Showing papers on "White dwarf published in 2000"

Nucleosynthesis in Chandrasekhar Mass Models for Type Ia Supernovae and Constraints on Progenitor Systems and Burning-Front Propagation

Abstract: We calculate explosive nucleosynthesis in Chandrasekhar mass models for Type Ia Supernovae(SNe Ia) to obtain new constraints on the rate of matter accretion onto the progenitor white dwarf and on the ignition density of central carbon deflagration. The calculated abundance of the Fe-group neutron-rich nuclei is highly sensitive to the electron captures taking place in the central layers. The yields obtained from a slow central deflagration, and from a fast deflagration or delayed detonation in the outer layers, are combined and put to comparison with solar isotopic abundances. We found that (1) to avoid too large ratios of $^{54}$Cr/$^{56}$Fe and $^{50}$Ti/$^{56}$Fe, the ignition density should be as low as \ltsim 2 \e9 \gmc, and that (2) to avoid the overproduction of $^{58}$Ni and $^{54}$Fe, either the flame speed should not exceed a few % of the sound speed in the central low $Y_e$ layers, or the progenitor star has to be metal-poor compared with solar. Such low central densities can be realized by a rapid accretion as fast as $\dot M$ \gtsim 1 $\times$ 10$^{-7}$M$_\odot$ yr$^{-1}$. In order to reproduce the solar abundance of $^{48}$Ca, one also needs progenitor systems that undergo ignition at higher densities. We also found that the total amount of $^{56}$Ni, the Si-Ca/Fe ratio, and the abundance of elements like Mn and Cr (incomplete Si-burning ashes), depend on the density of the deflagration-detonation transition in delayed detonations. Our nucleosynthesis results favor transition densities slightly below 2.2$\times 10^7$ g cm$^{-3}$.

Type Ia Supernova Explosions in Binary Systems: The Impact on the Secondary Star and Its Consequences

Abstract: One method of discriminating between the many Type Ia progenitor scenarios is by searching for contaminating hydrogen and helium stripped from the companion star. However, this requires an understanding of the effect of the impact of the supernova shell on different companion stars to predict the amount of mass stripped and its distribution in velocity and solid angle for the types of binary scenarios that have been proposed as Type Ia progenitor models. We present several high-resolution two-dimensional numerical simulations of the impact of a Type Ia supernova explosion with hydrogen-rich main-sequence, subgiant, and red giant companions. The binary parameters were chosen to represent several classes of single-degenerate Type Ia progenitor models that have been suggested in the literature. We use realistic stellar models and supernova debris profiles to represent each binary system. For each scenario, we explore the hydrodynamics of the supernova-secondary interaction, calculate the amount of stellar material stripped from the secondary and the kick delivered by the impact, and construct the velocity and solid angle distributions of the stripped material. We find that the main-sequence and subgiant companions lose 0.15-0.17 M? as a result of the impact of the supernova shell, 15% of their mass. The red giant companions lose 0.53-0.54 M?, 96%-98% of their envelopes. The characteristic velocity of the stripped hydrogen is less than 103 km s-1 for all the scenarios: 420-590 km s-1 for the red giant companions, 820 km s-1 for the main-sequence companion, and 890 km s-1 for the subgiant companion. The stripped hydrogen and helium contaminate a wide solid angle behind the companion: 115? from the downstream axis for the red giant, 66? for the main-sequence star, and 72? for the subgiant. With such low velocities, the bulk of the stripped hydrogen and helium is embedded within the low-velocity iron of the supernova ejecta. The hydrogen and helium may be visible in the late-time spectra as narrow emission lines.?????Although most of the stripped material is ejected at low velocities, all the numerical simulations yield a small high-velocity tail. The main-sequence, subgiant, and red giant companions are just under the upper limit determined by Della Valle et al. in 1996 from observations of SN 1990M taken near maximum light. The main-sequence companion receives a kick of 86 km s-1, and the subgiant receives a kick of 49 km s-1. In all cases, the kick to the remnant is smaller than the original orbital velocity. Because it is too small to intercept more than a negligible amount of momentum, the red giant core will not receive an appreciable kick.?????The impact of the supernova ejecta with the secondary star creates a hole in the supernova debris with an angular size of ~30? in the high-velocity ejecta and with an angular size of ~40? in the low-velocity ejecta. This corresponds to 7%-12% of the ejecta's surface. Because we explore binary scenarios that are close enough, or almost close enough, to be in Roche lobe overflow, the degree of asymmetry is similar for all the models. The asymmetry in the supernova debris could have observational consequences beyond the change in morphology of the supernova remnant. The asymmetry in the supernova atmosphere will result in distorted P Cygni profiles that might indicate the presence of the companion star, but it will be difficult to use the degree of asymmetry alone to discriminate between a main-sequence, subgiant, or red giant companion.?????The impact of the supernova shell will have consequences for the future evolution of the secondary star. After the impact, the main-sequence star is puffed up, much like a pre-main-sequence star. The luminosity will rise dramatically, to as high as ~5000 L?, as the extended envelope relaxes back into thermal equilibrium. The subgiant companion will follow a similar sequence of events. The star will not be contaminated by much supernova debris from the initial impact, but it may accrete low-velocity iron-group elements (or oxygen and silicon if the ejecta is radially mixed) at late times.?????A He pre-white dwarf will be left behind after a supernova explosion with a red giant companion. Almost all of the envelope will be ejected by the impact, but a residual amount of material (~0.02 M?) will form an extended, hydrogen-rich envelope around the degenerate core. The star will evolve away from the red giant branch on a timescale of 105-106 yr. On its post-red giant track, it may appear as an underluminous O or B star before passing through an sdO or sdB phase on its way to a standard He white dwarf cooling track. This binary scenario could be a possible pathway for the formation of a subset of single, low-mass He white dwarfs.

Magnetism in Isolated and Binary White Dwarfs

Abstract: Since the discovery of the first isolated magnetic white dwarf (MWD) Grw +70°8047 nearly 60 years ago, the number of stars belonging to this class has grown steadily. There are now some 65 isolated white dwarfs classified as magnetic, and a roughly equal number of MWDs are found in the close interacting binaries known as the magnetic cataclysmic variables (MCVs). The isolated MWDs comprise ~5% of all WDs, while the MCVs comprise ~25% of all CVs. The magnetic fields range from ~ 3 × 104–109 G in the former group with a distribution peaking at 1.6 × 107 G, and ~ 107–3 × 108 G in the latter group. The space density of isolated magnetic white dwarfs with fields in the range ~3 × 104–109 G is estimated to be ~1.5 × 10–4 pc–3. The MCVs have a space density that is about a hundred times smaller.About 80% of the isolated MWDs have almost pure H atmospheres and show only hydrogen lines in their spectra (the magnetic DAs), while the remainder show He i lines (the magnetic DBs) or molecular bands of C2 and CH (magnetic DQs) and have helium as the dominant atmospheric constituent, mirroring the situation in the nonmagnetic white dwarfs. The incidence of stars of mixed composition (H and He) appears to be higher among the MWDs.There is growing evidence based on trigonometric parallaxes, space motions, and spectroscopic analyses that the isolated MWDs tend as a class to have a higher mass than the nonmagnetic white dwarfs. The mean mass for 16 MWDs with well-constrained masses is 0.95 M⊙. Magnetic fields may therefore play a significant role in angular momentum and mass loss in the post-main-sequence phases of single star evolution affecting the initial-final mass relationship, a view supported by recent work on cluster MWDs. The progenitors of the vast majority of the isolated MWDs are likely to be the magnetic Ap and Bp stars. However, the discovery of two MWDs with masses within a few percent of the Chandrasekhar limit, one of which is also rapidly rotating (Pspin = 12 minutes), has led to the proposal that these may be the result of double-degenerate (DD) mergers. An intriguing possibility is that magnetism, through its effect on the initial-final mass relationship, may also favor the formation of more massive double degenerates in close binary evolution. The magnetic DDs may therefore be more likely progenitors of Type Ia supernovae.A subclass of the isolated MWDs appear to rotate slowly with no evidence of spectral or polarimetric variability over periods of tens of years, while others exhibit rapid rotation with coherent periods in the range of tens of minutes to hours or days. There is a strong suggestion of a bimodal period distribution. The "rapidly" rotating isolated MWDs may include as a subclass stars which have been spun up during a DD merger or a previous phase of mass transfer from a companion star.Zeeman spectroscopy and polarimetry, and cyclotron spectroscopy, have variously been used to estimate magnetic fields of the isolated MWDs and the MWDs in MCVs and to place strong constraints on the field structure. The surface field distributions tend in general to be strongly nondipolar and to a first approximation can be modeled by dipoles that are offset from the center by ~10%-30% of the stellar radius along the dipole axis. Other stars show extreme spectral variations with rotational phase which cannot be modeled by off-centered dipoles. More exotic field structures with spot-type field enhancements appear to be necessary. These field structures are even more intriguing and suggest that some of the basic assumptions inherent in most calculations of field evolution, such as force-free fields and free ohmic decay, may be oversimplistic.

Population synthesis for double white dwarfs I.Close detached systems

Abstract: We model the population of double white dwarfs in the Galaxy and find a better agreement with observations compared to earlier studies, due to two modifications. The first is the treatment of the first phase of unstable mass transfer and the second the modelling of the cooling of the white dwarfs. A satisfactory agreement with observations of the local sample of white dwarfs is achieved if we assume that the initial binary fraction is ~ 50% and that the lowest mass white dwarfs (M < 0.3 Msun) cool faster than the most recently published cooling models predict. With this model we find a Galactic birth rate of close double white dwarfs of 0.05 yr^{-1}, a birth rate of AM CVn systems of 0.005 yr^{-1}, a merger rate of pairs with a combined mass exceeding the Chandrasekhar limit (which may be progenitors of SNe Ia) of 0.003 yr^{-1} and a formation rate of planetary nebulae of 1 yr^{-1}. We estimate the total number of double white dwarfs in the Galaxy as 2.5 10^8. In an observable sample with a limiting magnitude V_lim = 15 we predict the presence of ~855 white dwarfs of which ~220 are close pairs. Of these 10 are double CO white dwarfs of which one has a combined mass exceeding the Chandrasekhar limit and will merge within a Hubble time.

Star cluster ecology IVa: Dissection of an open star cluster---photometry

Abstract: The evolution of star clusters is studied using N-body simulations in which the evolution of single stars and binaries are taken self-consistently into account. Initial conditions are chosen to represent relatively young Galactic open clusters, such as the Pleiades, Praesepe and the Hyades. The calculations include a realistic mass function, primordial binaries and the external potential of the parent Galaxy. Our model clusters are generally significantly flattened in the Galactic tidal field, and dissolve before deep core collapse occurs. The binary fraction decreases initially due to the destruction of soft binaries, but increases later because lower mass single stars escape more easily than the more massive binaries. At late times, the cluster core is quite rich in giants and white dwarfs. There is no evidence for preferential evaporation of old white dwarfs, on the contrary the formed white dwarfs are likely to remain in the cluster. Stars tend to escape from the cluster through the first and second Lagrange points, in the direction of and away from the Galactic center. Mass segregation manifests itself in our models well within an initial relaxation time. As expected, giants and white dwarfs are much more strongly affected by mass segregation than main-sequence stars. Open clusters are dynamically rather inactive. However, the combined effect of stellar mass loss and evaporation of stars from the cluster potential drives its dissolution on a much shorter timescale than if these effects are neglected. The often-used argument that a star cluster is barely older than its relaxation time and therefore cannot be dynamically evolved is clearly in error for the majority of star clusters.

The Formation of Very Narrow Waist Bipolar Planetary Nebulae

Abstract: We discuss the interaction of the slow wind blown by an asymptotic giant branch (AGB) star with a collimated fast wind (CFW) blown by its main-sequence or white dwarf companion, at orbital separations in the range of several AU a 200 AU. The CFW results from accretion of the AGB wind into an accretion disk around the companion. The fast wind is collimated by the accretion disk. We argue that such systems are the progenitors of bipolar planetary nebulae and bipolar symbiotic nebulae with a very narrow equatorial waist between the two polar lobes. The CFW wind will form two lobes along the symmetry axis and will further compress the slow wind near the equatorial plane, leading to the formation of a dense slowly expanding ring. Therefore, contrary to the common claim that a dense equatorial ring collimates the bipolar flow, we argue that in the progenitors of very narrow waist bipolar planetary nebulae, the CFW, through its interaction with the slow wind, forms the dense equatorial ring. Only later in the evolution, and after the CFW and slow wind cease, does the mass-losing star leave the AGB and blow a second, more spherical, fast wind. At this stage the flow structure becomes the one that is commonly assumed for bipolar planetary nebulae, i.e., collimation of the fast wind by the dense equatorial material. However, this results in the broadening of the waist in the equatorial plane and cannot by itself account for the presence of very narrow waists or jets. We conduct a population synthesis study of the formation of planetary nebulae in wide binary systems which quantitatively supports the proposed model. The population synthesis code follows the evolution of both stars and their arbitrarily eccentric orbit, including mass loss via stellar winds, for 5 ? 104 primordial binaries. We show the number of expected systems that blow a CFW is in accord with the number found from observations, to within the many uncertainties involved. Overall, we find that ~5% of all planetary nebulae are bipolars with very narrow waists. Our population synthesis not only supports the CFW model but more generally supports the binary model for the formation of bipolar planetary nebulae.

KPD 1930+2752: a candidate Type Ia supernova progenitor

Abstract: We present spectra of the pulsating sdB star KPD 1930+2752 which confirm that this star is a binary. The radial velocities measured from the Hα and He i 6678-A spectral lines vary sinusoidally with the same period (2 h 17 min) as the ellipsoidal variability seen by Billeres et al. The amplitude of the orbital motion (349.3±2.7 km s−1) combined with the canonical mass for sdB stars (0.5 M⊙) implies a total mass for the binary of 1.47±0.01 M⊙. The unseen companion star is almost certainly a white dwarf star. The binary will merge within ∼200 million years because of gravitational wave radiation. The accretion of helium and other elements heavier than hydrogen on to the white dwarf, which then exceeds the Chandrasekhar mass (1.4 M⊙), is a viable model for the cause of Type Ia supernovae. KPD 1930+2752 is the first star to be discovered that is a good candidate for the progenitor of a Type Ia supernova of this type, which will merge on an astrophysically interesting time-scale.

Gravitational waves from a compact star in a circular, inspiral orbit, in the equatorial plane of a massive, spinning black hole, as observed by LISA

Abstract: Results are presented from high-precision computations of the orbital evolution and emitted gravitational waves for a stellar-mass object spiraling into a massive black hole in a slowly shrinking, circular, equatorial orbit. The focus of these computations is inspiral near the innermost stable circular orbit (isco)—more particularly, on orbits for which the angular velocity Ω is 0.03≲Ω/Ωisco 10 at r0=1 Gpc during the last year of inspiral. The hole’s spin a has a factor of ∼10 influence on the range of M (at fixed μ) for which S/N>10, and the presence or absence of a white-dwarf–binary background has a factor of ∼3 influence. A comparison with predicted event rates shows strong promise for detecting these waves, but not beyond about 1 Gpc if the inspiraling object is a white dwarf or neutron star. This argues for a modest lowering of LISA’s noise floor. A brief discussion is given of the prospects for extracting information from the observed waves.

KPD1930+2752 - a candidate Type Ia supernova progenitor

Abstract: We present spectra of the pulsating sdB star KPD1930+2752 which confirm that this star is a binary. The radial velocities measured from the H-alpha and HeI6678 spectral lines vary sinusoidally with the same period (2h 17m) as the ellipsoidal variability seen by Billeres et al. (2000). The amplitude of the orbital motion (349.3+-2.7 km/s) combined with the canonical mass for sdB stars (0.5 solar masses) implies a total mass for the binary of 1.47+-0.01 solar masses The unseen companion star is almost certainly a white dwarf star. The binary will merge within about 200 million years due to gravitational wave radiation. The accretion of helium and other elements heavier than hydrogen onto the white dwarf which then exceeds the Chandrasekhar mass (1.4 solar masses) is a viable model for the cause of Type Ia supernovae. KPD1930+2752 is the first star to be discovered which is a good candidate for the progenitor of a Type Ia supernova of this type which will merge on an astrophysically interesting timescale.

Measuring the remnant mass function of the galactic bulge

Abstract: I show that by observing microlensing events both astrometrically and photometrically, the Space Interferometry Mission (SIM) can measure the mass function of stellar remnants in the Galactic bulge, including white dwarfs, neutron stars, and black holes. Neutron stars and black holes can be identified individually, while white dwarfs are detected statistically from the sharp peak in their mass function near M ~ 0.6 M☉. This peak is expected to be more than twice as high as the "background" of main-sequence microlenses. I estimate that of order 20% of the ~ 400 bulge microlensing events detected to date are due to remnants, but show that these are completely unrecognizable from their timescale distribution (the only observable that "normal" microlensing observations produce). To resolve the white dwarf peak, the SIM mass measurements must be accurate to ~ 5%, substantially better than is required to measure the mass function of the more smoothly distributed main sequence. Nevertheless, SIM could measure the masses of about 20 bulge remnants in 500 hours of observing time.

The Role of Electron Captures in Chandrasekhar-Mass Models for Type Ia Supernovae

Abstract: The Chandrasekhar-mass model for Type Ia supernovae (SNe Ia) has received increasing support from recent comparisons of observations with light-curve predictions and modeling of synthetic spectra. It explains SN Ia events via thermonuclear explosions of accreting white dwarfs in binary stellar systems, being caused by central carbon ignition when the white dwarf approaches the Chandrasekhar mass. As the electron gas in white dwarfs is degenerate, characterized by high Fermi energies for the high-density regions in the center, electron capture on intermediate-mass and Fe group nuclei plays an important role in explosive burning. Electron capture affects the central electron fraction Ye, which determines the composition of the ejecta from such explosions. Up to the present, astrophysical tabulations based on shell model matrix elements were available only for light nuclei in the sd-shell. Recently, new shell model Monte Carlo and large-scale shell model diagonalization calculations have also been performed for pf-shell nuclei. These lead in general to a reduction of electron capture rates in comparison with previous, more phenomenological, approaches. Making use of these new shell model-based rates, we present the first results for the composition of Fe group nuclei produced in the central regions of SNe Ia and possible changes in the constraints on model parameters like ignition densities ρign and burning front speeds vdef.

The age of the solar neighbourhood

Abstract: High-quality Hipparcos data for a complete sample of nearly 12 000 main-sequence and subgiant stars, together with Padua isochrones, are used to constrain the star formation history of the solar neigbourhood and the processes that stochastically accelerate disc stars. The velocity dispersion of a coeval group of stars is found to increase with time from ∼8 km s−1 at birth as t0.33. In the fits, the slope of the initial mass function (IMF) near 1 M⊙ proves to be degenerate with the rate at which the star formation rate declines. If the slope of the IMF is to lie near Salpeter's value, −2.35, the star formation rate has to be very nearly constant. The age of the solar neighbourhood is found to be 11.2±0.75 Gyr with remarkably little sensitivity to variations in the assumed metallicity distribution of old disc stars. This age is only a gigayear younger than the age of the oldest globular clusters when the same isochrones and distance scale are employed. It is compatible with current indications of the redshift of luminous galaxy formation only if there is a large cosmological constant. A younger age is formally excluded because it provides a poor fit to the number density of red stars. Since this density is subject to a significantly uncertain selection function, ages as low as 9 Gyr are plausible even though they lie outside our formal error bars.

The evolution of a rapidly accreting helium white dwarf to become a low-luminosity helium star

Abstract: We have examined the evolution of merged low-mass double white dwarfs which become low-luminosity (or high-gravity) extreme helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a helium 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. As the helium-burning shell moves inwards with repeating shell flashes, the effective temperature gradually increases as the star evolves towards the helium main sequence. When the mass interior to the helium-burning shell is approximately 0.25 M⊙, the star enters a regime where it is pulsationally unstable. We have obtained radial pulsation periods for these models. These models have properties very similar to those of the pulsating helium star V652 Her. We have compared the rate of period change of the theoretical models with that observed in V652 Her, as well as with its position on the Hertzsprung–Russell diagram. We conclude that the merger between two helium white dwarfs can produce a star with properties remarkably similar to those observed in at least one extreme helium star, and is a viable model for their evolutionary origin. Such helium stars will evolve to become hot subdwarfs close to the helium main sequence. We also discuss the number of low-luminosity helium stars in the Galaxy expected for our evolution scenario.

Formation of Millisecond Pulsars with Heavy White Dwarf Companions: Extreme Mass Transfer on Subthermal Timescales.

Abstract: We have performed detailed numerical calculations of the nonconservative evolution of close X-ray binary systems with intermediate-mass (2.0-6.0 M☉) donor stars and a 1.3 M☉ accreting neutron star. We calculated the thermal response of the donor star to mass loss in order to determine its stability and follow the evolution of the mass transfer. Under the assumption of the "isotropic reemission model," we demonstrate that in many cases it is possible for the binary to prevent a spiral-in and survive a highly super-Eddington mass transfer phase (1 /Edd < 105) on a subthermal timescale if the convective envelope of the donor star is not too deep. These systems thus provide a new formation channel for binary millisecond pulsars with heavy CO white dwarfs and relatively short orbital periods (3-50 days). However, we conclude that to produce a binary pulsar with a O-Ne-Mg white dwarf or Porb ~ 1 day (e.g., PSR B0655+64) the above scenario does not work, and a spiral-in phase is still considered the most plausible scenario for the formation of such a system.

The history of the cosmic supernova rate derived from the evolution of the host galaxies

Abstract: We make a prediction of the cosmic supernova rate history as a composite of the supernova rates in spiral and elliptical galaxies. We include the metallicity eUect on the evolution of Type Ia supernova (SN Ia) progenitors and construct detailed models for the evolutions of spiral and elliptical galaxies in clus- ters and the —eld to meet the latest observational constraints. In the cluster environment, the synthesized cosmic star formation rate (SFR) has an excess at corresponding to the early starburst in ellipticals z Z 3 and a shallower slope from the present to the peak at the redshift of z D 1.4 compared with Madaus plot. In the —eld environment, we assume that ellipticals form over a wide range of redshifts as 1 ( z ( The synthesized cosmic SFR has a broad peak around z D 3, which is in good agreement with the 4. observed one. The resultant cosmic SFRs lead to the following predictions for the cosmic SN Ia rate: (1) the SN Ia rate in spirals has a break at z D 2 because of the low-metallicity inhibition of SNe Ia, regard- less of whether the galaxies are in clusters or in the —eld; (2) at high redshifts, the SN Ia rate has a strong peak around z D 3 in clusters, whereas in the —eld much lower rate is expected, re—ecting the diUerence in the formation epochs of ellipticals. Subject headings: cosmology: theorygalaxies: abundancesgalaxies: evolution ¨ supernovae: general

Quark phases in neutron stars and a "third family" of compact stars as a signature for phase transitions

Abstract: The appearance of quark phases in the dense interior of neutron stars provides one possibility to soften the equation of state (EOS) of neutron star matter at high densities. This softening leads to more compact equilibrium configurations of neutron stars compared to pure hadronic stars of the same mass. We investigate the question to which amount the compactness of a neutron star can be attributed to the presence of a quark phase. For this purpose we employ several hadronic EOS in the framework of the relativistic mean-field (RMF) model and an extended MIT bag model to describe the quark phase. We find that - almost independent of the model parameters - the radius of a pure hadronic neutron star gets typically reduced by 20-30% if a pure quark phase in the center of the star does exist. For some EOS we furthermore find the possibility of a "third family" of compact stars which may exist besides the two known families of white dwarfs and neutron stars. We show how an experimental proof of the existence of a third family by mass and radius measurements may provide a unique signature for a phase transition inside neutron stars.

An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera

Abstract: We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE) stars observed in the IUE short-wavelength bandpass (1150-1980 A). Each low-resolution (R ~ 6 A) spectrum was visually inspected for source contamination and data quality, and then all good spectra were combined to form a single time-averaged spectrum for each star. Use of IUE Final Archive spectra processed with NEWSIPS reduces fixed pattern noise in individual spectra, allowing significant signal-to-noise ratio gains in our co-added spectra. For the TTS observed by IUE, we measured fluxes and uncertainties for 17 spectral features, including two continuum windows and four fluoresced H2 complexes. Thirteen of the 32 accreting TTS observed by IUE have detectable H2 emission, which until now had been reported only for T Tau. Using an empirical correlation between H2 and C IV line flux, we show that lack of sensitivity can account for practically all nondetections, suggesting that H2 fluorescence may be intrinsically strong in all accreting TTS systems. Comparison of IUE and GHRS spectra of T Tau show extended emission primarily, but not exclusively, in lines of H2. We also fit reddened main-sequence templates to 72 HAEBE stars, determining extinction and checking spectral types. Several of the HAEBE stars could not be fitted well or yielded implausibly low extinctions, suggesting the presence of a minority emission component hotter than the stellar photosphere, perhaps caused by white dwarf companions or heating in accretion shocks. We identified broad wavelength intervals in the far-UV that contain circumstellar absorption features ubiquitous in B5-A4 HAEBE stars, declining in prominence for earlier spectral types, perhaps caused by increasing ionization of metal resonance lines. For 61 HAEBE stars, we measured or set upper limits on a depth index that characterizes the strength of circumstellar absorption and compared this depth index with published IR properties.

The Ages of Very Cool Hydrogen-rich White Dwarfs

Abstract: The evolution of white dwarfs is essentially a cooling process that depends primarily on the energy stored in their degenerate cores and on the transparency of their envelopes. In this paper we compute accurate cooling sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres for the full range of masses of interest. For this purpose we use the most accurate available physical inputs for both the equation of state and opacities of the envelope and for the thermodynamic quantities of the degenerate core. We also investigate the role of the latent heat in the computed cooling sequences. We present separately cooling sequences in which the effects of phase separation of the carbon-oxygen binary mixture upon crystallization have been neglected, and the delay introduced in the cooling times when this mechanism is properly taken into account, in order to compare our results with other published cooling sequences which do not include a treatment of this phenomenon. We find that the cooling ages of very cool white dwarfs with pure hydrogen atmospheres have been systematically underestimated by roughly 1.5 Gyr at log(L/L☉) = -4.5 for an otherwise typical ~0.6 M☉ white dwarf, when phase separation is neglected. If phase separation of the binary mixture is included, then the cooling ages are further increased by roughly 10%. Cooling tracks and cooling isochrones in several color-magnitude diagrams are presented as well.

Formation of Millisecond Pulsars with Heavy White Dwarf Companions - Extreme Mass Transfer on Sub-Thermal Timescales

Abstract: We have performed detailed numerical calculations of the non-conservative evolution of close X-ray binary systems with intermediate-mass (2.0-6.0 M_sun) donor stars and a 1.3 M_sun accreting neutron star. We calculated the thermal response of the donor star to mass loss, in order to determine its stability and follow the evolution of the mass transfer. Under the assumption of the "isotropic re-emission model" we demonstrate that in many cases it is possible for the binary to prevent a spiral-in and survive a highly super-Eddington mass-transfer phase (1 << M_dot/M_Edd < 10^5) on a sub-thermal timescale, if the convective envelope of the donor star is not too deep. These systems thus provide a new formation channel for binary millisecond pulsars with heavy CO white dwarfs and relatively short orbital periods (3-50 days). However, we conclude that to produce a binary pulsar with a O-Ne-Mg white dwarf or P_orb ~1 day (e.g. PSR B0655+64) the above scenario does not work, and a spiral-in phase is still considered the most plausible scenario for the formation of such a system.

A Theoretical Light-Curve Model for the 1999 Outburst of U Scorpii

Abstract: A theoretical light curve for the 1999 outburst of U Scorpii is presented in order to obtain various physical parameters of the recurrent nova. Our U Sco model consists of a very massive white dwarf (WD) with an accretion disk and a lobe-filling, slightly evolved, main-sequence star (MS). The model includes a reflection effect by the companion and the accretion disk together with a shadowing effect on the companion by the accretion disk. The early visual light curve (with a linear phase of t ~ 1-15 days after maximum) is well reproduced by a thermonuclear runaway model on a very massive WD close to the Chandrasekhar limit (MWD = 1.37 ± 0.01 M⊙), in which optically thick winds blowing from the WD play a key role in determining the nova duration. The ensuing plateau phase (t ~ 15-30 days) is also reproduced by the combination of a slightly irradiated MS and a fully irradiated flaring-up disk with a radius ~1.4 times the Roche lobe size. The cooling phase (t ~ 30-40 days) is consistent with a low-hydrogen content of X ≈ 0.05 of the envelope for the 1.37 M⊙ WD. The best-fit parameters are the WD mass of MWD ~ 1.37 M⊙, the companion mass of MMS ~ 1.5 M⊙ (0.8-2.0 M⊙ is acceptable), the inclination angle of the orbit (i ~ 80°), and the flaring-up edge, the vertical height of which is ~0.30 times the accretion disk radius. The duration of the strong wind phase (t ~ 0-17 days) is very consistent with the BeppoSAX supersoft X-ray detection at t ~ 19-20 days because supersoft X-rays are self-absorbed by the massive wind. The envelope mass at the peak is estimated to be ~3 × 10-6 M⊙, which is indicates an average mass accretion rate of ~2.5 × 10-7 M⊙ yr-1 during the quiescent phase between 1987 and 1999. These quantities are exactly the same as those predicted in a new progenitor model of Type Ia supernovae.

Gravitational waves from cosmological compact binaries

Abstract: We consider gravitational waves emitted by various populations of compact binaries at cosmological distances. We use population synthesis models to characterize the properties of double neutron stars, double black holes and double white dwarf binaries as well as white dwarf-neutron star, white dwarf-black hole and black hole-neutron star systems. We use the observationally determined cosmic star formation history to reconstruct the redshift distribution of these sources and their merging rate evolution. The gravitational signals emitted by each source during its early-inspiral phase add randomly to produce a stochastic background in the low frequency band with spectral strain amplitude between 10^{-18} Hz^{-1/2} and 5 10^{-17} Hz^{-1/2} at frequencies in the interval [5 10^{-6}-5 10^{-5}] Hz. The overall signal which, at frequencies above 10^{-4}Hz, is largely dominated by double white dwarf systems, might be detectable with LISA in the frequency range [1-10] mHz and acts like a confusion limited noise component which might limit the LISA sensitivity at frequencies above 1 mHz.

Can differences in the nickel abundance in Chandrasekhar mass models explain the relation between brightness and decline rate of normal Type Ia Supernovae

Abstract: The use of Type Ia supernovae as distance indicators relies on the determination of their brightness. This is not constant, but it can be calibrated using an observed relation between the brightness and the properties of the optical light curve (decline rate, width, shape), which indicates that brighter SNe have broader, slower light curves. However, the physical basis for this relation is not yet fully understood. Among possible causes are different masses of the progenitor white dwarfs or different opacities in Chandrasekhar-mass explosions. We parametrise the Chandrasekhar-mass models presented by Iwamoto et al (1999), which synthesize different amounts of Ni, and compute bolometric light curves and spectra at various epochs. Since opacity in SNe Ia is due mostly to spectral lines, it should depend on the mass of Fe-peak elements synthesized in the explosion, and on the temperature in the ejecta. Bolometric light curves computed using these prescriptions for the optical opacity reproduce the relation between brightness and decline rate. Furthermore, when spectra are calculated, the change in colour between maximum and two weeks later allows the observed relation between M_B(Max) and Dm_{15}(B) to be reproduced quite nicely. Spectra computed at various epochs compare well with corresponding spectra of spectroscopically normal SNeIa selected to cover a similar range of Dm_{15}(B) values.

Cooling sequences and color-magnitude diagrams for cool white dwarfs with hydrogen atmospheres

Abstract: We present new cooling sequences, color-magnitude diagrams, and color-color diagrams for cool white dwarfs with pure hydrogen atmospheres down to an effective temperature Teff = 1500 K. We include a more detailed treatment of the physics of the fully ionized interior, particularly an improved discussion of the thermodynamics of the temperature-dependent ion-ion and ion-electron contributions of the quantum, relativistic electron-ion plasma. The present calculations also incorporate accurate boundary conditions between the degenerate core and the outermost layers as well as updated atmosphere models including the H2-H2 induced-dipole absorption. We examine the differences on the cooling time of the star arising from uncertainties in the initial carbon-oxygen profile and the core-envelope L-Tc relation. The maximum time delay due to crystallization-induced chemical fractionation remains substantial, from ~1.0 Gyr for 0.5 and 1.2 M☉ white dwarfs to ~1.5 Gyr for 0.6-0.8 M☉ white dwarfs, even with initial stratified composition profiles, and cannot be ignored in detailed white dwarf cooling calculations. These cooling sequences provide theoretical support to search for or identify old disk or halo hydrogen-rich white dwarfs by characterizing their mass and age from their observational signatures.

The Ionization of the Local Interstellar Medium, as Revealed by FUSE Observations of N, O and Ar toward White Dwarf Stars

Abstract: FUSE spectra of the white dwarf stars G191-B2B, GD 394, WD 2211-495 and WD 2331-475 cover the absorption features out of the ground electronic states of N I, N II, N III, O I and Ar I in the far ultraviolet, providing new insights on the origin of the partial ionization of the Local Interstellar Medium (LISM), and for the case of G191-B2B, the interstellar cloud that immediately surrounds the solar system. Toward these targets the interstellar abundances of Ar I, and sometimes N I, are significantly below their cosmic abundances relative to H I. In the diffuse interstellar medium, these elements are not likely to be depleted onto dust grains. Generally, we expect that Ar should be more strongly ionized than H (and also O and N whose ionizations are coupled to that of H via charge exchange reactions) because the cross section for the photoionization of Ar I is very high. Our finding that Ar I/H I is low may help to explain the surprisingly high ionization of He in the LISM found by other investigators. Our result favors the interpretation that the ionization of the local medium is maintained by a strong EUV flux from nearby stars and hot gases, rather than an incomplete recovery from a past, more highly ionized condition.

The Ionization of the Local Interstellar Medium as Revealed by Far Ultraviolet Spectroscopic Explorer Observations of N, O, and Ar toward White Dwarf Stars

Abstract: Far Ultraviolet Spectroscopic Explorer spectra of the white dwarf stars G191-B2B, GD 394, WD 2211-495, and WD 2331-475 cover the absorption features out of the ground electronic states of N I, N II, N III, O I, and Ar I in the far-ultraviolet, providing new insights on the origin of the partial ionization of the local interstellar medium (LISM) and, for the case of G191-B2B, the interstellar cloud that immediately surrounds the solar system. Toward these targets the interstellar abundances of Ar I, and sometimes N I, are significantly below their cosmic abundances relative to H I. In the diffuse interstellar medium, these elements are not likely to be depleted onto dust grains. Generally, we expect that Ar should be more strongly ionized than H (and also O and N, whose ionizations are coupled to that of H via charge-exchange reactions) because the cross section for the photoionization of Ar I is very high. Our finding that Ar I/H I is low may help to explain the surprisingly high ionization of He in the LISM found by other investigators. Our result favors the interpretation that the ionization of the local medium is maintained by a strong extreme-ultraviolet flux from nearby stars and hot gases, rather than an incomplete recovery from a past, more highly ionized condition.

The ages of very cool hydrogen-rich white dwarfs

Abstract: The evolution of white dwarfs is essentially a cooling process that depends primarily on the energy stored in their degenerate cores and on the transparency of their envelopes. In this paper we compute accurate cooling sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres for the full range of masses of interest. For this purpose we use the most accurate available physical inputs for both the equation of state and opacities of the envelope and for the thermodynamic quantities of the degenerate core. We also investigate the role of the latent heat in the computed cooling sequences. We present separately cooling sequences in which the effects of phase separation of the carbon-oxygen binary mixture upon crystallization have been neglected, and the delay introduced in the cooling times when this mechanism is properly taken into account, in order to compare our results with other published cooling sequences which do not include a treatment of this phenomenon. We find that the cooling ages of very cool white dwarfs with pure hydrogen atmospheres have been systematically underestimated by roughly 1.5 Gyr at log(L/Lo)=-4.5 for an otherwise typical 0.6 Mo white dwarf, when phase separation is neglected. If phase separation of the binary mixture is included then the cooling ages are further increased by roughly 10%. Cooling tracks and cooling isochrones in several color-magnitude diagrams are presented as well.

Discovery of High Proper-Motion Ancient White Dwarfs: Nearby Massive Compact Halo Objects?

Abstract: We present the discovery and spectroscopic identification of two very high proper-motion ancient white dwarf stars, found in a systematic proper-motion survey. Their kinematics and apparent magnitude clearly indicate that they are halo members, while their optical spectra are almost identical to the recently identified cool halo white dwarf WD 0346+246. Canonical stellar halo models predict a white dwarf volume density that is 2 orders of magnitude less than the ρ ~ 7 × 10-4 M☉ pc-3 inferred from this survey. With the caveat that the sample size is very small, it appears that a significant fraction, ~10%, of the local dark matter halo is in the form of very old, cool, white dwarfs.

Adiabatic Survey of Subdwarf B Star Oscillations. I. Pulsation Properties of a Representative Evolutionary Model

Abstract: We present the first results of a large, systematic adiabatic survey of the pulsation properties of models of subdwarf B (sdB) stars. This survey is aimed at providing the most basic theoretical data with which to analyze the asteroseismological properties of the recently discovered class of pulsating sdB stars (the EC 14026 stars). Such a theoretical framework has been lacking up to now. In this paper, the first of a series of three, an adiabatic pulsation code is used to compute, in the 80-1500 s period window, the radial (l = 0) and nonradial (from l = 1 up to l = 3) oscillation modes for a representative evolutionary model of subdwarf B stars. Quantities such as the periods, kinetic energies, first-order rotational splitting coefficients, eigenfunctions, and weight functions are given by the code, providing a complete set of very useful diagnostic tools with which to study the mode properties. The main goal is to determine how these quantities relate to the internal structure of B subdwarfs, a crucial and necessary step if one wants to eventually apply the tools of asteroseismology to EC 14026 stars. All modes (p, f, and g) were considered in order to build the most complete picture we can have on pulsations in these stars. In that context, we show that g-modes are essentially deep interior modes oscillating mainly in the radiative helium-rich core (but not in the convective nucleus), while p-modes are shallower envelope modes. We demonstrate that g-modes respond to a trapping/confinement phenomenon induced mainly by the He/H chemical transition between the H-rich envelope and the He-rich core of subdwarf B stars. This phenomenon is very similar in nature to the g-mode trapping and confinement mechanisms observed in pulsating white dwarf models. We emphasize that p-modes may also experience distortions of their period distribution due to this He/H transition, although these are not as pronounced as in the g-mode case. These phenomena are of great interest as they can potentially provide powerful tools for probing the internal structure of these objects, in particular, with respect to constraining the mass of their H-rich envelope. The results given in this first paper form the minimal background on pulsation mode characteristics in sdB stars. Upcoming discussions on additional mode properties in subdwarf B star models (Paper II and Paper III of this series) will strongly rely on these basic results since they provide essential guidance in understanding mode period behaviors as functions of B subdwarf stellar parameters and/or evolution.

Negative Poisson's Ratios for Extreme States of Matter

Abstract: Negative Poisson's ratios are predicted for body-centered-cubic phases that likely exist in white dwarf cores and neutron star outer crusts, as well as those found for vacuumlike ion crystals, plasma dust crystals, and colloidal crystals (including certain virus crystals). The existence of this counterintuitive property, which means that a material laterally expands when stretched, is experimentally demonstrated for very low density crystals of trapped ions. At very high densities, the large predicted negative and positive Poisson's ratios might be important for understanding the asteroseismology of neutron stars and white dwarfs and the effect of stellar stresses on nuclear reaction rates. Giant Poisson's ratios are both predicted and observed for highly strained coulombic photonic crystals, suggesting possible applications of large, tunable Poisson's ratios for photonic crystal devices.