A source of high-velocity white dwarfs

We investigate whether the recently observed population of high-velocity white dwarfs can be derived from a population of binaries residing initially within the thin disc of the Galaxy. In particular, we consider binaries where the primary is sufficiently massive to explode as a Type II supernova. A large fraction of such binaries are broken up when the primary then explodes as a supernova, owing to the combined effects of the mass loss from the primary and the kick received by the neutron star on its formation. For binaries where the primary evolves to fill its Roche lobe, mass transfer from the primary leads to the onset of a common envelope phase during which the secondary and the core of the primary spiral together as the envelope is ejected. Such binaries are the progenitors of X-ray binaries if they are not broken up when the primary explodes. For those systems that are broken up, a large number of the secondaries receive kick velocities ∼100–200 km s⁻¹ and subsequently evolve into white dwarfs. We compute trajectories within the Galactic potential for this population of stars and relate the birth rate of these stars over the entire Galaxy to those seen locally with high velocities relative to the local standard of rest (LSR) . We show that for a reasonable set of assumptions concerning the Galactic supernova rate and the binary population, our model produces a local number density of high-velocity white dwarfs compatible with that inferred from observations. We therefore propose that a population of white dwarfs originating in the thin disc may make a significant contribution to the observed population of high-velocity white dwarfs.     

Supernova kicks and misaligned Be star binaries

Be stars are rapidly spinning B stars surrounded by an outflowing disc of gas in Keplerian rotation. Be star/X-ray binary systems contain a Be star and a neutron star. They are found to have non-zero eccentricities and there is evidence that some systems have a misalignment between the spin axis of the star and the spin axis of the binary orbit. The eccentricities in these systems are caused by a kick to the neutron star during the supernova that formed it. Such kicks would also give rise to misalignments. In this paper, we investigate the extent to which the same kick distribution can give rise to both the observed eccentricity distribution and the observed misalignments. We find that a Maxwellian distribution of velocity kicks with a low velocity dispersion,  σ_k≈ 15 km s⁻¹  , is consistent with the observed eccentricity distribution but is hard to reconcile with the observed misalignments, typically   i ≥ 25°  . Alternatively, a higher velocity kick distribution,  σ_k= 265 km s⁻¹  , is consistent with the observed misalignments but not with the observed eccentricities, unless post-supernova circularization of the binary orbits has taken place. We discuss briefly how this might be achieved.     

Relation of Velocity Distribution and Mass for DA White Dwarfstwo

White dwarfs are the evolutionary endpoint of the low-and-medium mass stars. In the studies of white dwarfs, the mass of white dwarf is an important physical parameter. In this paper, we give an analysis about the velocity distribution of DA white dwarfs in the Sloan Digital Sky Survey (SDSS), and hope to find the relation between mass and velocity distribution of white dwarfs. We get the radial velocity and tangential velocity of every DA white dwarf according to their proper motion and spectral shift. Through analyzing the velocity distribution of DA white dwarfs, we find that the small-mass white dwarfs, which are produced from the single-star evolution channel, have a relatively large velocity dispersion.     

Magnetic fields and rotation in white dwarfs and neutron stars

Recent spectropolarimetric observations of Ap and Bp stars with improved sensitivity have suggested that most Ap and Bp stars are magnetic with dipolar fields of at least a few hundred gauss. These new estimates suggest that the range of magnetic fluxes found for the majority of magnetic white dwarfs is similar to that of main-sequence Ap–Bp stars, thus strengthening the empirical evidence for an evolutionary link between magnetism on the main sequence and magnetism in white dwarfs. We draw parallels between the magnetic white dwarfs and the magnetic neutron stars and argue that the observed range of magnetic fields in isolated neutron stars  ( B_p ∼ 10¹¹–10¹⁵ G)  could also be explained if their mainly O-type progenitors have effective dipolar fields in the range of a few gauss to a few kilogauss, assuming approximate magnetic flux conservation with the upper limit being consistent with the recent measurement of a field of   B_p ∼ 1100 G  for θ Orion C.
In the magnetic field–rotation diagram, the magnetic white dwarfs can be divided into three groups of different origin: a significant group of strongly magnetized slow rotators  ( P _rot∼ 50 –100 yr)  that have originated from single-star evolution, a group of strongly magnetized fast rotators  ( P _rot∼ 700 s)  , typified by EUVE J0317–853, that have originated from a merger, and a group of modest rotators ( P _rot∼ hours–days) of mixed origin (single-star and CV-type binary evolution). We propose that the neutron stars may similarly divide into distinct classes at birth , and suggest that the magnetars may be the counterparts of the slowly rotating high-field magnetic white dwarfs.     

Pulsar spin–velocity alignment from single and binary neutron star progenitors

The role of binary progenitors of neutron stars (NSs) in the apparent distribution of space velocities and spin–velocity alignment observed in young pulsars is studied. We performed a Monte Carlo synthesis of pulsar populations originated from single and binary stars with different assumptions about the NS natal kick (kick–spin alignment, kick amplitude and kick reduction in electron-capture supernovae in binary progenitors with initial main-sequence masses from the range  8–11 M_⊙  which experienced mass exchange due to Roche lobe overflow). The calculated spin–velocity alignment in pulsars is compared with data inferred from radio polarization measurements. The observed space velocity of pulsars is found to be mostly affected by the natal kick velocity form and its amplitude; the fraction of binaries is not important here for reasonably large kicks. The natal kick–spin alignment is found to strongly affect the spin–velocity correlation of pulsars. Comparison with the observed pulsar spin–velocity angles favours a sizeable fraction of binary progenitors and kick–spin angles  ∼5°–20°  .     

Accretion and dynamical interactions in small-N star-forming clusters: N= 5

We present results from high-resolution hydrodynamical simulations that explore the effects of small-scale clustering in star-forming regions. A large ensemble of small- N clusters with five stellar seeds have been modelled and the resulting properties of stars and brown dwarfs statistically derived and compared with observational data.
Close dynamical interactions between the protostars and competitive accretion driven by the cloud collapse are shown to produce a distribution of final masses that is bimodal, with most of the mass residing in the binary components. When convolved with a suitable core mass function, the final distribution of masses resembles the observed initial mass function, in both the stellar and substellar regimes. Binaries and single stars are found to constitute two kinematically distinct populations, with about half of the singles attaining velocities ≥2 km s⁻¹, which might deprive low-mass star-forming regions of their lightest members in a few crossing times. The eccentricity distribution of binaries and multiples is found to follow a distribution similar to that of observed long-period (uncircularized) binaries.
The results obtained support a mechanism in which a significant fraction of brown dwarfs form under similar circumstances as those of normal stars but are ejected from the common envelope of unstable multiple systems before their masses exceed the hydrogen burning limit. We predict that many close binary stars should have wide brown dwarf companions. Brown dwarfs, and, in general, very low-mass stars, would be rare as pure binary companions. The binary fraction should be a decreasing function of primary mass, with low-mass or substellar primaries being scarce. Where such binaries exist, they are expected either to be close enough (semimajor axis ∼10 au) to survive strong interactions with more massive binaries or to be born in very small molecular cloud cores.     

Neutron star spin–kick velocity correlation effect on binary neutron star coalescence rates and spin–orbit misalignment of the components

We study the effect of the neutron star spin–kick velocity alignment observed in young radio pulsars on the coalescence rate of binary neutron stars. Two scenarios are considered for neutron star formation: when the kick is always present, and when it is small or absent if a neutron star is formed in a binary system as a result of electron-capture degenerate core collapse. The effect is shown to be especially strong for large kick amplitudes and tight alignments, reducing the expected galactic rate of binary neutron star coalescence compared to calculations with randomly directed kicks. The spin–kick correlation also leads to a much narrower neutron star spin–orbit misalignment.     

A search for hidden white dwarfs in the ROSAT EUV survey — II. Discovery of a distant DA+F6/7V binary system in a direction of low-density neutral hydrogen

The ROSAT Wide Field Camera (WFC) survey of the extreme ultraviolet (EUV) has provided us with evidence for the existence of a previously unidentified sample of hot white dwarfs in unresolved, detached binary systems. These stars are invisible at optical wavelengths due to the close proximity of their much more luminous companions (spectral type K or earlier). However, for companions of spectral type ∼A5 or later the white dwarfs are easily visible at far-ultraviolet wavelengths, and can be identified in spectra taken by IUE . 16 such systems have been discovered in this way through ROSAT EUVE IUE observations, including four identified by us in Paper I. In the present paper we report the results of our continuing search during the final year of IUE operations. One new system, RE J0500−364 (DA+F6/7V), has been identified. This star appears to lie at a distance of ∼500−1000 pc, making it one of the most distant white dwarfs, if not the most distant, to be detected in the EUV surveys. The very low line-of-sight neutral hydrogen volume density to this object could place a lower limit on the length of the β CMa interstellar tunnel of diffuse gas, which stretches away from the Local Bubble in a similar direction to RE J0500−364. In this paper we also analyse a number of the stars observed where no white dwarf companion was found. Some of these objects show evidence for chromospheric and coronal activity. Finally, we present an analysis of the previously known WD+active F6V binary HD 27483 (Bo¨hm-Vitense 1993), and show that, at T  ≈ 22 000 K, the white dwarf may be contributing significantly to the observed EUV flux. If so, it is one of the coolest such stars to be detected in the EUV surveys.     

IRTF observations of white dwarfs with possible near-infrared excess

Near-infrared photometry and spectroscopy are obtained for a heterogeneous sample of nearby white dwarfs with possible excess flux as identified primarily in the Two Micron All Sky Survey. Among the sample of 43 stars are a number of white dwarfs that are either metal-rich, magnetic or binary suspects. With a few notable exceptions in four (or possibly five) distinct categories, the newly obtained JHK photometric data fail to corroborate the putative excesses, with  〈 K _IRTF− K _2MASS〉=+0.31  mag. Where available, Galaxy Evolution Explorer photometric data are used to better constrain the overall spectral energy distribution of the white dwarfs, enabling any excess near-infrared flux to stand out more readily against the expected stellar photosphere.
With superior data, a near-infrared photometric excess is confirmed at three metal-rich white dwarfs and ruled out at nine others. Several new binaries are confirmed or suggested; five white dwarf–red dwarf pairs and five double degenerates. Four apparently single magnetic white dwarfs – two DA and two DQp – display modest to strong near-infrared excess (relative to non-magnetic models), which may be better described as two effective temperatures owing to a redistribution of energy in highly magnetic or peculiar atmospheres.     

Merged binary white dwarf evolution: rapidly accreting carbon–oxygen white dwarfs and the progeny of extreme helium stars

We have examined the evolution of merged low-mass double white dwarfs that become luminous helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a carbon–oxygen (CO) white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. When the mass above the helium-burning shell becomes small enough, the star evolves blueward almost horizontally in the Hertzsprung–Russell diagram. The theoretical models for the merger of a 0.6-M_⊙ CO white dwarf with a 0.3-M_⊙ He white dwarf agree very well with the observed locations of extreme helium stars in the  log  T _eff–log  g   diagram, with their observed rates of blueward evolution, and with luminosities and masses obtained from their pulsations. Together with predicted merger rates for  CO+He  white dwarf pairs, the evolutionary time-scales are roughly consistent with the observed numbers of extreme helium stars. Predicted surface carbon and oxygen abundances can be consistent with the observed values if carbon and oxygen produced in the helium shell during a previous asymptotic giant branch phase are assumed to exist in the helium zone of the initial CO white dwarfs. These results establish the  CO+He  white dwarf merger as the best, if not only, viable model for the creation of extreme helium stars and, by association, the majority of R Coronae Borealis stars.     

The triple degenerate star WD 1704+481

WD 1704+481 is a visual binary in which both components are white dwarfs. We present spectra of the H α line of both stars which show that one component (WD 1704+481.2=Sanduleak B=GR 577) is a close binary with two white dwarf components. Thus, WD 1704+481 is the first known triple degenerate star. From radial velocity measurements of the close binary we find an orbital period of 0.1448 d, a mass ratio, q M _bright M _faint, of 0.70±0.03 and a difference in the gravitational redshifts of 11.5±2.3 km s⁻¹. The masses of the close pair of white dwarfs predicted by the mass ratio and gravitational redshift difference combined with theoretical cooling curves are 0.39±0.05 and 0.56±0.07 M_⊙. WD 1704+481 is therefore also likely to be the first example of a double degenerate in which the less massive white dwarf is composed of helium and the other white dwarf is composed of carbon and oxygen.     

The mass ratio distribution of short-period double degenerate stars

Short-period double degenerates (DDs) are close white dwarf–white dwarf binary stars which are the result of the evolution of interacting binary stars. We present the first definitive measurements of the mass ratio for two DDs, WD 0136+768 and WD 1204+450, and an improved measurement of the mass ratio for WD 0957−666. We compare the properties of the six known DDs with measured mass ratios to the predictions of various theoretical models. We confirm the result that standard models for the formation of DDs do not predict sufficient DDs with mass ratios close to 1. We also show that the observed difference in cooling ages between white dwarfs in DDs is a useful constraint on the initial mass ratio of the binary. A more careful analysis of the properties of the white dwarf pair WD 1704+481.2 leads us to conclude that the brighter white dwarf is older than its fainter companion. This is the opposite of the usual case for DDs and is caused by the more massive white dwarf being smaller and cooling faster. The mass ratio in the sense (mass of younger star)/(mass of older star) is then  1.43±0.06  rather than the value of  0.70±0.03  given previously.     

The fraction of double degenerates among DA white dwarfs

We present the results of a radial velocity survey designed to measure the fraction of double degenerates among DA white dwarfs. The narrow core of the H line was observed twice or more for 46 white dwarfs yielding radial velocities accurate to a few km s¹. This makes our survey the most sensitive to the detection of double degenerates undertaken to date. We found no new double degenerates in our sample, though H emission from distant companions is seen in two systems. Two stars known to be double degenerates prior to our observations are included in the analysis. We find a 95 per cent probability that the fraction of double degenerates among DA white dwarfs lies in the range [0.017, 0.19].     

Circinus X-1: survivor of a highly asymmetric supernova

We have analysed the kinematical parameters of Cir X-1 to constrain the nature of its companion star, the eccentricity of the binary and the pre-supernova parameter space. We argue that the companion is most likely to be a low-mass (≲2.0 M_⊙) unevolved star and that the eccentricity of the orbit is 0.94±0.04. We have evaluated the dynamical effects of the supernova explosion and we find it must have been asymmetric. On average , we find that a kick of ∼740 km s⁻¹ is needed to account for the recently measured radial velocity of +430 km s⁻¹ (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is ∼500 km s⁻¹. This is the largest kick needed to explain the motion of any observed binary system. If Cir X-1 is associated with the supernova remnant G321.9-0.3 then we find a limiting minimum age of this remnant of ∼60 000 yr. Furthermore, we predict that the companion star has lost ∼10 per cent of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion.     

Projected rotational velocities of WD 1614+136 and WD 1353+409: implications for the rate of Galactic Type Ia supernovae

The white dwarf stars WD 1614+136 and WD 1353+409 are not sufficiently massive to have formed through single-star evolution. However, observations to date have not yet found any evidence for binarity. It has therefore been suggested that these stars are the result of a merger. In this paper we place an upper limit of ≈ 50 km s⁻¹ on the projected rotational velocities of both stars. This suggests that, if these stars are the results of a merger, efficient angular momentum loss with accompanying mass loss must have occurred. If the same process occurs following the merging of more massive white dwarf stars, the predicted rate of Type Ia supernovae due to merging white dwarfs may have been greatly overestimated. Further observations to determine binarity in WD 1614+136 and WD 1353+409 are therefore encouraged.     

Hydrogen-model atmospheres for white dwarf stars

We present a detailed calculation of model atmospheres for DA white dwarfs. Our atmosphere code solves the atmosphere structure in local thermodynamic equilibrium with a standard partial linearization technique, which takes into account the energy transfer by radiation and convection. This code incorporates recent improved and extended data base of collision-induced absorption by molecular hydrogen. We analyse the thermodynamic structure and emergent flux of atmospheres in the range 2500 T _eff60 000 K and 6.5log  g 9.0. Bolometric correction and colour indices are provided for a subsample of the model grid. Comparison of the colours is made with published observational material and results of other recent model calculations.
Motivated by the increasing interest in helium-core white dwarfs, we analyse the photometric characteristics of these stars during their cooling, using evolutionary models recently available. Effective temperatures, surface gravities, masses and ages have been determined for some helium-core white dwarf candidates, and their possible binary nature is briefly discussed.     

The effect of photospheric heavy elements on the hot DA white dwarf temperature scale

Using the latest non-local thermodynamic equilibrium (non-LTE) synthetic spectra and stellar model calculations, we have evaluated the potential effect of the presence of heavy elements in the photospheres of hot H-rich DA white dwarfs. In particular, we have examined their influence on the effective temperature and surface gravity perceived from analysis of the Balmer line profiles. It is apparent that both the inclusion of non-LTE effects in the models and significant quantities of heavy elements act independently to lower the value of T _eff determined from a particular spectrum. Hence, the true effective temperatures of the heavy element-rich DA white dwarfs, currently estimated to be above 55 000 K, are apparently lower than previously reported from pure-H LTE analyses, by some 4000–7000 K. We do not see any similar influence on measurements of log g . This work concentrates on a group of relatively bright well-studied objects, for which heavy element abundances are known. As a consequence of this, establishment of correct temperatures for all other hot white dwarfs will require a programme of far-UV spectroscopy in order to obtain the essential compositional information. Since only stars with effective temperatures lying notionally in the range from ≈ 55 000 to 70 000 K (52 000–62 000 K when the non-LTE effects and heavy elements are taken into account) have been considered here, important questions remain regarding the magnitude of any similar effects in even hotter white dwarfs and pre-white dwarfs. The resulting implications for the plausibility of the evolutionary link between the main hot DA population and their proposed precursors, the H-rich central stars of planetary nebulae, need to be investigated.     

Full evolution of low-mass white dwarfs with helium and oxygen cores

We study the full evolution of low-mass white dwarfs with helium and oxygen cores. We revisit the age dichotomy observed in many white dwarf companions to millisecond pulsar on the basis of white dwarf configurations derived from binary evolution computations. We evolve 11 dwarf sequences for helium cores with final masses of 0.1604, 0.1869, 0.2026, 0.2495, 0.3056, 0.3333, 0.3515, 0.3844, 0.3986, 0.4160 and  0.4481 M_⊙  . In addition, we compute the evolution of five sequences for oxygen cores with final masses of 0.3515, 0.3844, 0.3986, 0.4160 and  0.4481 M_⊙  . A metallicity of   Z = 0.02  is assumed. Gravitational settling, chemical and thermal diffusion are accounted for during the white dwarf regime. Our study reinforces the result that diffusion processes are a key ingredient in explaining the observed age and envelope dichotomy in low-mass helium-core white dwarfs, a conclusion we arrived at earlier on the basis of a simplified treatment for the binary evolution of progenitor stars. We determine the mass threshold where the age dichotomy occurs. For the oxygen white dwarf sequences, we report the occurrence of diffusion-induced, hydrogen-shell flashes, which, as in the case of their helium counterparts, strongly influence the late stages of white dwarf cooling. Finally, we present our results as a set of white dwarf mass–radius relations for helium and oxygen cores.     

Radial velocity measurements of white dwarfs

We present 594 radial velocity measurements for 71 white dwarfs obtained during our search for binary white dwarfs and not reported elsewhere. We identify three excellent candidate binaries, which require further observations to confirm our preliminary estimates for their orbital periods, and one other good candidate. We investigate whether our data support the existence of a population of single, low-mass (≲0.5 M_⊙) white dwarfs (LMWDs). These stars are difficult to explain using standard models of stellar evolution. We find that a model with a mixed single/binary population is at least ~20 times more likely to explain our data than a pure binary population. This result depends on assumed period distributions for binary LMWDs, assumed companion masses and several other factors. Therefore, the evidence in favour of the existence of a population of single LMWDs is not sufficient, in our opinion, to firmly establish the existence of such a population, but does suggest that extended observations of LMWDs to obtain a more convincing result would be worthwhile.     

Two-Micron All-Sky Survey J01542930+0053266: a new eclipsing M dwarf binary system

We report on Two-Micron All-Sky Survey (2MASS) J01542930+0053266, a faint eclipsing system composed of two M dwarfs. The variability of this system was originally discovered during a pilot study of the 2MASS Calibration Point Source Working Data base. Additional photometry from the Sloan Digital Sky Survey yields an eight-passband light curve from which we derive an orbital period of  2.639 0157 ± 0.000 0016  d. Spectroscopic followup confirms our photometric classification of the system, which is likely composed of M0 and M1 dwarfs. Radial velocity measurements allow us to derive the masses  (M₁= 0.66 ± 0.03 M_⊙; M₂= 0.62 ± 0.03 M_⊙)  and radii  (R₁= 0.64 ± 0.08 R_⊙; R₂= 0.61 ± 0.09 R_⊙)  of the components, which are consistent with empirical mass–radius relationships for low-mass stars in binary systems. We perform Monte Carlo simulations of the light curves which allow us to uncover complicated degeneracies between the system parameters. Both stars show evidence of Hα emission, something not common in early-type M dwarfs. This suggests that binarity may influence the magnetic activity properties of low-mass stars; activity in the binary may persist long after the dynamos in their isolated counterparts have decayed, yielding a new potential foreground of flaring activity for next generation variability surveys.