like events should be common around white dwarfs. We discuss the potential implications of this discovery on transient surveys targeting white dwarfs, like the K2 mission and the Large Synoptic Survey Telescope.

### Evidence for Gas from a Disintegrating Extrasolar Asteroid

We report high-resolution spectroscopic observations of WD 1145+017 -- a white dwarf that recently has been found to be transitted by multiple asteroid-sized objects within its tidal radius. We have discovered numerous circumstellar absorption lines with linewidths of $\sim$ 300 km s$^{-1}$ from Mg, Ca, Ti, Cr, Mn, Fe and Ni, possibly from several gas streams produced by collisions among the actively disintegrating objects. The atmosphere of WD 1145+017 is polluted with 11 heavy elements, including O, Mg, Al, Si, Ca, Ti, V:, Cr, Mn, Fe and Ni. Evidently, we are witnessing the active disintegration and subsequent accretion of an extrasolar asteroid.

### Discovery of an eclipsing dwarf nova in the ancient nova shell Te 11

We report on the discovery of an eclipsing dwarf nova (DN) inside the peculiar, bilobed nebula Te 11. Modelling of high-speed photometry of the eclipse finds the accreting white dwarf to have a mass 1.18 M$_\odot$ and temperature 13 kK. The donor spectral type of M2.5 results in a distance of 330 pc, colocated with Barnard's loop at the edge of the Orion-Eridanus superbubble. The perplexing morphology and observed bow shock of the slowly-expanding nebula may be explained by strong interactions with the dense interstellar medium in this region. We match the DN to the historic nova of 483 CE in Orion and postulate that the nebula is the remnant of this eruption. This connection supports the millennia time scale of the post-nova transition from high to low mass-transfer rates. Te 11 constitutes an important benchmark system for CV and nova studies as the only eclipsing binary out of just three DNe with nova shells.

### Doppler-imaging of the planetary debris disc at the white dwarf SDSS J122859.93+104032.9

Debris discs which orbit white dwarfs are signatures of remnant planetary systems. We present twelve years of optical spectroscopy of the metal-polluted white dwarf SDSS J1228+1040, which shows a steady variation in the morphology of the 8600 {\AA} Ca II triplet line profiles from the gaseous component of its debris disc. We identify additional emission lines of O I, Mg I, Mg II, Fe II and Ca II in the deep co-added spectra. These emission features (including Ca H & K) exhibit a wide range in strength and morphology with respect to each other and to the Ca II triplet, indicating different intensity distributions of these ionic species within the disc. Using Doppler tomography we show that the evolution of the Ca II triplet profile can be interpreted as the precession of a fixed emission pattern with a period in the range 24-30 years. The Ca II line profiles vary on time-scales that are broadly consistent with general relativistic precession of the debris disc.

### A wide binary trigger for white dwarf pollution

Metal pollution in white dwarf atmospheres is likely to be a signature of remnant planetary systems. Most explanations for this pollution predict a sharp decrease in the number of polluted systems with white dwarf cooling age. Observations do not confirm this trend, and metal pollution in old (1-5 Gyr) white dwarfs is difficult to explain. We propose an alternative, time-independent mechanism to produce the white dwarf pollution. The orbit of a wide binary companion can be perturbed by Galactic tides, approaching close to the primary star for the first time after billions of years of evolution on the white dwarf branch. We show that such a close approach perturbs a planetary system orbiting the white dwarf, scattering planetesimals onto star-grazing orbits, in a manner that could pollute the white dwarf's atmosphere. Our estimates find that this mechanism is likely to contribute to metal pollution, alongside other mechanisms, in up to a few percent of an observed sample of white dwarfs with wide binary companions, independent of white dwarf age. This age independence is the key difference between this wide binary mechanism and others mechanisms suggested in the literature to explain white dwarf pollution. Current observational samples are not large enough to assess whether this mechanism makes a significant contribution to the population of polluted white dwarfs, for which better constraints on the wide binary population are required, such as those that will be obtained in the near future with Gaia.

### Discovery of near-ultraviolet counterparts to millisecond pulsars in the globular cluster 47 Tucanae

We report the discovery of the likely white dwarf companions to radio millisecond pulsars 47 Tuc Q and 47 Tuc S in the globular cluster 47 Tucanae. These blue stars were found in near-ultraviolet images from the Hubble Space Telescope for which we derived accurate absolute astrometry, and are located at positions consistent with the radio coordinates to within 0.016 arcsec (0.2sigma). We present near-ultraviolet and optical colours for the previously identified companion to millisecond pulsar 47 Tuc U, and we unambiguously confirm the tentative prior identifications of the optical counterparts to 47 Tuc T and 47 Tuc Y. For the latter, we present its radio-timing solution for the first time. We find that all five near-ultraviolet counterparts have U300-B390 colours that are consistent with He white dwarf cooling models for masses ~0.16-0.3 Msun and cooling ages within ~0.1-6 Gyr. The Ha-R625 colours of 47 Tuc U and 47 Tuc T indicate the presence of a strong Ha absorption line, as expected for white dwarfs with an H envelope.

### Discovery of near-ultraviolet counterparts to millisecond pulsars in the globular cluster 47 Tucanae [Replacement]

We report the discovery of the likely white dwarf companions to radio millisecond pulsars 47 Tuc Q and 47 Tuc S in the globular cluster 47 Tucanae. These blue stars were found in near-ultraviolet images from the Hubble Space Telescope for which we derived accurate absolute astrometry, and are located at positions consistent with the radio coordinates to within 0.016 arcsec (0.2sigma). We present near-ultraviolet and optical colours for the previously identified companion to millisecond pulsar 47 Tuc U, and we unambiguously confirm the tentative prior identifications of the optical counterparts to 47 Tuc T and 47 Tuc Y. For the latter, we present its radio-timing solution for the first time. We find that all five near-ultraviolet counterparts have U300-B390 colours that are consistent with He white dwarf cooling models for masses ~0.16-0.3 Msun and cooling ages within ~0.1-6 Gyr. The Ha-R625 colours of 47 Tuc U and 47 Tuc T indicate the presence of a strong Ha absorption line, as expected for white dwarfs with an H envelope.

### Growing White Dwarfs to the Chandrasekhar Limit: The Parameter Space of the Single Degenerate SNIa Channel

Can a white dwarf, accreting hydrogen-rich matter from a non-degenerate companion star, ever exceed the Chandrasekhar mass and explode as a type Ia supernova? We explore the range of accretion rates that allow a white dwarf (WD) to secularly grow in mass, and derive limits on the accretion rate and on the initial mass that will allow it to reach $1.4M_\odot$ --- the Chandrasekhar mass. We follow the evolution through a long series of hydrogen flashes, during which a thick helium shell accumulates. This determines the effective helium mass accretion rate for long-term, self-consistent evolutionary runs with helium flashes. We find that net mass accumulation always occurs despite helium flashes. Although the amount of mass lost during the first few helium shell flashes is a significant fraction of that accumulated prior to the flash, that fraction decreases with repeated helium shell flashes. Eventually no mass is ejected at all during subsequent flashes. This unexpected result occurs because of continual heating of the WD interior by the helium shell flashes near its surface. The effect of heating is to lower the electron degeneracy throughout the WD, and especially in the outer layers. This key result yields helium burning that is quasi-steady state, instead of explosive. We thus find a remarkably large parameter space within which long-term, self-consistent simulations show that a WD can grow in mass and reach the Chandrasekhar limit, despite its helium flashes.

### Swift J0525.6+2416 and IGR J04571+4527: two new hard X-ray selected magnetic cataclysmic variables identified with XMM-Newton

IGR J04571+4527 and Swift J0525.6+2416 are two hard X-ray sources detected in the Swift/BAT and INTEGRAL/IBIS surveys. They were proposed to be magnetic cataclysmic variables of the Intermediate Polar (IP) type, based on optical spectroscopy. IGR J04571+4527 also showed a 1218 s optical periodicity, suggestive of the rotational period of a white dwarf, further pointing towards an IP classification. We here present detailed X-ray (0.3-10 keV) timing and spectral analysis performed with XMM-Newton, complemented with hard X-ray coverage (15-70 keV) from Swift/BAT. These are the first high signal to noise observations in the soft X-ray domain for both sources, allowing us to identify the white dwarf X-ray spin period of Swift J0525.6+2416 (226.28 s), and IGR J04571+4527 (1222.6 s). A model consisting of multi-temperature optically thin emission with complex absorption adequately fits the broad-band spectrum of both sources. We estimate a white dwarf mass of about 1.1 and 1.0 solar masses for IGR J04571+4527 and Swift J0525.6+2416, respectively. The above characteristics allow us to unambiguously classify both sources as IPs, confirming the high incidence of this subclass among hard X-ray emitting Cataclysmic Variables.

### A second case of outbursts in a pulsating white dwarf observed by Kepler

We present observations of a new phenomenon in pulsating white dwarf stars: large-amplitude outbursts at timescales much longer than the pulsation periods. The cool (Teff = 11,010 K), hydrogen-atmosphere pulsating white dwarf PG 1149+057 was observed nearly continuously for more than 78.8 d by the extended Kepler mission in K2 Campaign 1. The target showed 10 outburst events, recurring roughly every 8 d and lasting roughly 15 hr, with maximum flux excursions up to 45% in the Kepler bandpass. We demonstrate that the outbursts affect the pulsations and therefore must come from the white dwarf. Additionally, we argue that these events are not magnetic reconnection flares, and are most likely connected to the stellar pulsations and the relatively deep surface convection zone. PG 1149+057 is now the second cool pulsating white dwarf to show this outburst phenomenon, after the first variable white dwarf observed in the Kepler mission, KIC 4552982. Both stars have the same effective temperature, within the uncertainties, and are among the coolest known pulsating white dwarfs of typical mass. These outbursts provide fresh observational insight into the red edge of the DAV instability strip and the eventual cessation of pulsations in cool white dwarfs.

### Constraining Neutrino Cooling using the Hot White Dwarf Luminosity Function in the Globular Cluster 47 Tucanae

We present Hubble Space Telescope observations of the upper part (T_eff> 10 000 K) of the white dwarf cooling sequence in the globular cluster 47 Tucanae and measure a luminosity function of hot white dwarfs. Comparison with previous determinations from large scale field surveys indicates that the previously determined plateau at high effective temperatures is likely a selection effect, as no such feature is seen in this sample. Comparison with theoretical models suggests that the current estimates of white dwarf neutrino emission (primarily by the plasmon channel) are accurate, and variations are restricted to no more than a factor of two globally, at 95% confidence. We use these constraints to place limits on various proposed exotic emission mechanisms, including a non-zero neutrino magnetic moment, formation of axions, and emission of Kaluza-Klein modes into extra dimensions.

### Effects of strong magnetic fields and rotation on white dwarf structure [Replacement]

In this work we compute models for relativistic white dwarfs in the presence of strong magnetic fields. These models possibly contribute to super-luminous SNIa. With an assumed axi-symmetric and poloidal magnetic field, we study the possibility of existence of super-Chandrasekhar magnetized white dwarfs by solving numerically the Einstein-Maxwell equations, by means of a pseudo-spectral method. We obtain a self-consistent rotating and non-rotating magnetized white dwarf models. According to our results, a maximum mass for a static magnetized white dwarf is 2.13 $\rm{M_{\odot}}$ in the Newtonian case and 2.09 $\rm{M_{\odot}}$ while taking into account general relativistic effects. Furthermore, we present results for rotating magnetized white dwarfs. The maximum magnetic field strength reached at the center of white dwarfs is of the order of $10^{15}\,$G in the static case, whereas for magnetized white dwarfs, rotating with the Keplerian angular velocity, is of the order of $10^{14}\,$G.

### Effects of strong magnetic fields and rotation on white dwarf structure

In this paper we compute relativistic stars models for the structure of white dwarfs under the influence of strong magnetic field and rotation. The magnetic field is assumed to be poloidal and axisymmetric. We find a maximum mass for a static magnetized white dwarf of about 2.13 $\rm{M_{\odot}}$ in the Newtonian case and a value of 2.09 $\rm{M_{\odot}}$ taking into account general relativistic effects. We also present properties of uniformly rotating white dwarfs and we show that the maximum mass is shifted from a mass of $\sim$ 1.40 $\rm{M_{\odot}}$ for non-rotating white dwarf to $\sim$ 1.45 $\rm{M_{\odot}}$ in the keplerian limit. We present also results for rotating magnetized white dwarfs calculated in a self$-$consistent way by solving the Maxwell and Einstein equations together. The maximum field strength obtained is about $10^{15}\,$G at the center of the star in the static and $10^{14}\,$G in the rotating case.

### Getting to know Classical Novae with Swift

Novae have been reported as transients for more than two thousand years. Their bright optical outbursts are the result of explosive nuclear burning of gas accreted from a binary companion onto a white dwarf. Novae containing a white dwarf close to the Chandrasekhar mass limit and accreting at a high rate are potentially the unknown progenitors of the type Ia supernovae used to measure the acceleration of the Universe. Swift X-ray observations have radically transformed our view of novae by providing dense monitoring throughout the outburst, revealing new phenomena in the super-soft X-rays from the still-burning white dwarf such as early extreme variability and half- to one-minute timescale quasi-periodic oscillations. The distinct evolution of this emission from the harder X-ray emission due to ejecta shocks has been clearly delineated. Soft X-ray observations allow the mass of the white dwarf, the mass burned and the mass ejected to be estimated. In combination with observations at other wavelengths, including the high spectral resolution observations of the large X-ray observatories, high resolution optical and radio imaging, radio monitoring, optical spectroscopy, and the detection of GeV gamma-ray emission from recent novae, models of the explosion have been tested and developed. I review nine novae for which Swift has made a significant impact; these have shown the signature of the components in the interacting binary system in addition to the white dwarf: the re-formed accretion disk, the companion star and its stellar wind.

### Unambiguous Detection of Reflection in Magnetic Cataclysmic Variables: Joint NuSTAR-XMM-Newton Observations of Three Intermediate Polars

In magnetic cataclysmic variables (CVs), X-ray emission regions are located close to the white dwarf surface, which is expected to reflect a significant fraction of intrinsic X-rays above 10 keV, producing a Compton reflection hump. However, up to now, a secure detection of this effect in magnetic CVs has largely proved elusive because of the limited sensitivity of non-imaging X-ray detectors. Here we report our analysis of joint NuSTAR/XMM-Newton observations of three magnetic CVs, V709 Cas, NY Lup, and V1223 Sgr. The improved hard X-ray sensitivity of the imaging NuSTAR data has resulted in the first robust detection of Compton hump in all three objects, with amplitudes of ~1 or greater in NY Lup, and likely <1.0 in the other two. We also confirm earlier report of a strong spin modulation above 10 keV in V709 Cas, and report the first detection of small spin amplitudes in the others. We interpret this as due to different height of the X-ray emitting region among these objects. A height of ~0.2 white dwarf radii provides a plausible explanation for the low reflection amplitude of V709 Cas. Since emission regions above both poles are visible at certain spin phases, this can also explain the strong hard X-ray spin modulation. A shock height of ~0.05 white dwarf radii can explain our results on V1223 Sgr, while the shock height in NY Lup appears negligible.

### The V471 Tauri System: A Multi-datatype Probe

V471 Tauri, a white dwarf--red dwarf eclipsing binary in the Hyades, is well known for stimulating development of common envelope theory, whereby novae and other cataclysmic variables form from much wider binaries by catastrophic orbit shrinkage. Our evaluation of a recent imaging search that reported negative results for a much postulated third body shows that the object could have escaped detection or may have actually been seen. The balance of evidence continues to favor a brown dwarf companion about 12 AU from the eclipsing binary. A recently developed algorithm finds unified solutions from three datatypes. New radial velocities (RVs) of the red dwarf and BV RCIC light curves are solved simultaneously along with white dwarf and red dwarf RVs from the literature, uvby data, the MOST mission light curve, and 40 years of eclipse timings. Precision-based weighting is the key to proper information balance among the various datasets. Timewise variation of modeled starspots allows unified solution of multiple data eras. Light curve amplitudes strongly suggest decreasing spottedness from 1976 to about 1980, followed by approximately constant spot coverage from 1981 to 2005. An explanation is proposed for lack of noticeable variation in 1981 light curves, in terms of competition between spot and tidal variations. Photometric spectroscopic distance is estimated. The red dwarf mass comes out larger than normal for a K2V star, and even larger than adopted in several structure and evolution papers. An identified cause for this result is that much improved red dwarf RVs curves now exist.

### An independent test of the photometric selection of white dwarf candidates using LAMOST DR3

In Gentile Fusillo et al. (2015) we developed a selection method for white dwarf candidates which makes use of photometry, colours and proper motions to calculate a probability of being a white dwarf (Pwd). The application of our method to the Sloan Digital Sky Survey (SDSS) data release 10 resulted in nearly 66,000 photometrically selected objects with a derived Pwd, approximately 21000 of which are high confidence white dwarf candidates. Here we present an independent test of our selection method based on a sample of spectroscopically confirmed white dwarfs from the LAMOST (Large Sky Area Multi-Fiber Spectroscopic Telescope) survey. We do this by cross matching all our $\sim$66,000 SDSS photometric white dwarf candidates with the over 4 million spectra available in the third data release of LAMOST. This results in 1673 white dwarf candidates with no previous SDSS spectroscopy, but with available LAMOST spectra. Among these objects we identify 309 genuine white dwarfs. We find that our Pwd can efficiently discriminate between confirmed LAMOST white dwarfs and contaminants. Our white dwarf candidate selection method can be applied to any multi-band photometric survey and in this work we conclusively confirm its reliability in selecting white dwarfs without recourse to spectroscopy. We also discuss the spectroscopic completeness of white dwarfs in LAMOST, as well as deriving effective temperatures, surface gravities and masses for the hydrogen-rich atmosphere white dwarfs in the newly identified LAMOST sample.

### Extensive photometry of the WZ Sge-type dwarf nova V455 And (HS2331+3905): detection of negative superhumps and coherence features of the short-period oscillations

We report the results of photometry of the WZ Sge-type dwarf nova V455 And. Observations were obtained over 19 nights in 2013 and 2014. The total duration of the observations was 96 h. We clearly detected three coherent oscillations with periods of 80.376+/-0.003 min, 40.5431+/-0.0004 min and 67.619685+/-0.000040 s. The 67.619685-s period can be the spin period of the white dwarf. The 40.5431-minute period is the first harmonic of the orbital period. The 80.376-minute oscillation can be a negative superhump because its period is 0.9% less than the orbital period. This oscillation was evident both in the data of 2013 and in the data of 2014. These results make V455 And a permanent superhump system which shows negative superhumps. This is also the first detection of persistent negative superhumps in a WZ Sge-type dwarf nova. In addition, the analysis of our data revealed incoherent oscillations with periods in the range 5-6 min, which were observed earlier and accounted for by non-radial pulsations of the white dwarf. Moreover, we clearly detected an oscillation with a period of 67.28+/-0.03 s, which was of a low degree of coherence. This oscillation conforms to the beat between the spin period of the white dwarf and the 3.5-h spectroscopic period, which was discovered earlier and accounted for by the free precession of the white dwarf. Because the 67.28-s period is shorter than the spin period and because the free precession of the white dwarf is retrograde, we account for the 67.28-s oscillation by the free precession of the white dwarf.

### An Extreme-AO Search for Giant Planets around a White Dwarf --VLT/SPHERE performance on a faint target GD 50 [Replacement]

CONTEXT. Little is known about the planetary systems around single white dwarfs although there is strong evidence that they do exist. AIMS. We performed a pilot study with the extreme-AO system on the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) on the Very Large Telescopes (VLT) to look for giant planets around a young white dwarf, GD 50. METHODS. We were awarded science verification time on the new ESO instrument SPHERE. Observations were made with the InfraRed Dual-band Imager and Spectrograph in classical imaging mode in H band. RESULTS. Despite the faintness of the target (14.2 mag in R band), the AO loop was closed and a strehl of 37\% was reached in H band. No objects were detected around GD 50. We achieved a 5-sigma contrast of 6.2, 8.0 and 8.25 mags at 0{\farcs}2, 0{\farcs}4 and 0{\farcs}6 and beyond, respectively. We exclude any substellar objects more massive than 4.0 M$_\textrm{J}$ at 6.2 AU, 2.9 M$_\textrm{J}$ at 12.4 AU and 2.8 M$_\textrm{J}$ at 18.6 AU and beyond. This rivals the previous upper limit set by Spitzer. We further show that SPHERE is the most promising instrument available to search for close-in substellar objects around nearby white dwarfs.

### An Extreme-AO Search for Giant Planets around a White Dwarf --VLT/SPHERE performance on a faint target GD 50

CONTEXT. Little is known about the planetary systems around single white dwarfs although there is strong evidence that they do exist. AIMS. We performed a pilot study with the extreme-AO system on the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) on the Very Large Telescopes (VLT) to look for giant planets around a young white dwarf, GD 50. METHODS. We were awarded science verification time on the new ESO instrument SPHERE. Observations were made with the InfraRed Dual-band Imager and Spectrograph in classical imaging mode in H band. RESULTS. Despite the faintness of the target (14.2 mag in R band), the AO loop was closed and a strehl of 37\% was reached in H band. No objects were detected around GD 50. We achieved a 5-sigma contrast of 6.2, 8.0 and 8.25 mags at 0{\farcs}2, 0{\farcs}4 and 0{\farcs}6 and beyond, respectively. We exclude any substellar objects more massive than 4.0 M$_\textrm{J}$ at 6.2 AU, 2.9 M$_\textrm{J}$ at 12.4 AU and 2.8 M$_\textrm{J}$ at 18.6 AU and beyond. This is the most stringent upper limit on a substellar object around any single white dwarf. We further show that SPHERE is the most promising instrument available to search for substellar objects around nearby white dwarfs.

### Dynamical masses of a nova-like variable on the edge of the period gap

We present the first dynamical determination of the binary parameters of an eclipsing SW Sextantis star in the 3-4 hour orbital period range during a low state. We obtained time-resolved optical spectroscopy and photometry of HS 0220+0603 during its 2004-2005 low brightness state, as revealed in the combined SMARTS, IAC80 and M1 Group long-term optical light curve. The optical spectra taken during primary eclipse reveal a secondary star spectral type of M5.5 $\pm$ 0.5 as derived from molecular band-head indices. The spectra also provide the first detection of a DAB white dwarf in a cataclysmic variable. By modelling its optical spectrum we estimate a white dwarf temperature of 30000 $\pm$ 5000 K. By combining the results of modelling the white dwarf eclipse from ULTRACAM light curves with those obtained by simultaneously fitting the emission- and absorption-line radial velocity curves and I-band ellipsoidal light curves, we measure the stellar masses to be M$_1 = 0.87 \pm 0.09$ M$_\odot$ and M$_2 = 0.47 \pm 0.05$ M$_\odot$ for the white dwarf and the M dwarf, respectively, and an inclination of the orbital plane of $i \approx 79^\mathrm{o}$. A radius of $0.0103 \pm 0.0007$ R$_\odot$ is obtained for the white dwarf. The secondary star in HS 0220+0603 is likely too cool and undersized for its mass.

### The Composition Of A Disrupted Extrasolar Planetesimal At SDSS J0845+2257 (Ton 345)

We present a detailed study of the metal-polluted DB white dwarf SDSS J0845+2257 (Ton 345). Using high-resolution HST/COS and VLT spectroscopy, we have detected hydrogen and eleven metals in the atmosphere of the white dwarf. The origin of these metals is almost certainly the circumstellar disc of dusty and gaseous debris from a tidally-disrupted planetesimal, accreting at a rate of 1.6E10 gs^-1. Studying the chemical abundances of the accreted material demonstrates that the planetesimal had a composition similar to the Earth, dominated by rocky silicates and metallic iron, with a low water content. The mass of metals within the convection zone of the white dwarf corresponds to an asteroid of at least ~130-170 km in diameter, although the presence of ongoing accretion from the debris disc implies that the planetesimal was probably larger than this. While a previous abundance study of the accreted material has shown an anomalously high mass fraction of carbon (15 percent) compared to the bulk Earth, our independent analysis results in a carbon abundance of just 2.5 percent. Enhanced abundances of core material (Fe, Ni) suggest that the accreted object may have lost a portion of its mantle, possibly due to stellar wind stripping in the asymptotic giant branch. Time-series spectroscopy reveals variable emission from the orbiting gaseous disc, demonstrating that the evolved planetary system at SDSS J0845+2257 is dynamically active.

### The Composition Of A Disrupted Extrasolar Planetesimal At SDSS J0845+2257 (Ton 345) [Replacement]

We present a detailed study of the metal-polluted DB white dwarf SDSS J0845+2257 (Ton 345). Using high-resolution HST/COS and VLT spectroscopy, we have detected hydrogen and eleven metals in the atmosphere of the white dwarf. The origin of these metals is almost certainly the circumstellar disc of dusty and gaseous debris from a tidally-disrupted planetesimal, accreting at a rate of 1.6E10 gs^-1. Studying the chemical abundances of the accreted material demonstrates that the planetesimal had a composition similar to the Earth, dominated by rocky silicates and metallic iron, with a low water content. The mass of metals within the convection zone of the white dwarf corresponds to an asteroid of at least ~130-170 km in diameter, although the presence of ongoing accretion from the debris disc implies that the planetesimal was probably larger than this. While a previous abundance study of the accreted material has shown an anomalously high mass fraction of carbon (15 percent) compared to the bulk Earth, our independent analysis results in a carbon abundance of just 2.5 percent. Enhanced abundances of core material (Fe, Ni) suggest that the accreted object may have lost a portion of its mantle, possibly due to stellar wind stripping in the asymptotic giant branch. Time-series spectroscopy reveals variable emission from the orbiting gaseous disc, demonstrating that the evolved planetary system at SDSS J0845+2257 is dynamically active.

### The Expanding Fireball of Nova Delphini 2013

A classical nova occurs when material accreting onto the surface of a white dwarf in a close binary system ignites in a thermonuclear runaway. Complex structures observed in the ejecta at late stages could result from interactions with the companion during the common envelope phase. Alternatively, the explosion could be intrinsically bipolar, resulting from a localized ignition on the surface of the white dwarf or as a consequence of rotational distortion. Studying the structure of novae during the earliest phases is challenging because of the high spatial resolution needed to measure their small sizes. Here we report near-infrared interferometric measurements of the angular size of Nova Delphini 2013, starting from one day after the explosion and continuing with extensive time coverage during the first 43 days. Changes in the apparent expansion rate can be explained by an explosion model consisting of an optically thick core surrounded by a diffuse envelope. The optical depth of the ejected material changes as it expands. We detect an ellipticity in the light distribution, suggesting a prolate or bipolar structure that develops as early as the second day. Combining the angular expansion rate with radial velocity measurements, we derive a geometric distance to the nova of 4.54 +/- 0.59 kpc from the Sun.

### Dark Matter Triggers of Supernovae

The transit of primordial black holes through a white dwarf causes localized heating around the trajectory of the black hole through dynamical friction. For sufficiently massive black holes, this heat can initiate runaway thermonuclear fusion causing the white dwarf to explode as a supernova. The shape of the observed distribution of white dwarfs with masses up to $1.25 M_{\odot}$ rules out primordial black holes with masses $\sim 10^{19}$ gm - $10^{20}$ gm as a dominant constituent of the local dark matter density. Black holes with masses as large as $10^{24}$ gm will be excluded if recent observations by the NuStar collaboration of a population of white dwarfs near the galactic center are confirmed. Black holes in the mass range $10^{20}$ gm - $10^{22}$ gm are also constrained by the observed supernova rate, though these bounds are subject to astrophysical uncertainties. These bounds can be further strengthened through measurements of white dwarf binaries in gravitational wave observatories. The mechanism proposed in this paper can constrain a variety of other dark matter scenarios such as Q balls, annihilation/collision of large composite states of dark matter and models of dark matter where the accretion of dark matter leads to the formation of compact cores within the star. White dwarfs, with their astronomical lifetimes and sizes, can thus act as large space-time volume detectors enabling a unique probe of the properties of dark matter, especially of dark matter candidates that have low number density. This mechanism also raises the intriguing possibility that a class of supernova may be triggered through rare events induced by dark matter rather than the conventional mechanism of accreting white dwarfs that explode upon reaching the Chandrasekhar mass.

### Dark Matter Triggers of Supernovae [Cross-Listing]

The transit of primordial black holes through a white dwarf causes localized heating around the trajectory of the black hole through dynamical friction. For sufficiently massive black holes, this heat can initiate runaway thermonuclear fusion causing the white dwarf to explode as a supernova. The shape of the observed distribution of white dwarfs with masses up to $1.25 M_{\odot}$ rules out primordial black holes with masses $\sim 10^{19}$ gm - $10^{20}$ gm as a dominant constituent of the local dark matter density. Black holes with masses as large as $10^{24}$ gm will be excluded if recent observations by the NuStar collaboration of a population of white dwarfs near the galactic center are confirmed. Black holes in the mass range $10^{20}$ gm - $10^{22}$ gm are also constrained by the observed supernova rate, though these bounds are subject to astrophysical uncertainties. These bounds can be further strengthened through measurements of white dwarf binaries in gravitational wave observatories. The mechanism proposed in this paper can constrain a variety of other dark matter scenarios such as Q balls, annihilation/collision of large composite states of dark matter and models of dark matter where the accretion of dark matter leads to the formation of compact cores within the star. White dwarfs, with their astronomical lifetimes and sizes, can thus act as large space-time volume detectors enabling a unique probe of the properties of dark matter, especially of dark matter candidates that have low number density. This mechanism also raises the intriguing possibility that a class of supernova may be triggered through rare events induced by dark matter rather than the conventional mechanism of accreting white dwarfs that explode upon reaching the Chandrasekhar mass.

### Dark Matter Triggers of Supernovae [Cross-Listing]

The transit of primordial black holes through a white dwarf causes localized heating around the trajectory of the black hole through dynamical friction. For sufficiently massive black holes, this heat can initiate runaway thermonuclear fusion causing the white dwarf to explode as a supernova. The shape of the observed distribution of white dwarfs with masses up to $1.25 M_{\odot}$ rules out primordial black holes with masses $\sim 10^{19}$ gm - $10^{20}$ gm as a dominant constituent of the local dark matter density. Black holes with masses as large as $10^{24}$ gm will be excluded if recent observations by the NuStar collaboration of a population of white dwarfs near the galactic center are confirmed. Black holes in the mass range $10^{20}$ gm - $10^{22}$ gm are also constrained by the observed supernova rate, though these bounds are subject to astrophysical uncertainties. These bounds can be further strengthened through measurements of white dwarf binaries in gravitational wave observatories. The mechanism proposed in this paper can constrain a variety of other dark matter scenarios such as Q balls, annihilation/collision of large composite states of dark matter and models of dark matter where the accretion of dark matter leads to the formation of compact cores within the star. White dwarfs, with their astronomical lifetimes and sizes, can thus act as large space-time volume detectors enabling a unique probe of the properties of dark matter, especially of dark matter candidates that have low number density. This mechanism also raises the intriguing possibility that a class of supernova may be triggered through rare events induced by dark matter rather than the conventional mechanism of accreting white dwarfs that explode upon reaching the Chandrasekhar mass.