scispace - formally typeset
Search or ask a question

Showing papers on "White dwarf published in 2005"


Journal ArticleDOI
TL;DR: In this paper, the mass distribution of DA white dwarfs from the Palomar Green (PG) survey was derived and the luminosity function of the sample, weighted by 1/Vmax, was obtained and compared with other determinations.
Abstract: Spectrophotometric observations at high signal-to-noise ratio were obtained of a complete sample of 348 DA white dwarfs from the Palomar Green (PG) Survey. Fits of observed Balmer lines to synthetic spectra calculated from pure-hydrogen model atmospheres were used to obtain robust values of Teff, log g, masses, radii, and cooling ages. The luminosity function of the sample, weighted by 1/Vmax, was obtained and compared with other determinations. Incompleteness of the sample due to selection by photographic mu - mb color and magnitude limits was found to be a serious problem, and an attempt is made to correct for this. The mass distribution of the white dwarfs is derived, after important corrections for the radii of the white dwarfs in this magnitude-limited survey and for the cooling time scales. This distribution has (1) a "peak" component centered near 0.6 M⊙, (2) a low-mass component centered near 0.4 M⊙, and (3) a high-mass component above about 0.8 M⊙. The formation rate of DA white dwarfs from the PG is estimated to be 0.6 × 10-12 pc-3 yr-1. Of these, 75% are from the peak component, 10% from the low-mass component, and 15% from the high-mass component. The low-mass component requires binary evolution for 100% of the objects, with a degenerate companion likely in the majority of cases. Comparison with predictions from a theoretical study of the white dwarf formation rate for single stars indicates that ≥80% of the high-mass component requires a different origin, presumably mergers of lower mass double degenerate stars. The need for a binary channel may not be as great for the massive, very hot white dwarfs found in the EUV all-sky surveys. In an Appendix, we even suggest that an enhanced density of the massive white dwarfs at lower Galactic latitudes might be due to some of them being the progeny of B stars in Gould's Belt. In order to estimate the recent formation rate of all white dwarfs in the local Galactic disk, corrections for incompleteness of the PG, addition of the DB-DO white dwarfs, and allowance for stars hidden by luminous binary companions had to be applied to enhance the rate. An overall formation rate of white dwarfs recently in the local Galactic disk of (1 ± 0.25) × 10-12 pc-3 yr-1 is obtained. Admittedly, the systematic errors in this estimate are difficult to quantify. Two recent studies of samples of nearby Galactic planetary nebulae lead to estimates around twice as high. Difficulties in reconciling these determinations are discussed.

490 citations


01 Dec 2005
TL;DR: In this article, four ground-based photometric systems with respect to the Hubble Space Telescope (HST) absolute flux scale, defined by Vega and four fundamental DA white dwarfs, are compared with the respective observed magnitudes of larger sets of DA white stars that have well-determined effective temperatures and surface gravities.
Abstract: We have calibrated four major ground-based photometric systems with respect to the Hubble Space Telescope (HST) absolute flux scale, which is defined by Vega and four fundamental DA white dwarfs. These photometric systems include the Johnson-Kron-Cousins UBVRI, the Stromgren uvby filters, the Two Micron All Sky Survey JHKs, and the Sloan Digital Sky Survey ugriz filters. Synthetic magnitudes are calculated from model white dwarf spectra folded through the published filter response functions; these magnitudes in turn are absolutely calibrated with respect to the HST flux scale. Effective zero-magnitude fluxes and zero-point offsets of each system are determined. In order to verify the external observational consistency, as well as to demonstrate the applicability of these definitions, the synthetic magnitudes are compared with the respective observed magnitudes of larger sets of DA white dwarfs that have well-determined effective temperatures and surface gravities and span a wide range in both of these parameters.

307 citations


Journal ArticleDOI
Laura Greggio1
TL;DR: In this paper, an effective formalism to couple the rate of SNIa explosions from a single burst of star formation and the star formation history is presented, which rests upon the definition of the realization probability of the SNI event (A Ia ) and the distribution function of the delay times (f Ia (τ)).
Abstract: The aim of this paper is to provide a handy tool to compute the impact of type la SN (SNIa) events on the evolution of stellar systems. An effective formalism to couple the rate of SNIa explosions from a single burst of star formation and the star formation history is presented, which rests upon the definition of the realization probability of the SNIa event (A Ia ) and the distribution function of the delay times (f Ia (τ)). It is shown that the current SNIa rate in late type galaxies constrains A Ia to be on the order of 10 -3 (i.e. 1 SNIa every 1000 M ○. of gas turned into stars), while the comparison of the current rates in early and late type galaxies implies that f Ia ought to be more populated at short delays. The paper presents analytical formulations for the description of the J Ia function for the most popular models of SNIa progenitors, namely Single Degenerates (Chandrasekhar and Sub-Chandrasekhar exploders), and Double Degenerates. These formulations follow entirely from general considerations on the evolutionary behavior of stars in binary systems, modulo a schematization of the outcome of the phases of mass exchange, and compare well with the results of population synthesis codes, for the same choice of parameters. The derivation presented here offers an immediate astrophysical interpretation of the shape of the f Ia functions, and have a built in parametrization of the key properties of the alternative candidates. The important parameters appear to be the minimum and maximum masses of the components of the binary systems giving rise to a SNIa explosions, the distribution of the primary mass and of the mass ratios in these systems, the distribution of the separations of the DD systems at their birth. The various models for the progenitors correspond to markedly different impact on the large scales; correspondingly, the model for the progenitor can be constrained by examining the relevant observations. Among these, the paper concentrates on the trend of the current SNIa rate with parent galaxy type. The recent data by Mannucci et al. (2005, A&A, 433, 807) favor the DD channel over the SD one, which tends to predict a too steep distribution function of the delay times. The SD scenario can be reconciled with the observations only if the distribution of the mass ratios in the primordial binaries is flat and the accretion efficiency onto the WD is close to 100%. The various models are characterized by different timescales for the Fe release from a single burst stellar population. In particular the delay time within which half of the SNIa events from such a population have occurred, ranges between 0.3 and 3 Gyr, for a wide variety of hypothesis on the progenitors.

292 citations


Journal ArticleDOI
TL;DR: In this article, the authors studied a Type Ia supernova explosion using large-scale three-dimensional numerical simulations based on reactive fluid dynamics with a simplified mechanism for nuclear reactions and energy release.
Abstract: We study a Type Ia supernova explosion using large-scale three-dimensional numerical simulations based on reactive fluid dynamics with a simplified mechanism for nuclear reactions and energy release. The initial deflagration stage of the explosion involves a subsonic turbulent thermonuclear flame propagating in the gravitational field of an expanding white dwarf. The deflagration produces an inhomogeneous mixture of unburned carbon and oxygen with intermediate-mass and iron-group elements in central parts of the star. During the subsequent detonation stage, a supersonic detonation wave propagates through the material unburned by the deflagration. The total energy released in this delayed-detonation process, (1.3-1.6) × 1051 ergs, is consistent with a typical range of kinetic energies obtained from observations. In contrast to the deflagration model, which releases only about 0.6 × 1051 ergs, the delayed-detonation model does not leave carbon, oxygen, and intermediate-mass elements in central parts of a white dwarf. This removes the key disagreement between three-dimensional simulations and observations, and makes a delayed detonation the mostly likely mechanism for Type Ia supernova explosions.

269 citations


Journal ArticleDOI
TL;DR: In this paper, the relativistic orbital decay of a low-mass, circular binary pulsar system with a helium white dwarf secondary was detected using high precision pulse timing measurements.
Abstract: PSR J0751+1807 is a millisecond pulsar in a circular 6 hr binary system with a helium white dwarf secondary. Through high precision pulse timing measurements with the Arecibo and Effelsberg radio telescopes, we have detected the decay of its orbit due to emission of gravitational radiation. This is the first detection of the relativistic orbital decay of a low-mass, circular binary pulsar system. The measured rate of change in orbital period, corrected for acceleration biases, is dP_b/dt=(-6.4+-0.9)x10^-14. Interpreted in the context of general relativity, and combined with measurement of Shapiro delay, it implies a pulsar mass of 2.1+-0.2 solar masses, the most massive pulsar measured. This adds to the emerging trend toward relatively high neutron star masses in neutron star--white dwarf binaries. Additionally, there is some evidence for an inverse correlation between pulsar mass and orbital period in these systems. We consider alternatives to the general relativistic analysis of the data, and we use the pulsar timing data to place limits on violations of the strong equivalence principle.

254 citations


Journal ArticleDOI
TL;DR: In this paper, the formation and evolution of hydrogen-deficient post-AGB white dwarfs is studied in the context of a double-diffusive mixing-length theory of convection.
Abstract: We explore the formation and evolution of hydrogen-deficient post-AGB white dwarfs. To this end, we compute the complete evolution of an initially 2.7 Mstar from the zero-age main sequence through the thermally pulsing and mass-loss phases to the white dwarf stage. Particular attention is given to the chemical abundance changes during the whole evolution. A time-dependent scheme for the simultaneous treatment of abundance changes caused by nuclear reactions, diffusive overshoot- ing, salt fingers and convection is considered. We employed the double-diffusive mixing-length theory of convection for fluids with composition gradients. The study can therefore be considered as a test of its performance in low-mass stars. Also, time- dependent element diffusion for multicomponent gases is taken into account during the white dwarf evolution. The evolutionary stages corresponding to the last helium thermal pulse on the early white-dwarf cooling branch and the following born-again episode are carefully explored. Relevant aspects for PG 1159 stars and DB white dwarf evolution are studied in the framework of these new evolutionary models that take into account the history of the white dwarf progenitor. The scope of the calculations is extended to the domain of the helium-rich, carbon-contaminated DQ white dwarfs with the aim of exploring the plausibility of the evolutionary connection PG 1159-DB-DQ. In this regard, the implications for the double-layered chemical structure in pulsating DB white dwarfs is investigated. We examine the consequences of mass-loss episodes during the PG 1159 stage for the chemical stratification of the outer layer of DB and DQ white dwarfs.

243 citations


Journal ArticleDOI
TL;DR: In this paper, a two-dimensional differentially rotating white dwarfs model with rotation laws resembling those of the one-dimensional models of the previous work is presented, which is applicable for a wide range of angular velocity profiles, including solid body rotation.
Abstract: A recent study indicated that the inner cores of rapidly accreting (M > 10 -7 M ○. yr -1 ) CO white dwarfs may rotate differentially, with a shear fate near the threshold value for the onset of the dynamical shear instability. Such differentially rotating white dwarfs have critical masses for thermonuclear explosion or electron-capture induced collapse that significantly exceed the canonical Chandrasekhar limit. Here, we construct two-dimensional differentially rotating white dwarf models with rotation laws resembling those of the one-dimensional models of the previous work. We derive analytic relations between the white dwarf mass, its angular momentum, and its rotational-, gravitational- and binding energy. We show that these relations are applicable for a wide range of angular velocity profiles, including solid body rotation. Taken at a central density of 2 x 10 9 g cm -3 they specify initial models for the thermonuclear explosion of rotating CO white dwarfs. At ρ c = 10 10 g cm -3 and 4 x 10 9 g cm -3 , they give criteria for the electron-capture induced collapse of rotating CO and ONeMg white dwarfs, respectively. We demonstrate that pre-explosion and pre-collapse conditions of both rigidly and differentially rotating white dwarfs are well established by the present work, which may facilitate future multi-dimensional simulations of type la supernova explosions and studies of the formation of millisecond pulsars and gamma-ray bursts from collapsing white dwarfs. Our results lead us to suggest various possible evolutionary scenarios for progenitors of type la supernovae, leading to a new paradigm of a variable mass of exploding white dwarfs, at values well above the classical Chandrasekhar mass. Based on our 2D-models, we argue that the supernova peak brightness is proportional to the white dwarf mass, which could explain various aspects of the diversity of type la supernovae, such as their variation in brightness, the dependence of their mean luminosity on the host galaxy type, and the weak correlation between ejecta velocity and peak brightness.

238 citations


Journal ArticleDOI
TL;DR: In this article, the formation and evolution of hydrogen-deficient post-AGB white dwarfs was studied. But the authors focused on the chemical abundance changes during the whole evolution, and employed the double-diffusive mixing-length theory for fluids with composition gradients.
Abstract: This paper is designed to explore the formation and evolution of hydrogen-deficient post-AGB white dwarfs. To this end, we compute the complete evolution of an initially 2.7 M_sun star from the zero-age main sequence through the thermally pulsing and mass-loss phases to the white dwarf stage. Particular attention is given to the chemical abundance changes during the whole evolution. A time-dependent scheme for the simultaneous treatment of abundance changes caused by nuclear reactions, diffusive overshooting, salt fingers and convection is considered. We employed the double-diffusive mixing-length theory of convection for fluids with composition gradients (Grossman & Taam 1996). The study can therefore be considered as a test of its performance in low-mass stars. Also, time-dependent element diffusion for multicomponent gases is taken into account during the white dwarf evolution. The evolutionary stages corresponding to the last helium thermal pulse on the early white-dwarf cooling branch and the following born-again episode are carefully explored. Relevant aspects for PG1159 stars and DB white dwarf evolution are studied in the frame of these new evolutionary models that take into account the history of the white dwarf progenitor. The scope of the calculations is extended to the domain of the helium-rich, carbon-contaminated DQ white dwarfs with the aim of exploring the plausibility of the evolutionary connection PG1159-DB-DQ. In this regard, the implications for the double-layered chemical structure in pulsating DB white dwarfs is investigated. Another aspect of the investigation concerned the consequences of mass-loss episodes during the PG1159 stage for the chemical stratification of the outer layer of DB and DQ white dwarfs.

226 citations


Journal ArticleDOI
TL;DR: The results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC 6397, focusing on the cluster's white dwarf cooling sequence, are presented in this paper.
Abstract: We present the results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC 6397, focussing attention on the cluster's white dwarf cooling sequence. This sequence is shown to extend over 5 mag in depth, with an apparent cutoff at magnitude F814W ~ 27.6. We demonstrate, using both artificial star tests and the detectability of background galaxies at fainter magnitudes, that the cutoff is real and represents the truncation of the white dwarf luminosity function in this cluster. We perform a detailed comparison between cooling models and the observed distribution of white dwarfs in color and magnitude, taking into account uncertainties in distance, extinction, white dwarf mass, progenitor lifetimes, binarity, and cooling model uncertainties. After marginalizing over these variables, we obtain values for the cluster distance modulus and age of μ0 = 12.02 ± 0.06 and Tc = 11.47 ± 0.47 Gyr (95% confidence limits). Our inferred distance and white dwarf initial-final mass relations are in good agreement with other independent determinations, and the cluster age is consistent with, but more precise than, prior determinations made using the main-sequence turnoff method. In particular, within the context of the currently accepted ΛCDM cosmological model, this age places the formation of NGC 6397 at a redshift z ~ 3, at a time when the cosmological star formation rate was approaching its peak.

226 citations


Journal ArticleDOI
TL;DR: In this paper, the authors presented results of a near-infrared imaging survey for low-mass stellar and substellar companions to white dwarfs, which was capable of directly detecting companions at orbital separations between ~100 and 5000 AU with masses as low as 0.05 M?.
Abstract: This paper presents results of a near-infrared imaging survey for low-mass stellar and substellar companions to white dwarfs. A wide-field proper-motion survey of 261 white dwarfs was capable of directly detecting companions at orbital separations between ~100 and 5000 AU with masses as low as 0.05 M?, while a deep near-field search of 86 white dwarfs was capable of directly detecting companions at separations between ~50 and 1100 AU with masses as low as 0.02 M?. Additionally, all white dwarf targets were examined for near-infrared excess emission, a technique capable of detecting companions at arbitrarily close separations down to masses of 0.05 M?. No brown dwarf candidates were detected, which implies a brown dwarf companion fraction of <0.5% for white dwarfs. In contrast, the stellar companion fraction of white dwarfs as measured by this survey is 22%, uncorrected for bias. Moreover, most of the known and suspected stellar companions to white dwarfs are low-mass stars whose masses are only slightly greater than the masses of brown dwarfs. Twenty previously unknown stellar companions were detected, five of which are confirmed or likely white dwarfs themselves, while 15 are confirmed or likely low-mass stars. Similar to the distribution of cool field dwarfs as a function of spectral type, the number of cool unevolved dwarf companions peaks at mid-M type. Based on the present work, relative to this peak, field L dwarfs appear to be roughly 2-3 times more abundant than companion L dwarfs. Additionally, there is no evidence that the initial companion masses have been altered by post-main-sequence binary interactions.

221 citations


Journal ArticleDOI
TL;DR: In this paper, the authors determined the masses and radii of the progenitors of white dwarfs in binary systems from fits to detailed stellar evolution models and used these to reconstruct the mass-transfer phase in which the white dwarf was formed.
Abstract: We determine the possible masses and radii of the progenitors of white dwarfs in binaries from fits to detailed stellar evolution models and use these to reconstruct the mass-transfer phase in which the white dwarf was formed. We confirm the earlier finding that in the first phase of mass transfer in the binary evolution leading to a close pair of white dwarfs, the standard common-envelope formalism (the alpha-formalism) equating the energy balance in the system (implicitly assuming angular momentum conservation) does not work. An algorithm equating the angular momentum balance (implicitly assuming energy conservation) can explain the observations. This conclusion is now based on 10 observed systems rather than three. With the latter algorithm (the gamma-algorithm) the separation does not change much for approximately equal-mass binaries. Assuming constant efficiency in the standard alpha-formalism and a constant value of gamma, we investigate the effect of both methods on the change in separation in general and conclude that when there is observational evidence for strong shrinkage of the orbit, the gamma-algorithm also leads to this. We then extend our analysis to all close binaries with at least one white dwarf component and reconstruct the mass-transfer phases that lead to these binaries. In this way we find all possible values of the efficiency of the standard alpha-formalism and of gamma that can explain the observed binaries for different progenitor and companion masses. We find that all observations can be explained with a single value of gamma, making the gamma-algorithm a useful tool to predict the outcome of common-envelope evolution. We discuss the consequences of our findings for different binary populations in the Galaxy, including massive binaries, for which the reconstruction method cannot be used.

Journal ArticleDOI
TL;DR: In this paper, the discovery of an unbound hyper-velocity star, US 708, in the Milky Way halo, with a heliocentric radial velocity of +708 ± 15 km s -1.
Abstract: We report the discovery of an unbound hyper-velocity star, US 708, in the Milky Way halo, with a heliocentric radial velocity of +708 ± 15 km s -1 . A quantitative NLTE model atmosphere analysis of optical spectra obtained with LRIS at the Keck I telescope shows that US 708 is an extremely helium-rich (N He /N H = 10) subluminous O type star with T eff = 44 500 K, logy = 5.23 at a distance of 19 kpc. Its Galactic rest frame velocity is at least 751 km s -1 , much higher than the local Galactic escape velocity indicating that the star is unbound to the Galaxy. It has been suggested that such hyper-velocity stars can be formed by the tidal disruption of a binary through interaction with the super-massive black hole (SMBH) at the Galactic centre (GC). Numerical kinemalical experiments are carried out to reconstruct the path from the GC. US 708 needs about 32 Myrs to travel from the GC to its present position, less than its evolutionary lifetime. Its predicted proper motion μ α cos δ = -2.3 mas yr -1 and μ δ = -2.4 mas yr -1 should be measurable by future space missions. We conjecture that US 708 is formed by the merger of two helium white dwarfs in a close binary induced by the interaction with the SMBH in the GC and then escaped.

Journal ArticleDOI
TL;DR: In this paper, the spectral energy distribution (SED) of the white dwarf G29-38 has a continuum peak around 4.5 μm and a 9-11 μm emission feature 1.25 times brighter than the continuum.
Abstract: We present new observations of the white dwarf G29-38 with the camera (4.5 and 8 μm), photometer (24 μm), and spectrograph (5.5-14 μm) of the Spitzer Space Telescope. This star has an exceptionally large infrared excess, amounting to 3% of the bolometric luminosity. The spectral energy distribution (SED) has a continuum peak around 4.5 μm and a 9-11 μm emission feature 1.25 times brighter than the continuum. A mixture of amorphous olivine and a small amount of forsterite in an emitting region 1-5 R☉ from the star can reproduce the shape of the 9-11 μm feature. The SED also appears to require amorphous carbon to explain the hot continuum. Our new measurements support the idea that a relatively recent disruption of a comet or asteroid created the cloud.

Journal ArticleDOI
TL;DR: In this article, a sample of white dwarfs is selected from SDSS DR3 imaging data using their reduced proper motions, based on improved proper motions from the combined USNO-B combined data, and a luminosity function is constructed covering the range 7 < M_bol < 16.
Abstract: A sample of white dwarfs is selected from SDSS DR3 imaging data using their reduced proper motions, based on improved proper motions from SDSS plus USNO-B combined data. Numerous SDSS and followup spectra (Kilic et al. 2005) are used to quantify completeness and contamination of the sample; kinematic models are used to understand and correct for velocity-dependent selection biases. A luminosity function is constructed covering the range 7 < M_bol < 16, and its sensitivity to various assumptions and selection limits is discussed. The white dwarf luminosity function based on 6000 stars is remarkably smooth, and rises nearly monotonically to M_bol = 15.3. It then drops abruptly, although the small number of low-luminosity stars in the sample and their unknown atmospheric composition prevent quantitative conclusions about this decline. Stars are identified that may have high tangential velocities, and a preliminary luminosity function is constructed for them.

Journal ArticleDOI
TL;DR: The spectral energy distribution of the white dwarf G 29-38 with the camera (4.5 and 8 microns), photometer (24 microns, and spectrograph (5.5-14 microns) of the Spitzer Space Telescope was observed in this article.
Abstract: We present new observations of the white dwarf G 29-38 with the camera (4.5 and 8 microns), photometer (24 microns), and spectrograph (5.5-14 microns) of the Spitzer Space Telescope. This star has an exceptionally large infrared excess amounting to 3% of the bolometric luminosity. The spectral energy distribution has a continuum peak around 4.5 micros and a 9-11 micron emission feature 1.25 times brighter than the continuum. A mixture of amorphous olivine and a small amount of forsterite in an emitting region 1-5 Rsun from the star can reproduce the shape of the 9-11 micron feature. The spectral energy distribution also appears to require amorphous carbon to explain the hot continuum. Our new measurements support the idea that a relatively recent disruption of a comet or asteroid created the cloud.

Journal ArticleDOI
TL;DR: In this article, high-velocity features (HVFs) such as those seen in the near-maximum spectra of some Type Ia supernovae (SNe Ia; e.g., SN 2000cx) have been searched for in the available SN Ia spectra observed earlier than 1 week before B maximum.
Abstract: Evidence of high-velocity features (HVFs) such as those seen in the near-maximum spectra of some Type Ia supernovae (SNe Ia; e.g., SN 2000cx) has been searched for in the available SN Ia spectra observed earlier than 1 week before B maximum. Recent observational efforts have doubled the number of SNe Ia with very early spectra. Remarkably, all SNe Ia with early data (seven in our Research Training Network sample and 10 from other programs) show signs of such features, to a greater or lesser degree, in Ca II IR and some also in the Si II λ6355 line. HVFs may be interpreted as abundance or density enhancements. Abundance enhancements would imply an outer region dominated by Si and Ca. Density enhancements may result from the sweeping up of circumstellar material (CSM) by the highest velocity SN ejecta. In this scenario, the high incidence of HVFs suggests that a thick disk and/or a high-density companion wind surrounds the exploding white dwarf, as may be the case in single degenerate systems. Large-scale angular fluctuations in the radial density and abundance distribution may also be responsible: this could originate in the explosion and would suggest a deflagration as the more likely explosion mechanism. CSM interaction and surface fluctuations may coexist, possibly leaving different signatures on the spectrum. In some SNe, the HVFs are narrowly confined in velocity, suggesting the ejection of blobs of burned material.

Posted Content
TL;DR: In this article, the authors investigated the observability of macronovae powered by neutron decay and by radioactive nickel and found that they can be observed with current 10m class telescopes over a range of one hour to one day following the burst.
Abstract: There is now good evidence linking short-hard GRBs with both elliptical and spiral galaxies at relatively low redshifts, redshift of about 02 This contrasts with the average redshift of about 2 of long-duration events, which also occur only in star-forming galaxies The diversity of hosts is reminiscent of type Ia supernovae, which are widely (but not universally) believed to originate from the coalescence of white dwarfs By analogy, it has been postulated that short-hard bursts originate from neutron star mergers Mergers, as well as stellar core-collapse events (type II SNe and long-duration GRBs) are accompanied by long-lived sub-relativistic components powered by radioactive decay of unstable elements produced in the explosion It is therefore interesting to explore whether short duration events also have ejecta powered by radioactivity (ie that are supernova-like) Observations already inform us that any supernova like component in the first few well studied short hard bursts must be fainter than those typical of type Ia or core-collapse supernovae Rather than refer to weaker supernova-like component as ``mini-super nova'', an etymologically indefensible term, I use the term {\it macronova} I investigate the observability of macronovae powered by neutron decay and by radioactive Nickel Separately, I note that a macronova will reprocess energetic emission arising from a long lived central source I find that surprisingly interesting limits on the basic parameters of macronovae can be obtained provided observations are obtained with current 10-m class telescopes over a range of one hour to one day following the burst

Journal ArticleDOI
TL;DR: In this article, ground-based measurements with millijansky-level sensitivity of GD 362 were reported to be a second single white dwarf with an infrared excess, which can be explained by emission from a passive, flat, opaque dust disk that lies within the Roche radius of the white dwarf.
Abstract: Eighteen years after an infrared excess was discovered associated with the white dwarf G29-38, we report ground-based measurements (JHKsKL'N') with millijansky-level sensitivity of GD 362 that show it to be a second single white dwarf with an infrared excess. As a first approximation, the excess around GD 362, which amounts to ~3% of the total stellar luminosity, can be explained by emission from a passive, flat, opaque dust disk that lies within the Roche radius of the white dwarf. The dust may have been produced by the tidal disruption of a large parent body such as an asteroid. Accretion from this circumstellar disk could account for the remarkably high abundance of metals in the star's photosphere.

Journal ArticleDOI
TL;DR: In this paper, the authors search for faint Ca II lines in the spectra of about 800 apparently single white dwarfs observed at high resolution for the SPY (ESO Supernova Progenitor Survey) survey.
Abstract: We search for faint Ca II lines in the spectra of about 800 apparently single white dwarfs observed at high resolution for the SPY (ESO Supernova Progenitor Survey) survey. Photospheric Ca is detected in 24 DAZ; in 25 mostly hot objects the observed lines must be interstellar. We also rediscover 9 strong H and K lines in helium-dominated atmospheres and discover for the first time faint lines in 8 DB. Most of these also show faint hydrogen lines and are thus classified DBAZ. The distribution of metal abundances is discussed and compared with the predictions of the accretion/diffusion scenario. We argue that the observations are easier to understand in a scenario of continuous ongoing accretion with rates varying with the conditions of the ambient medium, rather than with the strongly idealized "two phase accretion/diffusion scenario" of Dupuis et al. (1992, 1993a,b).

Journal ArticleDOI
TL;DR: In this article, the first set of definitive trigonometric parallaxes and proper motions from the Cerro Tololo Inter-American Observatory Parallax Investigation (COTI) were presented.
Abstract: We present the first set of definitive trigonometric parallaxes and proper motions from the Cerro Tololo Inter-American Observatory Parallax Investigation Full astrometric reductions for the program are discussed, including methods of reference star selection, differential color refraction corrections, and conversion of relative to absolute parallax Using data acquired at the 09 m telescope at CTIO, full astrometric solutions and VRIJHKs photometry are presented for 36 red and white dwarf stellar systems with proper motions faster than 10 yr-1 Of these, 33 systems have their first ever trigonometric parallaxes, which comprise 41% of MOTION systems (those reported to have proper motions greater than 10 yr-1) south of δ = 0° that have no parallaxes Four of the systems are new members of the RECONS 10 pc sample for which the first accurate trigonometric parallaxes are published here: DENIS J1048-3956 (404 ± 003 pc), GJ 1128 (LHS 271, 653 ± 010 pc), GJ 1068 (LHS 22, 697 ± 009 pc), and GJ 1123 (LHS 263, 902 ± 016 pc) In addition, two red subdwarf–white dwarf pairs, LHS 193AB and LHS 300AB, are identified The white dwarf secondaries fall in a previously uncharted region of the H-R diagram

Journal ArticleDOI
TL;DR: In this article, the authors present an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations, and they find that DQ stars form a remarkably well-defined sequence in a carbon abundance versus effective temperature diagram; below Teff ~ 10,000 K, carbon pollution decreases monotonically with decreasing effective temperature.
Abstract: We present an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations. In particular, we revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz and discuss the astrophysical implications on the temperature scale and mean mass, as well as the chemical evolution of these stars. We also analyze 40 new DQ stars discovered in the First Data Release of the Sloan Digital Sky Survey (SDSS). Our analysis confirms that effective temperatures (Teff) derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models. Similarly, the mean mass of the trigonometric parallax sample, M = 0.62 M?, is significantly lower than that obtained from pure helium models, M = 0.73 M?, and more consistent with the spectroscopic mean mass of DB stars, M = 0.59 M?, the most likely progenitors of DQ white dwarfs. We find that DQ stars form a remarkably well-defined sequence in a carbon abundance versus effective temperature diagram; below Teff ~ 10,000 K, carbon pollution decreases monotonically with decreasing effective temperature. Improved evolutionary models including diffusion and connecting to the PG 1159 phase are used to infer a typical value for the thickness of the helium layer MHe/M between 10-3 and 10-2, compatible with the predictions of post-AGB models. Several DQ stars in our sample, however, show larger than average carbon abundances. We argue that these DQ stars are all massive white dwarfs and could represent the high-mass tail of the white dwarf mass distribution, with their hotter counterparts corresponding to the hot DQ stars reported recently by Liebert et al. The number distribution of DQ white dwarfs as a function of effective temperature clearly shows a sudden drop at about Teff ~ 7000 K and an abrupt cutoff at Teff ~ 6000 K. The existence of this cutoff is now statistically more significant with the addition of the SDSS stars. The physical mechanism responsible for this cutoff is still unknown, even though it is believed to be somehow related to the existence of the so-called C2H stars at lower temperatures.

Journal ArticleDOI
TL;DR: In this paper, the authors report the discovery of excess K-band radiation from the massive DAZ white dwarf star GD 362, which cannot be explained by a stellar or substellar companion.
Abstract: We report the discovery of excess K-band radiation from the massive DAZ white dwarf star GD 362. Combining infrared photometric and spectroscopic observations, we show that the excess radiation cannot be explained by a stellar or substellar companion, and is likely to be caused by a debris disk. This would be only the second such system known, discovered 18 years after G29-38, the only single white dwarf currently known to be orbited by circumstellar dust. Both of these systems favor a model with accretion from a surrounding debris disk to explain the metal abundances observed in DAZ white dwarfs. Nevertheless, observations of more DAZs in the mid-infrared are required to test if this model can explain all DAZs.

Journal ArticleDOI
TL;DR: In this paper, the Coulomb barrier penetration in dense environments and the astrophysical S factor at low energies were analyzed for all five different nuclear burning regimes in dense matter (two thermonuclear regimes, two pycnonuclear ones, and the intermediate regime).
Abstract: In this paper we analyze the nuclear fusion rates among equal nuclei for all five different nuclear burning regimes in dense matter (two thermonuclear regimes, two pycnonuclear ones, and the intermediate regime). The rate is determined by Coulomb barrier penetration in dense environments and by the astrophysical S factor at low energies. We evaluate previous studies of the Coulomb barrier problem and propose a simple phenomenological formula for the reaction rate that covers all cases. The parameters of this formula can be varied to take into account current theoretical uncertainties in the reaction rate. The results are illustrated for the example of the 12 C+ 12 C fusion reaction. This reaction is important for the understanding of nuclear burning in evolved stars, in exploding white dwarfs producing type Ia supernovas, and in accreting neutron stars. The S factor at stellar energies depends on a reliable fit and extrapolation of the experimental data. We calculate the energy dependence of the S factor by using a recently developed parameter-free model for the nuclear interaction, taking into account the effects of the Pauli nonlocality. For illustration, we analyze the efficiency of carbon burning in a wide range of densities and temperatures of stellar matter with the emphasis on carbon ignition at densities ρ > ∼ 10 9 gc m −3 .

Journal ArticleDOI
TL;DR: In this paper, the authors re-evaluate the ensemble of data that determines the initial-final mass relationship (IFMR) and argue that the IFMR can be characterized by a mean IFMR about which there is an intrinsic scatter.
Abstract: Recent studies of white dwarfs in open clusters have provided new constraints on the initial-final mass relationship (IFMR) for main-sequence stars with masses in the range 2.5-6.5 M ○. . We re-evaluate the ensemble of data that determines the IFMR and argue that the IFMR can be characterized by a mean IFMR about which there is an intrinsic scatter. We investigate the consequences of the IFMR for the observed mass distribution of field white dwarfs using population synthesis calculations. We show that while a linear IFMR predicts a mass distribution that is in reasonable agreement with the recent results from the Palomar-Green survey, the data are better fitted by an IFMR with some curvature. Our calculations indicate that a significant (∼28) percentage of white dwarfs originating from a single star evolution has masses in excess of ∼0.8 M ○. , obviating the necessity for postulating the existence of a dominant population of high-mass white dwarfs that arise from binary star mergers.

Journal ArticleDOI
TL;DR: In this paper, the Caltech-Parkes-Swinburne Recorder II was used to estimate the mass of the binary millisecond pulsar PSR J1909-3744.
Abstract: We report on nearly 2 years of timing observations of the low-mass binary millisecond pulsar PSR J1909-3744 with the Caltech-Parkes-Swinburne Recorder II, a new instrument that gives unprecedented timing precision. Daily observations give a weighted rms residual of 74 ns, indicating an extremely low level of systematic error. We have greatly improved on the previous parallax and proper motion measurements of PSR J1909-3744, yielding a distance of 1.14 kpc and transverse velocity of 200 km s-1. The system's orbital eccentricity is just (1.35 ± 12) × 10-7, the smallest yet recorded. Since their discovery, the masses of the rapidly rotating millisecond pulsars have remained a mystery, with the recycling hypothesis arguing for heavy objects, and the accretion-induced collapse of a white dwarf more consistent with neutron stars less than the Chandrashkar limit. Fortuitously, PSR J1909-3744 is an edge-on system, and our data have allowed the measurement of the range and shape of the Shapiro delay to high accuracy, giving the first precise determination of a millisecond pulsar mass to date, mp = 1.438 ± 0.024 M☉. The mass of PSR J1909-3744 is at the upper edge of the range observed in mildly recycled pulsars in double neutron star systems, consistent with the recycling hypothesis. It appears that the production of millisecond pulsars is possible with the accretion of <0.2 M☉.

Journal ArticleDOI
TL;DR: In this article, the relativistic Shapiro delay, perturbations of pulsar orbital elements due to secular and annual motion of the Earth, and the pulsar's parallax were measured with uncertainties as small as 200 ns.
Abstract: We report on 12 years of observations of PSR J1713+0747, a pulsar in a 68 day orbit with a white dwarf. Pulse times of arrival were measured with uncertainties as small as 200 ns. The timing data yielded measurements of the relativistic Shapiro delay, perturbations of pulsar orbital elements due to secular and annual motion of the Earth, and the pulsar's parallax, as well as pulse spin-down, astrometric, and Keplerian measurements. The observations constrain the masses of the pulsar and secondary star to be m1 = 1.3 ± 0.2 M☉ and m2 = 0.28 ± 0.03 M☉, respectively (68% confidence). Combining the theoretical orbital period-core mass relation with the observational constraints yields a somewhat higher pulsar mass, m1 = 1.53 M☉. The parallax is π = 0.89 ± 0.08 mas, corresponding to a distance of 1.1 ± 0.1 kpc; the precision of the parallax measurement is limited by uncertainties in the electron content of the solar wind. The transverse velocity is unusually small, 33 ± 3 km s-1. We find significant timing noise on timescales of several years, but no more than expected by extrapolating timing noise statistics from the slow pulsar population. With the orientation of the binary orbit fully measured, we are able to improve on previous tests of equivalence principle violations.

Journal ArticleDOI
TL;DR: In this paper, the authors report detailed follow-up observations of the cataclysmic variable HS 2331+3905, identified as an emissionline object in the Hamburg Quasar Survey.
Abstract: We report detailed follow-up observations of the cataclysmic variable HS 2331+3905, identified as an emissionline object in the Hamburg Quasar Survey. An orbital period of 81.08 min is unambiguously determined from the detection of eclipses in the light curves of HS 2331+3905. A second photometric period is consistently detected at P 83.38 min,∼2.8% longer than P orb , which we tentatively relate to the presence of permanent superhumps. High time resolution photometry exhibits short-timescale variability on time scales of 5−6 min which we interpret as non-radial white dwarf pulsations, as well as a coherent signal at 1.12 min, which is likely to be the white dwarf spin period. A large-amplitude quasi-sinusoidal radial velocity modulation of the Balmer and Helium lines with a period ∼3.5 h is persistently detected throughout three seasons of time-resolved spectroscopy. However, this spectroscopic period, which is in no way related to the orbital period, is not strictly coherent but drifts in period and/or phase on time scales of a few days. Modeling the far-ultraviolet to infrared spectral energy distribution of HS 2331+3905, we determine a white dwarf temperature of T eff 10 500 K (assuming M wd = 0.6 M ), close to the ZZ Ceti instability strip of single white dwarfs. The spectral model implies a distance of d = 90 ± 15 pc, and a low value for the distance is supported by the large proper motion of the system, µ = 0.14 yr −1 . The non-detection of molecular bands and the low J, H, and K fluxes of HS 2331+3905 make this object a very likely candidate for a brown-dwarf donor.

Journal ArticleDOI
TL;DR: The radio source FIRST J102347.6+003841 as discussed by the authors was presented as the first radio-selected cataclysmic variable star and showed a superficially normal, mid-G type photosphere, with no detectable emission lines.
Abstract: The radio source FIRST J102347.6+003841 was presented as the first radio-selected cataclysmic variable star. In the discovery paper, Bond et al. (2002) show a spectrum consistent with a magnetic AM Her–type system, or polar, featuring strong Balmer lines, He I and He II emission lines, and a light curve with rapid, irregular flickering. In contrast, Woudt, Warner, and Pretorius found a smoothly varying light curve with a period near 4.75 hr and one minimum per orbit, indicating that the state of the system had changed dramatically. We present time-resolved spectra showing a superficially normal, mid-G type photosphere, with no detectable emission lines. The absorption-line radial velocity varies sinusoidally, with semiamplitude 268 ± 4 km s-1, on the orbital period, which is refined to 0.198094(2) days. At this orbital period, the secondary's spectral type is atypically early, suggesting an unusual evolutionary history. We also obtained photometry around the orbit in B, V, and I. The light curve resembles that observed by Woudt, Warner, and Pretorius, and the colors are modulated in a manner consistent with a heating effect. A simple illumination model matches the observations strikingly well, with a Roche lobe–filling secondary near Teff = 5650 K being illuminated by a primary with an isotropic luminosity of ~2 L⊙. The modest amplitude of the observed modulation constrains the orbital inclination i ~ 55° or less, unless the gravity darkening is artificially reduced. Combining the low i with the secondary's velocity amplitude gives a primary star mass above the Chandrasekhar limit when conventional gravity darkening is assumed. We consider the robustness of this conclusion and examine the possibility that the compact object in this system is not a white dwarf, in which case this is not actually a cataclysmic variable. On close examination, FIRST J102347.6+003841 defies easy classification.

Journal ArticleDOI
TL;DR: In this article, the authors present results from their ongoing follow-up observations of double white dwarf binaries detected in the ESO SN Ia Progenitor Surveys (SPY), discuss their observing strategy and data analysis and present the orbital solutions of five close double white dwarfs binaries: HE0320−1917, HE1511−0448, WD0326−273, WD1013−010 and WD1210+140.
Abstract: We present results from our ongoing follow-up observations of double white dwarf binaries detected in the ESO SN Ia Progenitor SurveY (SPY). We discuss our observing strategy and data analysis and present the orbital solutions of five close double white dwarf binaries: HE0320−1917, HE1511−0448, WD0326−273, WD1013−010 and WD1210+140. Their periods range from 0.44 to 3.22 days. In none of these systems we find any spectral lines originating from the companion. This rules out main sequence companions and indicates that the companion white dwarfs are significantly older and cooler than the bright component. Infrared photometry suggests the presence of a cool, helium-rich white dwarf companion in the binary WD 0326−273. We briefly discuss the consequences of our findings for our understanding of the formation and evolution of double white dwarfs.

Journal ArticleDOI
TL;DR: In this article, an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations is presented, and it is shown that effective temperatures derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models.
Abstract: We present an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations. In particular, we revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz, and discuss the astrophysical implications on the temperature scale and mean mass, as well as the chemical evolution of these stars. We also analyze 40 new DQ stars discovered in the first data release of the Sloan Digital Sky Survey. Our analysis confirms that effective temperatures derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models. Similarly the mean mass of the trigonometric parallax sample, = 0.62 Mo, is significantly lower than that obtained from pure helium models, = 0.73 Mo, and more consistent with the spectroscopic mean mass of DB stars, = 0.59 Mo, the most likely progenitors of DQ white dwarfs. We find that DQ stars form a remarkably well defined sequence in a carbon abundance versus effective temperature diagram; below Teff~10,000 K, carbon pollution decreases monotonically with decreasing effective temperature. Improved evolutionary models including diffusion and connecting to the PG 1159 phase are used to infer a typical value for the thickness of the helium layer M_He/M_* between 10^{-3} and 10^{-2}, compatible with the predictions of post-AGB models.