# Posts Tagged white dwarf

## Recent Postings from white dwarf

### Discovery of ZZ Cetis in detached white dwarf plus main-sequence binaries

We present the first results of a dedicated search for pulsating white dwarfs (WDs) in detached white dwarf plus main-sequence binaries. Candidate systems were selected from a catalogue of WD+MS binaries, based on the surface gravities and effective temperatures of the WDs. We observed a total of 26 systems using ULTRACAM mounted on ESO’s 3.5m New Technology Telescope (NTT) at La Silla. Our photometric observations reveal pulsations in seven WDs of our sample, including the first pulsating white dwarf with a main-sequence companion in a post common envelope binary, SDSSJ1136+0409. Asteroseismology of these new pulsating systems will provide crucial insight into how binary interactions, particularly the common envelope phase, affect the internal structure and evolution of WDs. In addition, our observations have revealed the partially eclipsing nature of one of our targets, SDSSJ1223-0056.

### Evidence for an Anhydrous Carbonaceous Extrasolar Minor Planet

Using Keck/HIRES, we report abundances of 11 different elements heavier than helium in the spectrum of Ton 345, a white dwarf that has accreted one of its own minor planets. This particular extrasolar planetesimal which was at least 60% as massive as Vesta appears to have been carbon-rich and water-poor; we suggest it was compositionally similar to those Kuiper Belt Objects with relatively little ice.

### The Binary Companion of Young, Relativistic Pulsar J1906+0746

PSR J1906+0746 is a young pulsar in the relativistic binary with the second-shortest known orbital period, of 3.98 hours. We here present a timing study based on five years of observations, conducted with the 5 largest radio telescopes in the world, aimed at determining the companion nature. Through the measurement of three post-Keplerian orbital parameters we find the pulsar mass to be 1.291(11) M_sol, and the companion mass 1.322(11) M_sol respectively. These masses fit well in the observed collection of double neutron stars, but are also compatible with other white dwarfs around young pulsars such as J1906+0746. Neither radio pulsations nor dispersion-inducing outflows that could have further established the companion nature were detected. We derive an HI-absorption distance, which indicates that an optical confirmation of a white dwarf companion is very challenging. The pulsar is fading fast due to geodetic precession, limiting future timing improvements. We conclude that young pulsar J1906+0746 is likely part of a double neutron star, or is otherwise orbited by an older white dwarf, in an exotic system formed through two stages of mass transfer.

### Formation of redbacks via accretion induced collapse

We examine the growing class of binary millisecond pulsars known as redbacks. In these systems the pulsar’s companion has a mass between 0.1 and about 0.5 solar masses in an orbital period of less than 1.5 days. All show extended radio eclipses associated with circumbinary material. They do not lie on the period-companion mass relation expected from the canonical intermediate-mass X-ray binary evolution in which the companion filled its Roche lobe as a red giant and has now lost its envelope and cooled as a white dwarf. The redbacks lie closer to, but usually at higher period than, the period-companion mass relation followed by cataclysmic variables and low-mass X-ray binaries. In order to turn on as a pulsar mass accretion on to a neutron star must be sufficiently weak, considerably weaker than expected in systems with low-mass main-sequence companions driven together by magnetic braking or gravitational radiation. If a neutron star is formed by accretion induced collapse of a white dwarf as it approaches the Chandrasekhar limit some baryonic mass is abruptly lost to its binding energy so that its effective gravitational mass falls. We propose that redbacks form when accretion induced collapse of a white dwarf takes place during cataclysmic variable binary evolution because the loss of gravitational mass makes the orbit expand suddenly so that the companion no longer fills its Roche lobe. Once activated, the pulsar can ablate its companion and so further expand the orbit and also account for the extended eclipses in the radio emission of the pulsar that are characteristic of these systems. The whole period-companion mass space occupied by the redbacks can be populated in this way.

### Late-time near-infrared observations of SN 2005df

We present late-time ($200-400$ days) near-infrared spectral evolution for the Type Ia supernova SN 2005df. The spectra show numerous strong emission features of [CoII], [CoIII], and [FeII] throughout the $0.8-1.8$\mu m region. As the spectrum ages, the cobalt features fade as would be expected from the decay of $^{56}$Co to $^{56}$Fe. We show that the strong and isolated [FeII] emission line at $1.644$\mu m provides a unique tool to analyze near-infrared spectra of Type Ia supernovae. Normalization of spectra to this line allows separation of features produced by stable versus unstable isotopes of iron group elements. We develop a new method of determining the initial central density, $\rho_c$, and the magnetic field, $B$, of the white dwarf using the width of the $1.644$\mu m line. The line width is sensitive because of electron capture in the early stages of burning, which increases as a function of density. The sensitivity of the line width to $B$ increase with time and the effects of the magnetic field shift towards later times with decreasing $\rho_c$. The initial central density for SN 2005df is measured as $\rho_c=0.9(\pm0.2)$ (in $10^9$g/cm$^3$), which corresponds to a white dwarf close to the Chandrasekhar mass ($\rm M_{Ch}$) with $\rm M_{WD}=1.313(\pm0.034)$M$_{\odot}$ and systematic error less than $0.04$M$_{\odot}$. Within $\rm M_{Ch}$ explosions, however, the central density found for SN 2005df is very low for a H-accretor, possibly suggesting a helium star companion or a tidally-disrupted white dwarf companion. As an alternative, we suggest mixing of the central region. We find some support for high initial magnetic fields of strength $10^6$G for SN 2005df, however, $0$G cannot be ruled out because of noise in the spectra combined with low $\rho_c$.

### Testing Fundamental Particle Physics with the Galactic White Dwarf Luminosity Function [Cross-Listing]

Recent determinations of the white dwarf luminosity function (WDLF) from very large surveys have extended our knowledge of the WDLF to very high luminosities. It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to test the possible properties and existence of fundamental weakly interacting subelectronvolt particles. This, together with the availability of new full evolutionary white dwarf models that are reliable at high luminosities, have opened the possibility of testing particle emission in the core of very hot white dwarfs. We use the available WDLFs from the Sloan Digital Sky Survey and the SuperCOSMOS Sky Survey to constrain the values of the neutrino magnetic dipole moment ($\mu_\nu$) and the axion-electron coupling constant ($g_{ae}$) of DFSZ-axions.

### Testing Fundamental Particle Physics with the Galactic White Dwarf Luminosity Function

Recent determinations of the white dwarf luminosity function (WDLF) from very large surveys have extended our knowledge of the WDLF to very high luminosities. It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to test the possible properties and existence of fundamental weakly interacting subelectronvolt particles. This, together with the availability of new full evolutionary white dwarf models that are reliable at high luminosities, have opened the possibility of testing particle emission in the core of very hot white dwarfs. We use the available WDLFs from the Sloan Digital Sky Survey and the SuperCOSMOS Sky Survey to constrain the values of the neutrino magnetic dipole moment ($\mu_\nu$) and the axion-electron coupling constant ($g_{ae}$) of DFSZ-axions.

### Do the constants of nature couple to strong gravitational fields?

Recently, white dwarf stars have found a new use in the fundamental physics community. Many prospective theories of the fundamental interactions of Nature allow traditional constants, like the fine structure constant $\alpha$, to vary in some way. A study by Berengut et al. (2013) used the Fe/Ni V line measurements made by Preval et al. (2013) from the hot DA white dwarf G191-B2B, in an attempt to detect any variation in $\alpha$. It was found that the Fe V lines indicated an increasing alpha, whereas the Ni V lines indicated a decreasing alpha. Possible explanations for this could be misidentification of the lines, inaccurate atomic data, or wavelength dependent distortion in the spectrum. We examine the first two cases by using a high S/N reference spectrum from the hot sdO BD+28$^{\circ}$4211 to calibrate the Fe/Ni V atomic data. With this new data, we re-evaluate the work of Berengut et al. (2013) to derive a new constraint on the variation of alpha in a gravitational field.

### Do the constants of nature couple to strong gravitational fields? [Cross-Listing]

Recently, white dwarf stars have found a new use in the fundamental physics community. Many prospective theories of the fundamental interactions of Nature allow traditional constants, like the fine structure constant $\alpha$, to vary in some way. A study by Berengut et al. (2013) used the Fe/Ni V line measurements made by Preval et al. (2013) from the hot DA white dwarf G191-B2B, in an attempt to detect any variation in $\alpha$. It was found that the Fe V lines indicated an increasing alpha, whereas the Ni V lines indicated a decreasing alpha. Possible explanations for this could be misidentification of the lines, inaccurate atomic data, or wavelength dependent distortion in the spectrum. We examine the first two cases by using a high S/N reference spectrum from the hot sdO BD+28$^{\circ}$4211 to calibrate the Fe/Ni V atomic data. With this new data, we re-evaluate the work of Berengut et al. (2013) to derive a new constraint on the variation of alpha in a gravitational field.

### Do the constants of nature couple to strong gravitational fields? [Replacement]

Recently, white dwarf stars have found a new use in the fundamental physics community. Many prospective theories of the fundamental interactions of Nature allow traditional constants, like the fine structure constant $\alpha$, to vary in some way. A study by Berengut et al. (2013) used the Fe/Ni V line measurements made by Preval et al. (2013) from the hot DA white dwarf G191-B2B, in an attempt to detect any variation in $\alpha$. It was found that the Fe V lines indicated an increasing alpha, whereas the Ni V lines indicated a decreasing alpha. Possible explanations for this could be misidentification of the lines, inaccurate atomic data, or wavelength dependent distortion in the spectrum. We examine the first two cases by using a high S/N reference spectrum from the hot sdO BD+28$^{\circ}$4211 to calibrate the Fe/Ni V atomic data. With this new data, we re-evaluate the work of Berengut et al. (2013) to derive a new constraint on the variation of alpha in a gravitational field.

### Do the constants of nature couple to strong gravitational fields? [Replacement]

Recently, white dwarf stars have found a new use in the fundamental physics community. Many prospective theories of the fundamental interactions of Nature allow traditional constants, like the fine structure constant $\alpha$, to vary in some way. A study by Berengut et al. (2013) used the Fe/Ni V line measurements made by Preval et al. (2013) from the hot DA white dwarf G191-B2B, in an attempt to detect any variation in $\alpha$. It was found that the Fe V lines indicated an increasing alpha, whereas the Ni V lines indicated a decreasing alpha. Possible explanations for this could be misidentification of the lines, inaccurate atomic data, or wavelength dependent distortion in the spectrum. We examine the first two cases by using a high S/N reference spectrum from the hot sdO BD+28$^{\circ}$4211 to calibrate the Fe/Ni V atomic data. With this new data, we re-evaluate the work of Berengut et al. (2013) to derive a new constraint on the variation of alpha in a gravitational field.

### The white dwarf cooling sequence of 47 Tucanae

47 Tucanae is one of the most interesting and well observed and theoretically studied globular clusters. This allows us to study the reliability of our understanding of white dwarf cooling sequences, to confront different methods to determine its age, and to assess other important characteristics, like its star formation history. Here we present a population synthesis study of the cooling sequence of the globular cluster 47 Tucanae. In particular, we study the distribution of effective temperatures, the shape of the color-magnitude diagram, and the corresponding magnitude and color distributions. We do so using an up-to-date population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. We find a good agreement between our theoretical models and the observed data. Thus, our study, rules out previous claims that there are still missing physics in the white dwarf cooling models at moderately high effective temperatures. We also derive the age of the cluster using the termination of the cooling sequence, obtaining a good agreement with the age determinations using the main-sequence turn-off. Finally, we find that the star formation history of the cluster is compatible with that btained using main sequence stars, which predict the existence of two distinct populations. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main sequence stars provides an unique tool to model the properties of globular clusters.

### Testing the planetary models of HU Aquarii

We present new eclipse observations of the polar (i.e. semi-detached magnetic white dwarf + M-dwarf binary) HU Aqr, and mid-egress times for each eclipse, which continue to be observed increasingly early. Recent eclipses occurred more than 70 seconds earlier than the prediction from the latest model that invoked a single circumbinary planet to explain the observed orbital period variations, thereby conclusively proving this model to be incorrect. Using ULTRACAM data, we show that mid-egress times determined for simultaneous data taken at different wavelengths agree with each other. The large variations in the observed eclipse times cannot be explained by planetary models containing up to three planets, because of poor fits to the data as well as orbital instability on short time scales. The peak-to-peak amplitude of the O-C diagram of almost 140 seconds is also too great to be caused by Applegate’s mechanism, movement of the accretion spot on the surface of the white dwarf, or by asynchronous rotation of the white dwarf. What does cause the observed eclipse time variations remains a mystery.

### The substellar companion in the eclipsing white dwarf binary SDSS J141126.20+200911.1

We present high time resolution SDSS-$g’$ and SDSS-$z’$ light curves of the primary eclipse in SDSS J141126.20+200911.1, together with time-resolved X-Shooter spectroscopy and near-infrared $JHK_{s}$ photometry. Our observations confirm the substellar nature of the companion, making SDSS J141126.20+200911.1 the first eclipsing white dwarf/brown dwarf binary known. We measure a (white dwarf model dependent) mass and radius for the brown dwarf companion of $M_{2} = 0.050 \pm 0.002$ $M_{\odot}$ and $R_{2} = 0.072 \pm 0.004$ $M_{\odot}$, respectively. The lack of a robust detection of the companion light in the $z’$-band eclipse constrains the spectral type of the companion to be later than L5. Comparing the NIR photometry to the expected white dwarf flux reveals a clear $K_s$-band excess, suggesting a spectral type in the range L7-T1. The radius measurement is consistent with the predictions of evolutionary models, and suggests a system age in excess of three Gyr. The low companion mass is inconsistent with the inferred spectral type of L7-T1, instead predicting a spectral type nearer T5. This indicates that irradiation of the companion in SDSS J1411 could be causing a significant temperature increase, at least on one hemisphere.

### Revealing the pulsational properties of the V777 Her star KUV 05134+2605 by its long-term monitoring

Context: KUV 05134+2605 is one of the 21 pulsating DB white dwarfs (V777 Her or DBV variables) known so far. The detailed investigation of the short-period and low-amplitude pulsations of these relatively faint targets requires considerable observational efforts from the ground, long-term single-site or multisite observations. The observed amplitudes of excited modes undergo short-term variations in many cases, which makes the determination of pulsation modes difficult. Methods: We re-analysed the data already published, and collected new measurements. We compared the frequency content of the different datasets from the different epochs and performed various tests to check the reliability of the frequency determinations. The mean period spacings were investigated with linear fits to the observed periods, Kolmogorov-Smirnov and Inverse Variance significance tests, and Fourier analysis of different period sets, including a Monte Carlo test simulating the effect of alias ambiguities. We employed fully evolutionary DB white dwarf models for the asteroseismic investigations. Results: We identified 22 frequencies between 1280 and 2530 microHz. These form 12 groups, which suggests at least 12 possible frequencies for the asteroseismic investigations. Thanks to the extended observations, KUV 05134+2605 joined the group of rich white dwarf pulsators. We identified one triplet and at least one doublet with a ~9 microHz frequency separation, from which we derived a stellar rotation period of 0.6 d. We determined the mean period spacings of ~31 and ~18 s for the modes we propose as dipole and quadrupole, respectively. We found an excellent agreement between the stellar mass derived from the l=1 period spacing and the period-to-period fits, all providing M_* = 0.84-0.85 M_Sun solutions. Our study suggests that KUV 05134+2605 is the most massive amongst the known V777 Her stars.

### Fermi Establishes Classical Novae as a Distinct Class of Gamma-Ray Sources

A classical nova results from runaway thermonuclear explosions on the surface of a white dwarf that accretes matter from a low-mass main-sequence stellar companion. In 2012 and 2013, three novae were detected in gamma rays and stood in contrast to the first gamma-ray detected nova V407 Cygni 2010, which belongs to a rare class of symbiotic binary systems. Despite likely differences in the compositions and masses of their white dwarf progenitors, the three classical novae are similarly characterized as soft spectrum transient gamma-ray sources detected over 2-3 week durations. The gamma-ray detections point to unexpected high-energy particle acceleration processes linked to the mass ejection from thermonuclear explosions in an unanticipated class of Galactic gamma-ray sources.

### ALMA and Herschel Observations of the Prototype Dusty and Polluted White Dwarf G29-38

ALMA Cycle 0 and Herschel PACS observations are reported for the prototype, nearest, and brightest example of a dusty and polluted white dwarf, G29-38. These long wavelength programs attempted to detect an outlying, parent population of bodies at 1-100 AU, from which originates the disrupted planetesimal debris that is observed within 0.01 AU and which exhibits L_IR/L = 0.039. No associated emission sources were detected in any of the data down to L_IR/L ~ 1e-4, generally ruling out cold dust masses greater than 1e24 – 1e25 g for reasonable grain sizes and properties in orbital regions corresponding to evolved versions of both asteroid and Kuiper belt analogs. Overall, these null detections are consistent with models of long-term collisional evolution in planetesimal disks, and the source regions for the disrupted parent bodies at stars like G29-38 may only be salient in exceptional circumstances, such as a recent instability. A larger sample of polluted white dwarfs, targeted with the full ALMA array, has the potential to unambiguously identify the parent source(s) of their planetary debris.

### Hubble Space Telescope Imaging and Spectroscopy of the Sirius-Like Triple Star System HD 217411

We present Hubble Space Telescope imaging and spectroscopy of HD 217411, a G3 V star associated with the extreme ultraviolet excess source (EUV 2RE J2300-07.0). This star is revealed to be a triple system with a G 3V primary (HD 217411 A) separated by ~1.1" from a secondary that is in turn composed of an unresolved K0 V star (HD 217411 Ba) and a hot DA white dwarf (HD 217411 Bb). The hot white dwarf dominates the UV flux of the system. However; it is in turn dominated by the K0 V component beyond 3000 {\AA}. A revised distance of 143 pc is estimated for the system. A low level photometric modulation having a period of 0.61 days has also been observed in this system along with a rotational velocity on the order of 60 km s-1 in the K0 V star. Together both observations point to a possible wind induced spin up of the K0 V star during the AGB phase of the white dwarf. The nature of all three components is discussed as are constraints on the orbits, system age and evolution.

### Radius constraints from high-speed photometry of 20 low-mass white dwarf binaries

We carry out high-speed photometry on 20 of the shortest-period, detached white dwarf binaries known and discover systems with eclipses, ellipsoidal variations (due to tidal deformations of the visible white dwarf), and Doppler beaming. All of the binaries contain low-mass white dwarfs with orbital periods less than 4 hr. Our observations identify the first eight tidally distorted white dwarfs, four of which are reported for the first time here, which we use to put empirical constraints on the mass-radius relationship for extremely low-mass (<0.30 Msun) white dwarfs. We also detect Doppler beaming in several of these binaries, which confirms the high-amplitude radial-velocity variability. All of these systems are strong sources of gravitational radiation, and long-term monitoring of those that display ellipsoidal variations can be used to detect spin-up of the tidal bulge due to orbital decay.

### Early 56Ni decay {\gamma}-rays from SN2014J suggest an unusual explosion

Type-Ia supernovae result from binary systems that include a carbon-oxygen white dwarf, and these thermonuclear explosions typically produce 0.5 M_solar of radioactive 56Ni. The 56Ni is commonly believed to be buried deeply in the expanding supernova cloud. Surprisingly, in SN2014J we detected the lines at 158 and 812 keV from 56Ni decay ({\tau}~8.8 days) earlier than the expected several-week time scale, only ~20 days after the explosion, and with flux levels corresponding to roughly 10% of the total expected amount of 56Ni. Some mechanism must break the spherical symmetry of the supernova, and at the same time create a major amount of 56Ni at the outskirts. A plausible explanation is that a belt of helium from the companion star is accreted by the white dwarf, where this material explodes and then triggers the supernova event.

### Early 56Ni decay {\gamma}-rays from SN2014J suggest an unusual explosion [Replacement]

Type-Ia supernovae result from binary systems that include a carbon-oxygen white dwarf, and these thermonuclear explosions typically produce 0.5 M_solar of radioactive 56Ni. The 56Ni is commonly believed to be buried deeply in the expanding supernova cloud. Surprisingly, in SN2014J we detected the lines at 158 and 812 keV from 56Ni decay ({\tau}~8.8 days) earlier than the expected several-week time scale, only ~20 days after the explosion, and with flux levels corresponding to roughly 10% of the total expected amount of 56Ni. Some mechanism must break the spherical symmetry of the supernova, and at the same time create a major amount of 56Ni at the outskirts. A plausible explanation is that a belt of helium from the companion star is accreted by the white dwarf, where this material explodes and then triggers the supernova event.

### Revisiting the axion bounds from the Galactic white dwarf luminosity function [Cross-Listing]

It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to check for the possible existence of DFSZ-axions, a proposed but not yet detected type of weakly interacting particles. With the aim of deriving new constraints on the axion mass, we compute in this paper new theoretical WDLFs on the basis of WD evolving models that incorporate for the feedback of axions on the thermal structure of the white dwarf. We find that the impact of the axion emission into the neutrino emission can not be neglected at high luminosities ($M_{\rm Bol}\lesssim 8$) and that the axion emission needs to be incorporated self-consistently into the evolution of the white dwarfs when dealing with axion masses larger than $m_a\cos^2\beta\gtrsim 5$ meV (i.e. axion-electron coupling constant $g_{ae}\gtrsim 1.4\times 10^{-13}$). We went beyond previous works by including 5 different derivations of the WDLF in our analysis. Then we have performed $\chi^2$-tests to have a quantitative measure of the assessment between the theoretical WDLFs —computed under the assumptions of different axion masses and normalization methods— and the observed WDLFs of the Galactic disk. While all the WDLF studied in this work disfavour axion masses in the range suggested by asteroseismology ($m_a\cos^2\beta\gtrsim 10$ meV; $g_{ae}\gtrsim 2.8\times 10^{-13}$) lower axion masses can not be discarded from our current knowledge of the WDLF of the Galactic Disk. A larger set of completely independent derivations of the WDLF of the galactic disk as well as a detailed study of the uncertainties of the theoretical WDLFs is needed before quantitative constraints on the axion-electron coupling constant can be made.

### Revisiting the axion bounds from the Galactic white dwarf luminosity function

It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to check for the possible existence of DFSZ-axions, a proposed but not yet detected type of weakly interacting particles. With the aim of deriving new constraints on the axion mass, we compute in this paper new theoretical WDLFs on the basis of WD evolving models that incorporate for the feedback of axions on the thermal structure of the white dwarf. We find that the impact of the axion emission into the neutrino emission can not be neglected at high luminosities ($M_{\rm Bol}\lesssim 8$) and that the axion emission needs to be incorporated self-consistently into the evolution of the white dwarfs when dealing with axion masses larger than $m_a\cos^2\beta\gtrsim 5$ meV (i.e. axion-electron coupling constant $g_{ae}\gtrsim 1.4\times 10^{-13}$). We went beyond previous works by including 5 different derivations of the WDLF in our analysis. Then we have performed $\chi^2$-tests to have a quantitative measure of the assessment between the theoretical WDLFs —computed under the assumptions of different axion masses and normalization methods— and the observed WDLFs of the Galactic disk. While all the WDLF studied in this work disfavour axion masses in the range suggested by asteroseismology ($m_a\cos^2\beta\gtrsim 10$ meV; $g_{ae}\gtrsim 2.8\times 10^{-13}$) lower axion masses can not be discarded from our current knowledge of the WDLF of the Galactic Disk. A larger set of completely independent derivations of the WDLF of the galactic disk as well as a detailed study of the uncertainties of the theoretical WDLFs is needed before quantitative constraints on the axion-electron coupling constant can be made.

### Revisiting the axion bounds from the Galactic white dwarf luminosity function [Replacement]

It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to check for the possible existence of DFSZ-axions, a proposed but not yet detected type of weakly interacting particles. With the aim of deriving new constraints on the axion mass, we compute in this paper new theoretical WDLFs on the basis of WD evolving models that incorporate for the feedback of axions on the thermal structure of the white dwarf. We find that the impact of the axion emission into the neutrino emission can not be neglected at high luminosities ($M_{\rm Bol}\lesssim 8$) and that the axion emission needs to be incorporated self-consistently into the evolution of the white dwarfs when dealing with axion masses larger than $m_a\cos^2\beta\gtrsim 5$ meV (i.e. axion-electron coupling constant $g_{ae}\gtrsim 1.4\times 10^{-13}$). We went beyond previous works by including 5 different derivations of the WDLF in our analysis. Then we have performed $\chi^2$-tests to have a quantitative measure of the assessment between the theoretical WDLFs —computed under the assumptions of different axion masses and normalization methods— and the observed WDLFs of the Galactic disk. While all the WDLF studied in this work disfavour axion masses in the range suggested by asteroseismology ($m_a\cos^2\beta\gtrsim 10$ meV; $g_{ae}\gtrsim 2.8\times 10^{-13}$) lower axion masses can not be discarded from our current knowledge of the WDLF of the Galactic Disk. A larger set of completely independent derivations of the WDLF of the galactic disk as well as a detailed study of the uncertainties of the theoretical WDLFs is needed before quantitative constraints on the axion-electron coupling constant can be made.

### Revisiting the axion bounds from the Galactic white dwarf luminosity function [Replacement]

It has been shown that the shape of the luminosity function of white dwarfs (WDLF) is a powerful tool to check for the possible existence of DFSZ-axions, a proposed but not yet detected type of weakly interacting particles. With the aim of deriving new constraints on the axion mass, we compute in this paper new theoretical WDLFs on the basis of WD evolving models that incorporate for the feedback of axions on the thermal structure of the white dwarf. We find that the impact of the axion emission into the neutrino emission can not be neglected at high luminosities ($M_{\rm Bol}\lesssim 8$) and that the axion emission needs to be incorporated self-consistently into the evolution of the white dwarfs when dealing with axion masses larger than $m_a\cos^2\beta\gtrsim 5$ meV (i.e. axion-electron coupling constant $g_{ae}\gtrsim 1.4\times 10^{-13}$). We went beyond previous works by including 5 different derivations of the WDLF in our analysis. Then we have performed $\chi^2$-tests to have a quantitative measure of the assessment between the theoretical WDLFs —computed under the assumptions of different axion masses and normalization methods— and the observed WDLFs of the Galactic disk. While all the WDLF studied in this work disfavour axion masses in the range suggested by asteroseismology ($m_a\cos^2\beta\gtrsim 10$ meV; $g_{ae}\gtrsim 2.8\times 10^{-13}$) lower axion masses can not be discarded from our current knowledge of the WDLF of the Galactic Disk. A larger set of completely independent derivations of the WDLF of the galactic disk as well as a detailed study of the uncertainties of the theoretical WDLFs is needed before quantitative constraints on the axion-electron coupling constant can be made.

### The Signature of Single-Degenerate Accretion Induced Collapse [Replacement]

The accretion induced collapse (AIC) of a white dwarf to a neutron star has long been suggested as a natural theoretical outcome in stellar evolution, but there has never been a direct detection of such an event. This is not surprising since the small amount of radioactive nickel synthesized ($\sim10^{-3}\,M_\odot$) implies a relatively dim optical transient. Here we argue that a particularly strong signature of an AIC would occur for an oxygen-neon-magnesium (ONeMg) white dwarf accreting from a star that is experiencing Roche-lobe overflow as it becomes a red giant. In such cases, the $\sim10^{50}\,{\rm erg}$ explosion from the AIC collides with and shock-heats the surface of the extended companion, creating an X-ray flash lasting $\sim1\,{\rm hr}$ followed by an optical signature that peaks at an absolute magnitude of $\sim -16$ to $-18$ and lasts for a few days to a week. These events would be especially striking in old stellar environments where hydrogen-rich supernova-like, transients would not normally be expected. Although the rate of such events is not currently known, we describe observing strategies that could be utilized with high cadence surveys that should either detect these events or place strong constraints on their rates.

### The Signature of Single-Degenerate Accretion Induced Collapse

The accretion induced collapse (AIC) of a white dwarf to a neutron star has long been suggested as a natural theoretical outcome in stellar evolution, but there has never been a direct detection of such an event. This is not surprising since the small amount of radioactive nickel synthesized ($\sim10^{-3}\,M_\odot$) implies a relatively dim optical transient. Here we argue that a particularly strong signature of an AIC would occur for an oxygen-neon-magnesium (ONeMg) white dwarf accreting from a star that is experiencing Roche-lobe overflow as it becomes a red giant. In such cases, the $\sim10^{50}\,{\rm erg}$ explosion from the AIC collides with and shock-heats the surface of the extended companion, creating an X-ray flash lasting $\sim1\,{\rm hr}$ followed by an optical signature that peaks at an absolute magnitude of $\sim -16$ to $-18$ and lasts for a few days to a week. These events would be especially striking in old stellar environments where hydrogen-rich supernova-like, transients would not normally be expected. Although the rate of such events is not currently known, we describe observing strategies that could be utilized with high cadence surveys that should either detect these events or place strong constraints on their rates.

### A parameter study of the eclipsing CV in the Kepler field, KIS J192748.53+444724.5

We present high-speed, three-colour photometry of the eclipsing dwarf nova KIS J192748.53+444724.5 which is located in the Kepler field. Our data reveal sharp features corresponding to the eclipses of the accreting white dwarf followed by the bright spot where the gas stream joins the accretion disc. We determine the system parameters via a parameterized model of the eclipse fitted to the observed lightcurve. We obtain a mass ratio of q = 0.570 +/- 0.011 and an orbital inclination of 84.6 +/- 0.3 degrees. The primary mass is M_w = 0.69 +/- 0.07 Msun. The donor star’s mass and radius are found to be M_d = 0.39 +/- 0.04 Msun and R_d = 0.43 +/- 0.01 Rsun, respectively. From the fluxes of the white dwarf eclipse we find a white dwarf temperature of T_w = 23000 +/- 3000 K, and a photometric distance to the system of 1600 +/- 200 pc, neglecting the effects of interstellar reddening. The white dwarf temperature in KISJ1927 implies the white dwarf is accreting at an average rate of Mdot = (1.4 +/- 0.8) x10e-9 Msun/yr, in agreement with estimates of the secular mass loss rate from the donor.

### A 1.05 $M_\odot$ Companion to PSR J2222-0137: The Coolest Known White Dwarf?

The recycled pulsar PSR J2222-0137 is one of the closest known neutron stars, with a parallax distance of $267_{-0.9}^{+1.2}\,$pc and an edge-on orbit. We measure the Shapiro delay in the system through pulsar timing with the Green Bank Telescope, deriving a low pulsar mass ($1.20\pm0.14$ $M_\odot$) and a high companion mass ($1.05\pm0.06$ $M_\odot$) consistent with either a low-mass neutron star or a high-mass white dwarf. We can largely reject the neutron star hypothesis on the basis of the system’s extremely low eccentricity (3e-4) – too low to have been the product of two supernovae under normal circumstances. However, despite deep optical and near-infrared searches with SOAR and the Keck telescopes we have not discovered the optical counterpart of the system. This is consistent with the white dwarf hypothesis only if the effective temperature is <3000 K, a limit that is robust to distance, mass, and atmosphere uncertainties. This would make the companion to PSR J2222-0137 one of the coolest white dwarfs ever observed. For the implied age to be consistent with the age of the Milky Way requires the white dwarf to have already crystallized and entered the faster Debye-cooling regime.

### IP Eri: A surprising long-period binary system hosting a He white dwarf

We determine the orbital elements for the K0 IV + white dwarf (WD) system IP Eri, which appears to have a surprisingly long period of 1071 d and a significant eccentricity of 0.25. Previous spectroscopic analyses of the WD, based on a distance of 101 pc inferred from its Hipparcos parallax, yielded a mass of only 0.43 M$_\odot$, implying it to be a helium-core WD. The orbital properties of IP Eri are similar to those of the newly discovered long-period subdwarf B star (sdB) binaries, which involve stars with He-burning cores surrounded by extremely thin H envelopes, and are therefore close relatives to He WDs. We performed a spectroscopic analysis of high-resolution spectra from the HERMES/Mercator spectrograph and concluded that the atmospheric parameters of the K0 component are $T_{\rm eff} = 4960$ K, $\log{g} = 3.3$, [Fe/H] = 0.09 and $\xi = 1.5$ km/s. The detailed abundance analysis focuses on C, N, O abundances, carbon isotopic ratio, light (Na, Mg, Al, Si, Ca, Ti) and s-process (Sr, Y, Zr, Ba, La, Ce, Nd) elements. We conclude that IP Eri abundances agree with those of normal field stars of the same metallicity. The long period and non-null eccentricity indicate that this system cannot be the end product of a common-envelope phase; it calls instead for another less catastrophic binary-evolution channel presented in detail in a companion paper (Siess et al. 2014).

### IP Eri: A surprising long-period binary system hosting a He white dwarf [Replacement]

We determine the orbital elements for the K0 IV + white dwarf (WD) system IP Eri, which appears to have a surprisingly long period of 1071 d and a significant eccentricity of 0.25. Previous spectroscopic analyses of the WD, based on a distance of 101 pc inferred from its Hipparcos parallax, yielded a mass of only 0.43 M$_\odot$, implying it to be a helium-core WD. The orbital properties of IP Eri are similar to those of the newly discovered long-period subdwarf B star (sdB) binaries, which involve stars with He-burning cores surrounded by extremely thin H envelopes, and are therefore close relatives to He WDs. We performed a spectroscopic analysis of high-resolution spectra from the HERMES/Mercator spectrograph and concluded that the atmospheric parameters of the K0 component are $T_{\rm eff} = 4960$ K, $\log{g} = 3.3$, [Fe/H] = 0.09 and $\xi = 1.5$ km/s. The detailed abundance analysis focuses on C, N, O abundances, carbon isotopic ratio, light (Na, Mg, Al, Si, Ca, Ti) and s-process (Sr, Y, Zr, Ba, La, Ce, Nd) elements. We conclude that IP Eri abundances agree with those of normal field stars of the same metallicity. The long period and non-null eccentricity indicate that this system cannot be the end product of a common-envelope phase; it calls instead for another less catastrophic binary-evolution channel presented in detail in a companion paper (Siess et al. 2014).

### One possible solution of peculiar type Ia supernovae explosions caused by a charged white dwarf

Recently astrophysics observation reveals the existence of some super luminous type Ia supernovae. One natural explanation of such peculiar phenomenon is to require the progenitor of such supernova to be a highly super-Chandrasekhar mass white dwarf. Along this line, in this paper, we propose a possible mechanism to explain this phenomenon based on charged white dwarf. In particular, by choosing suitable new variables and a representative charge distribution, an analytic solution is obtained. The stability issue is also discussed, remarkably, it turns out that the charged white dwarf configuration can be dynamical stable. Moreover, we investigate the general relativistic effects and it is shown that the general relativistic effects can be negligible when the mass of charged white dwarf below about $3M_\odot$.

### One possible solution of peculiar type Ia supernovae explosions caused by a charged white dwarf [Cross-Listing]

Recently astrophysics observation reveals the existence of some super luminous type Ia supernovae. One natural explanation of such peculiar phenomenon is to require the progenitor of such supernova to be a highly super-Chandrasekhar mass white dwarf. Along this line, in this paper, we propose a possible mechanism to explain this phenomenon based on charged white dwarf. In particular, by choosing suitable new variables and a representative charge distribution, an analytic solution is obtained. The stability issue is also discussed, remarkably, it turns out that the charged white dwarf configuration can be dynamical stable. Moreover, we investigate the general relativistic effects and it is shown that the general relativistic effects can be negligible when the mass of charged white dwarf below about $3M_\odot$.

### One possible solution of peculiar type Ia supernovae explosions caused by a charged white dwarf [Replacement]

Recent astrophysics observation reveals the existence of some super luminous type Ia supernovae. One natural explanation of such a peculiar phenomenon is to require the progenitor of such a supernova to be a highly super-Chandrasekhar mass white dwarf. Along this line, in this paper, we propose a possible mechanism to explain this phenomenon based on a charged white dwarf. In particular, by choosing suitable new variables and a representative charge distribution, an analytic solution is obtained. The stability issue is also discussed, remarkably, it turns out that the charged white dwarf configuration can be dynamically stable. Moreover, we investigate the general relativistic effects and it is shown that the general relativistic effects can be negligible when the mass of the charged white dwarf is below about $3M_\odot$.

### One possible solution of peculiar type Ia supernovae explosions caused by a charged white dwarf [Replacement]

Recent astrophysics observation reveals the existence of some super luminous type Ia supernovae. One natural explanation of such a peculiar phenomenon is to require the progenitor of such a supernova to be a highly super-Chandrasekhar mass white dwarf. Along this line, in this paper, we propose a possible mechanism to explain this phenomenon based on a charged white dwarf. In particular, by choosing suitable new variables and a representative charge distribution, an analytic solution is obtained. The stability issue is also discussed, remarkably, it turns out that the charged white dwarf configuration can be dynamically stable. Moreover, we investigate the general relativistic effects and it is shown that the general relativistic effects can be negligible when the mass of the charged white dwarf is below about $3M_\odot$.

### Precision asteroseismology of the pulsating white dwarf GD 1212 using a two-wheel-controlled Kepler spacecraft

We present a preliminary analysis of the cool pulsating white dwarf GD 1212, enabled by more than 11.5 days of space-based photometry obtained during an engineering test of the two-reaction-wheel-controlled Kepler spacecraft. We detect at least 19 independent pulsation modes, ranging from 828.2-1220.8 s, and at least 17 nonlinear combination frequencies of those independent pulsations. Our longest uninterrupted light curve, 9.0 days in length, evidences coherent difference frequencies at periods inaccessible from the ground, up to 14.5 hr, the longest-period signals ever detected in a pulsating white dwarf. These results mark some of the first science to come from a two-wheel-controlled Kepler spacecraft, proving the capability for unprecedented discoveries afforded by extending Kepler observations to the ecliptic.

### Mass-radius relation of strongly magnetized white dwarfs: nearly independent of Landau quantization [Replacement]

We compute static equilibria of white dwarf stars containing strong poloidal magnetic field, and present the modification of white dwarf mass-radius relation caused by the magnetic field. We find that a maximum white dwarf mass of about $1.9$~$M\odot$ may be supported if the interior field is as strong as approximately $10^{10}$ T. This mass is over 30 per cent larger than the traditional Chandrasekhar Limit. The equation of state of electron degenerate matter can be strongly modified due to Landau quantization at such high magnetic fields. We find, however, that this does not significantly affect the structure of the white dwarf.

### Mass Radius Relation of Strongly Magnetized White Dwarfs and the Effects of Landau Quantization

We compute static equilibria of White Dwarf stars containing strong poloidal magnetic field, and present the modification of White Dwarf mass-radius relation caused by the magnetic field. We find that a maximum White Dwarf mass of $\sim 1.9 M_{\odot}$ may be supported if the interior field is as strong as $\sim10^{10}$ T. This mass is over 30 % larger than the traditional Chandrasekhar Limit. The equation of state of electron degenerate matter can be strongly modified due to Landau quantization at such high magnetic fields. We find, however, that this does not significantly affect the structure of the White Dwarf.

### On the origin of the peculiar cataclysmic variable AE Aquarii

The nova-like variable AE Aquarii is a close binary system containing a red dwarf and a magnetized white dwarf rotating with the period of 33 seconds. A short spin period of the white dwarf is caused by an intensive mass exchange between the system components during a previous epoch. We show that a high rate of disk accretion onto the white dwarf surface resulted in temporary screening of its magnetic field and spin-up of the white dwarf to its present spin period. Transition of the white dwarf to the ejector state occurred at a final stage of the spin-up epoch after its magnetic field had emerged from the accreted plasma due to diffusion. In the frame of this scenario AE Aqr represents a missing link in the chain of Polars evolution and the white dwarf resembles a recycled pulsar.

### Stellar laboratories III. New Ba V, Ba VI, and Ba VII oscillator strengths and the barium 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. Reliable Ba V – VII oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE0503-289 and to determine their photospheric Ba abundances. We newly calculated Ba V – VII oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Ba lines exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE0503-289. For the first time, we identified highly ionized Ba in the spectra of hot white dwarfs. We detected Ba VI and Ba VII lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE0503-289. The Ba VI / Ba VII ionization equilibrium is well reproduced with the previously determined effective temperature of 70000 K and surface gravity of $\log g = 7.5$. The Ba abundance is $3.5 \pm 0.5 \times 10^{-4}$ (mass fraction, about 23000 times the solar value). In the FUSE spectrum of G191-B2B, we identified the strongest Ba VII line (at 993.41 \AA) only, and determined a Ba abundance of $4.0 \pm 0.5 \times 10^{-6}$ (about 265 times solar). Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Ba VI – VII line profiles in two white dwarfs’ (G191-B2B and RE0503-289) far-ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed to determine the photospheric Ba abundance of these two stars precisely.

### KOI-3278: A Self-Lensing Binary Star System

Over 40% of Sun-like stars are bound in binary or multistar systems. Stellar remnants in edge-on binary systems can gravitationally magnify their companions, as predicted 40 years ago. By using data from the Kepler spacecraft, we report the detection of such a "self-lensing" system, in which a 5-hour pulse of 0.1% amplitude occurs every orbital period. The white dwarf stellar remnant and its Sun-like companion orbit one another every 88.18 days, a long period for a white dwarf-eclipsing binary. By modeling the pulse as gravitational magnification (microlensing) along with Kepler’s laws and stellar models, we constrain the mass of the white dwarf to be ~63% of the mass of our Sun. Further study of this system, and any others discovered like it, will help to constrain the physics of white dwarfs and binary star evolution.

### Evaporation and Accretion of Extrasolar Comets Following White Dwarf Kicks

Several lines of observational evidence suggest that white dwarfs receive small birth kicks due to anisotropic mass loss. If other stars possess extrasolar analogues to the Solar Oort cloud, the orbits of comets in such clouds will be scrambled by white dwarf natal kicks. Although most comets will be unbound, some will be placed on low angular momentum orbits vulnerable to sublimation or tidal disruption. The dusty debris from these comets will manifest itself as a debris disk temporarily visible around newborn white dwarfs; examples of such disks may already have been seen in the Helix Nebula, and around several other young WDs. Future observations with the James Webb Space Telescope will distinguish this hypothesis from alternatives such as a dynamically excited Kuiper Belt analogue. If interpreted as indeed being cometary in origin, the observation that >15% of young WDs possess such disks provides indirect evidence that low mass gas giants (thought necessary to produce an Oort cloud) are common in the outer regions of extrasolar planetary systems. Hydrogen abundances in the atmospheres of older white dwarfs can, if sufficiently low, also be used to place constraints on the joint parameter space of natal kicks and exo-Oort cloud models.

### The 2011 Outburst of Recurrent Nova T Pyx: X-ray Observations Expose the White Dwarf Mass and Ejection Dynamics

The recurrent nova T Pyx underwent its sixth historical outburst in 2011, and became the subject of an intensive multi-wavelength observational campaign. We analyze data from the Swift and Suzaku satellites to produce a detailed X-ray light curve augmented by epochs of spectral information. X-ray observations yield mostly non-detections in the first four months of outburst, but both a super-soft and hard X-ray component rise rapidly after Day 115. The super-soft X-ray component, attributable to the photosphere of the nuclear-burning white dwarf, is relatively cool (~45 eV) and implies that the white dwarf in T Pyx is significantly below the Chandrasekhar mass (~1 M_sun). The late turn-on time of the super-soft component yields a large nova ejecta mass (>~10^-5 M_sun), consistent with estimates at other wavelengths. The hard X-ray component is well fit by a ~1 keV thermal plasma, and is attributed to shocks internal to the 2011 nova ejecta. The presence of a strong oxygen line in this thermal plasma on Day 194 requires a significantly super-solar abundance of oxygen and implies that the ejecta are polluted by white dwarf material. The X-ray light curve can be explained by a dual-phase ejection, with a significant delay between the first and second ejection phases, and the second ejection finally released two months after outburst. A delayed ejection is consistent with optical and radio observations of T Pyx, but the physical mechanism producing such a delay remains a mystery.

### New approaches to SNe Ia progenitors

Although Type Ia supernovae (SNe Ia) are a major tool in cosmology and play a key role in the chemical evolution of galaxies, the nature of their progenitor systems (apart from the fact that they must be close binaries containing at least one white dwarf) remains largely unknown. In the last decade, considerable efforts have been made, both observationally and theoretically, to solve this problem. Observations have, however, revealed a previously unsuspected variety of events, ranging from very underluminous outbursts to clearly overluminous ones, and spanning a range well outside the peak luminosity–decline rate of the light curve relationship, used to make calibrated candles of the SNe Ia. On the theoretical side, new explosion scenarios, such as violent mergings of pairs of white dwarfs, have been explored. We review those recent developments, emphasizing the new observational findings, but also trying to tie them to the different scenarios and explosion mechanisms proposed thus far.

### New approaches to SNe Ia progenitors [Replacement]

Although Type Ia supernovae (SNe Ia) are a major tool in cosmology and play a key role in the chemical evolution of galaxies, the nature of their progenitor systems (apart from the fact that they must contain at least one white dwarf, that explodes) remains largely unknown. In the last decade, considerable efforts have been made, both observationally and theoretically, to solve this problem. Observations have, however, revealed a previously unsuspected variety of events, ranging from very underluminous outbursts to clearly overluminous ones, and spanning a range well outside the peak luminosity–decline rate of the light curve relationship, used to make calibrated candles of the SNe Ia. On the theoretical side, new explosion scenarios, such as violent mergings of pairs of white dwarfs, have been explored. We review those recent developments, emphasizing the new observational findings, but also trying to tie them to the different scenarios and explosion mechanisms proposed thus far.

### Absorption non-symmetric ion-atom processes in helium-rich white dwarf atmospheres

In this work the processes of absorption charge-exchange and photo-association in He+H$^{+}$ collisions together with the process of ion HeH$^{+}$ photo-dissociation are considered as factors of influence on the opacity of the atmospheres of helium-rich white dwarfs in the far UV and EUV region. It is shown that they should be taken into account even in the cases of the atmospheres of white dwarfs with H:He =$10^{-5}$. Than, it is established that in the cases of white dwarfs with H:He $\gtrsim 10^{-4}$, particulary when H:He $\approx 10^{-3}$, these processes have to be included \emph{ab initio} in the corresponding models of their atmospheres, since in the far UV and EUV region they become dominant with respect to the known symmetric ion-atom absorption processes.

### Mid-Infrared High-Contrast Imaging of HD 114174 B : An Apparent Age Discrepancy in a "Sirius-Like" Binary System

We present new observations of the faint "Sirius-like" companion discovered to orbit HD 114174. Previous attempts to image HD 114174 B at mid-infrared wavelengths using NIRC2 at Keck have resulted in a non-detection. Our new L’-band observations taken with the Large Binocular Telescope and LMIRCam recover the companion ($\Delta L$ = 10.15 $\pm$ 0.15 mag, $\rho$ = 0.675” $\pm$ 0.016”) with a high signal-to-noise ratio (10 $\sigma$). This measurement represents the deepest L’ high-contrast imaging detection at sub-arcsecond separations to date, including extrasolar planets. We confirm that HD 114174 B has near-infrared colors consistent with the interpretation of a cool white dwarf ($J-L’$ = 0.76 $\pm$ 0.19 mag, $K-L’$ = 0.64 $\pm$ 0.20). New model fits to the object’s spectral energy distribution indicate a temperature $T_{\rm eff}$ = 4260 $\pm$ 360 K, surface gravity log g = 7.94 $\pm$ 0.03, a cooling age t$_{c} \approx$ 7.8 Gyr, and mass $M$ = 0.54 $\pm$ 0.01 $M_{\odot}$. We find that the cooling age given by theoretical atmospheric models do not agree with the age of HD 114174 A derived from both isochronological and gyrochronological analyses. We speculate on possible scenarios to explain the apparent age discrepancy between the primary and secondary. HD 114174 B is a nearby benchmark white dwarf that will ultimately enable a dynamical mass estimate through continued Doppler and astrometric monitoring. Efforts to characterize its physical properties in detail will test theoretical atmospheric models and improve our understanding of white dwarf evolution, cooling, and progenitor masses.

### Detection of white dwarf companions to blue stragglers in the open cluster NGC 188: direct evidence for recent mass transfer

Several possible formation pathways for blue straggler stars have been developed recently, but no one pathway has yet been observationally confirmed for a specific blue straggler. Here we report the first findings from a Hubble Space Telescope ACS/SBC far-UV photometric program to search for white dwarf companions to blue straggler stars. We find three hot and young white dwarf companions to blue straggler stars in the 7-Gyr open cluster NGC 188, indicating that mass transfer in these systems ended less than 300 Myr ago. These companions are direct and secure observational evidence that these blue straggler stars were formed through mass transfer in binary stars. Their existence in a well-studied cluster environment allows for observational constraints of both the current binary system and the progenitor binary system, mapping the entire mass transfer history.

### Spectroscopy of the enigmatic short-period cataclysmic variable IR Com in an extended low state [Replacement]

We report the occurrence of a deep low state in the eclipsing short-period cataclysmic variable IR Com, lasting more than two years. Spectroscopy obtained in this state shows the system as a detached white dwarf plus low-mass companion, indicating that accretion has practically ceased. The spectral type of the companion derived from the SDSS spectrum is M6-7, somewhat later than expected for the orbital period of IR Com. Its radial velocity amplitude, K_2=419.6+-3.4 km/s, together with the inclination of 75-90deg implies 0.8Msun<Mwd<1.0Msun. We estimate the white dwarf temperature to be ~15000K, and the absence of Zeeman splitting in the Balmer lines rules out magnetic fields in excess of ~5 MG. IR Com still defies an unambiguous classification, in particular the occurrence of a deep, long low state is so far unique among short-period CVs that are not strongly magnetic.

### Spectroscopy of the enigmatic short-period cataclysmic variable IR Com in an extended low state

We report the occurrence of a deep low state in the eclipsing short-period cataclysmic variable IR Com, lasting more than two years. Spectroscopy obtained in this state shows the system as a detached white dwarf plus low-mass companion, indicating that accretion has practically ceased. The spectral type of the companion is M6-7, suggesting a mass of 0.15-0.20 Msun. Its radial velocity amplitude, K_2=419.6+/-3.4 km/s, together with the inclination of 75deg – 90deg implies 0.91Msun<Mwd<1.05Msun. We estimate the white dwarf temperature to be ~15000K, and the absence of Zeeman splitting in the Balmer lines rules out magnetic fields in excess of ~5MG. While all the binary and stellar parameters are typical for a CV near the lower edge of the orbital period gap, the long-term behaviour of IR Com defies its classification, in particular the occurrence of a deep, long low state is so far unique among short-period CVs that are not strongly magnetic.