Close White Dwarf Research Articles

The single-degenerate channel for the progenitors of Type Ia supernovae

We have carried out a detailed study of one of the most favoured evolutionary channels for the production of Type Ia supernova (SN Ia) progenitors, the single-degenerate channel (CO + MS), where a carbon/oxygen (CO) white dwarf (WD) accretes matter from an unevolved or slightly evolved non-degenerate star until it reaches the Chandrasekhar mass limit. Employing Eggleton's stellar evolution code and adopting the prescription of Hachisu et al. for the accretion efficiency, we performed binary stellar evolution calculations for about 2300 close WD binary systems and mapped out the initial parameters in the orbital period–secondary mass (P–M2) plane (for a range of WD masses) which lead to a successful Type Ia supernova. We obtained accurate, analytical fitting formulae to describe this parameter range which can be used for binary population synthesis (BPS) studies. The contours in the P–M2 plane differ from those obtained by Hachisu et al. for low-mass CO WDs, which are more common than massive CO WDs. We confirm that WDs with a mass as low as 0.67 M⊙ can accrete efficiently and reach the Chandrasekhar limit. We have implemented these results in a BPS study to obtain the birth rates for SNe Ia and the evolution of birth rates with time of SNe Ia for both a constant star formation rate and a single starburst. The birth rates are lower than (but comparable to) those inferred observationally.

The Optical Identification of Close White Dwarf Binaries in the Laser Interferometer Space Antenna Era

The Laser Interferometer Space Antenna (LISA) is expected to detect close white dwarf binaries (CWDBs) through their gravitational radiation. Around 3000 binaries will be spectrally resolved at frequencies greater than 3 mHz, and their positions on the sky will be determined to an accuracy ranging from a few tens of arcminutes to a degree or more. Because of the small binary separation, the optical light curves of 30% of these CWDBs are expected to show eclipses, giving a unique signature for identification in follow-up studies of the LISA error boxes. While the precise optical location improves binary parameter determination with LISA data, the optical light curve captures additional physics of the binary, including the individual sizes of the stars in terms of the orbital separation. To optically identify a substantial fraction of CWDBs and thus localize them very accurately, a rapid monitoring campaign is required, capable of imaging a square degree or more in a reasonable time, at intervals of 10-100 s, to magnitudes between 20 and 25. While the detectable fraction can be up to many tens of percent of the total resolved LISA CWDBs, the exact fraction is uncertain because of unknowns related to the WD spatial distribution and potentially interesting physics, such as induced tidal heating of the WDs due to their small orbital separation.

Editorial

The 4th International LISA Symposium was held at the National Science Foundation Physics Frontier Center for Gravitational Wave Physics at The Pennsylvania State University on 19-24 July 2002. This special issue of Classical and Quantum Gravity is the proceedings of this meeting.LISA - the Laser Interferometer Space Antenna - is part of an international effort to open a new window on the universe. Not all things radiate light, but everything gravitates. Observations of the gravitational waves radiated by black holes and compact binary star systems, in our galaxy and beyond, can reveal details about these systems and their environments that are otherwise inaccessible.The international effort, of which LISA is a part, includes ground-based detectors, and the relationship between LISA and its ground-based detector 'cousins' was an important theme for this Symposium. LISA will observe gravitational waves in the 0.1 mHz to 0.1 Hz band, complementing observations made by ground-based detectors in the 10 Hz to several KHz band. Together they will explore nearly six decades of bandwidth in the gravitational-wave sky. LISA in particular will observe the gravitational waves radiated by the coalescence of black holes at the centres of colliding galaxies, and the inspiral of compact neutron stars or stellar-mass black holes onto these black holes, virtually anywhere in the universe. It will take a census of neutron star or close white dwarf binaries in our own galaxy and observe the formation of large black holes from the very first structures to form and collapse in our universe. In doing all these things, it will shed new 'light' on the first structures to form in the universe, explore the evolution of galaxies and the roles that black holes play in their structure, test relativity near the 'edges' of a black hole, and deepen our understanding of stellar and binary system evolution.A successful conference - and this LISA Symposium, like its predecessors, was very successful - does not just happen. It is the result of the work of many individuals and teams working together on scientific organization, travel, visa, lodging and financial logistics and planning. When all these things come together just right, as they did in July of 2002, it enables the speakers and participants to make something truly remarkable happen. It is my pleasure to thank the local organizing and scientific organizing committees, the accommodating staff of the Penn State Conference Center, and Penn State Transportation Services for their help and cooperation. I would like to single out for special thanks and appreciation the tireless Karen Brewster, whose energy, vigour and focus overcame all logistical obstacles, freeing the participants to focus on the exciting science that LISA will enable. In the end, however, it is the speakers and the participants who make a successful meeting. I look forward to the continued growth of the LISA Community and to meeting you all at the 5th International LISA Symposium, to be held at ESTEC in July 2004.Lee Samuel FinnPenn State University

Long-term operation of the Laser Interferometer Space Antenna and Galactic close white dwarf binaries

The binary confusion noise spectrum in the Laser Interferometer Space Antenna (LISA) band depends strongly on the observational period and abundance of Galactic close white dwarf binaries (CWDBs). We have investigated how the number of the resolved Galactic CWDBs varies with the operation period of LISA, and found that the resolved number would typically grow by a factor of 5 when the operation period increases from 1 to 10 yr. We have also made a similar estimation for the number of CWDBs, the chirp signal of which can be measured using matched filtering analysis.

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.

Stochasticbackgrounds at LISA frequencies

We review some recent investigations on the cumulative signal producedby extragalactic populations of gravitational wave sources at frequencieswhich might be interesting for LISA. In particular, we consider thecontribution of extragalactic compact binary systems, accounting for thebinaries which have been formed since the onset of galaxy formation inthe Universe. The resulting gravitational signal appears to be largelydominated by close white dwarf binaries and represents a significantconfusion noise component for LISA at frequencies between 1 and 10 mHz(Schneider et al 2001 Mon. Not. R. Astron. Soc. 324 797).Going further in the high redshift Universe, we consider the gravitationalwave emission from the first stars, which are assumed to be very massiveobjects (VMOs) (Schneider et al 2000Mon. Not. R. Astron. Soc. 317 385).If the final fate of VMOs is to collapse,leaving very massive black hole remnants,then the gravitational waves emittedduring each collapse would be seen as astochastic background. The predictedsignal in a critical-density cold darkmatter universe might be interesting forLISA. The expected emission rate isroughly 4000 events/yr, resulting in astationary, discrete sequence of bursts,i.e. a shot-noise signal.

Low frequency gravitational waves from black hole MACHO binaries

The intensity of low frequency gravitational waves from black hole MACHO binaries is studied. First we estimate the gravitational wave background produced by black hole MACHO binaries in the Milky Way halo as well as the cosmological gravitational wave background produced by the extragalactic black hole MACHO binaries. It is found that the cosmological gravitational wave background due to black hole MACHO binaries is larger than the halo background unless an extreme model of the halo is assumed, while it is smaller than the background due to close white dwarf binaries at $\nu_{gw} \siml 10^{-2.5}$ Hz if the actual space density of white dwarfs is maximal. This cosmological background due to black hole MACHO binaries is well below the observational constraints from the pulsar timing, quasar proper motions and so on. We find that one year observation by LISA will be able to detect gravitational waves from at least several hundreds of nearby independent black hole MACHO binaries whose amplitudes exceed these backgrounds. This suggests that LISA will be able to pin down various properties of primordial black hole MACHOs together with the results of LIGO-VIRGO-TAMA-GEO network. Furthermore, it may be possible to draw a map of the mass distribution of our halo, since LISA can determine the position and the distance to individual sources consisted of black hole MACHO binaries. Therefore, LISA may open a new field of the gravitational wave astronomy.

Stellar Masses, Kinematics, and White Dwarf Composition for Three Close DA+dMe Binaries

We determine the mass functions and mass ratios for three close white dwarf plus red dwarf binaries (EUVE J0720-317, 1016-053, and 2013+400). Hubble Space Telescope Goddard High-Resolution Spectrograph spectra of the He II ?1640 and C IV ?1550 spectral lines trace the white dwarf orbital motion, and Hamilton Spectrograph echelle spectra (Lick Observatory) and lower dispersion spectra trace the red dwarf orbital motion. The data sets allow us to measure orbital periods and velocities, as well as the white dwarf gravitational redshifts. The red dwarf and white dwarf mass estimates obtained from a combination of independent mass measurements for the white dwarf stars and our new orbital elements help constrain probable evolutionary outcomes. We find that EUVE J0720-317 will probably come into contact within a Hubble time and that the mass transfer will be unstable on a dynamical time. We also conclude that the much lower secondary masses in EUVE J1016-053 and EUVE J2013+400 exclude the possibility of significant interaction in these systems. We also present new helium and carbon photospheric abundance measurements in the three white dwarfs. The white dwarf atmospheric composition may show the effects of accretion of red dwarf mass-loss material onto its surface. Finally, we study the kinematics of the systems, and we also show that the white dwarf in EUVE?J1016-053 is part of a quadruple system.

Helium and Carbon‐Oxygen White Dwarfs in Close Binaries

We present a scenario model of the population of close binaries in which the brighter component is a helium or a carbon-oxygen white dwarf. The other, dimmer component can be a helium white dwarf, a carbon-oxygen white dwarf, or a low-mass (≤0.3 M☉) main-sequence star. The model takes into account the effects of observational selection related to the cooling of white dwarfs and the relative brightness of components. The total birthrate of such binaries in our Galaxy is estimated to be ~0.17 yr-1. In the model, in 63% of all cases, the dimmer component is also a white dwarf. In 82% of the systems that consist of two close white dwarfs, the brighter component is a helium white dwarf. This explains why, in at least seven out of the eight white dwarf pairs found in recent years, the primary is composed of helium. We estimate that close white dwarf pairs may constitute of the total sample of observed white dwarfs. Systems that have a total mass exceeding 1.4 M☉ and in which the components are close enough to merge in a Hubble time may constitute ~1/40 of all close white dwarf pairs. This means that the sample of observed white dwarf pairs must be at least quadrupled before one may hope to find a hypothetical Type Ia supernova (SN Ia) precursor. The total number of SN Ia precursors is estimated to exceed the observable number by a factor of about 20. The merger frequency of close binary helium white dwarfs in the Galaxy is estimated to be ~0.02 yr-1. This number is consistent with the fact that, in the total sample of known white dwarf pairs, there are three in which the components are close enough to merge in a Hubble time. In one system (WD 1101+364), the merger may result in the formation of a helium subdwarf (nondegenerate helium star), and in two others (WD 2331+290 and WD 0957-666), probably in the formation of a hydrogen-deficient subgiant and possibly later of an R CrB star. The predicted merger frequency is also consistent with the fact that two out of seven white dwarfs selected for their low mass are apparently single, while five are in close binaries.

The formation of bipolar planetary nebulae and close white dwarf binaries

The formation of bipolar planetary nebulae and close white dwarf binaries

Single and binary star evolution

After presenting a general account of the observed global properties of single stars of low, intermediate, and high mass, together with their theoretical Hertzsprung-Russell diagram evolution, attention is given to the observed properties of various evolved close binaries and to an assessment of the value of comparisons between observation and crude theory in characterizing the physics of mass transfer within interacting binary systems. Detailed consideration is then undertaken of such topics as stellar evolution in globular clusters, interior star changes due to nucleosynthesis and mixing, asymptotic giant branch stars of intermediate mass, the response of white dwarfs in binary systems to mass accretion, and scenarios for binary star evolution tending toward close white dwarf pairs.

Gravitational radiation from the Galaxy

The spectral flux of gravitational radiation incident on earth from the Galactic W UMa binaries, unevolved binaries, cataclysmic binaries, neutron star binaries, black hole-neutron star binaries, and close white dwarf binary (CWDB) systems is calculated. The peak values for the strain amplitude hv produced by the binaries are: log hv = -17.0/sq rt Hz at log v = -4.40 Hz for W UMa's; log hv = -16.5/sq rt Hz at log v = -5.50 Hz for unevolved binaries; log hv = -18.2/sq rt Hz at log v = -5.15 Hz for neutron star binaries; log hv = -17.0/sq rt Hz at log v = -4.7 Hz for black hole-neutron star binaries; log hv = -18.0 /sq rt Hz at log v = -4.10 Hz for cataclysmic binaries, and log hv = -27.0/sq rt Hz at log v = -4.0 Hz for CWDBs. The gravitational flux at ultralow frequencies is emitted mainly by unevolved binaries. The integrated flux incident on earth is about 2.4 x 10 to the -9th ergs/sq cm/s.

Astronomy: Formation and fate of interacting binaries

Astronomy: Formation and fate of interacting binaries