Close Double White Dwarfs Research Articles

Magnetic dynamos in white dwarfs – III. Explaining the occurrence of strong magnetic fields in close double white dwarfs

ABSTRACT The origin of strong ($\stackrel{\gt }{\scriptstyle \sim }1\,\mathrm{ MG}$) magnetic fields in white dwarfs has been a puzzle for decades. Recently, a dynamo mechanism operating in rapidly rotating and crystallizing white dwarfs has been suggested to explain the occurrence rates of strong magnetic fields in white dwarfs with close low-mass main-sequence star companions. Here, we investigate whether the same mechanism may produce strong magnetic fields in close double white dwarfs. The only known strongly magnetic white dwarf that is part of a close double white dwarf system, the magnetic component of NLTT 12758, is rapidly rotating and likely crystallizing and therefore the proposed dynamo mechanism represents an excellent scenario for the origin of its magnetic field. Presenting a revised formation scenario for NLTT 12758, we find a natural explanation for the rapid rotation of the magnetic component. We furthermore show that it is not surprising that strong magnetic fields have not been detected in all other known double white dwarfs. We therefore conclude that the incidence of magnetic fields in close double white dwarfs supports the idea that a rotation- and crystallization-driven dynamo plays a major role in the generation of strong magnetic fields in white dwarfs.

Convection in common envelopes and the formation of double white dwarfs

AbstractFormation of close double white dwarfs likely requires the initial binary system to evolve through two successive common envelope (CE) phases. A prominent method for describing CE outcomes involves defining an ejection efficiency, αeff, which quantifies the fraction of orbital energy available to unbind the envelope. Reproducing observed post-CE orbital parameters has proven difficult for numerical simulations, as the companion’s decaying orbit fails to eject the envelope. The ejection failure seen in numerical simulations may be resolved with a proper treatment of convection, whereby the binary orbit shrinks before energy can drive ejection. Where the orbital decay timescale exceeds the convective transport timescale, the energy released during inspiral is carried to the stellar surface and radiated away. By including convection, we produce sub-day post-CE orbital periods, a result consistent with observations. We comment on the effects of convection for the population of double white dwarfs that evolve through two CEs.

The double helium-white dwarf channel for the formation of AM CVn binaries

Most close double helium white dwarfs will merge within a Hubble time due to orbital decay by gravitational wave radiation. However, a significant fraction with low mass ratios will survive for a long time as a consequence of stable mass transfer. Such stable mass transfer between two helium white dwarfs (HeWDs) provides one channel for the production of AM CVn binary stars. In previous calculations of double HeWD progenitors, the accreting HeWD was treated as a point mass. We have computed the evolution of 16 double HeWD models in order to investigate the consequences of treating the evolution of both components in detail. We find that the boundary between binaries having stable and unstable mass transfer is slightly modified by this approach. By comparing with observed periods and mass ratios, we redetermine masses of eight known AM CVn stars by our double HeWDs channel, i.e. HM Cnc, AM CVn, V406 Hya, J0926, J1240, GP Com, Gaia14aae and V396 Hya.We propose that central spikes in the triple-peaked emission spectra of J1240, GP Com and V396 Hya and the surface abundance ratios of N/C/O in GP Com can be explained by the stable double HeWD channel. The mass estimates derived from our calculations are used to discuss the predicted gravitational wave signal in the context of the Laser Interferometer Space Antenna (LISA) project.

Evolution of Angular Momentum and Orbital Period Changes in Close Double White Dwarf Binaries

We have presented the evolution of angular momentum and orbital period changes between the component spins and the orbit in close double white dwarf binaries undergoing mass transfer through direct impact accretion over a broad range of orbital parameter space. This work improves upon similar earlier studies in a number of ways: First, we calculate self-consistently the angular momentum of the orbit at all times. This includes gravitational, tides and mass transfer effects in the orbital evolution of the component structure models, and allow the Roche lobe radius of the donor star and the rotational angular velocities of both components to vary, and account for the exchange of angular momentum between the spins of the white dwarfs and the orbit. Second, we investigate the mass transfer by modeling the ballistic motion of a point mass ejected from the center of the donor star through the inner Lagrangian point. Finally, we ensure that the angular momentum is conserved, which requires the donor star spin to vary self-consistently. With these improvements, we calculate the angular momentum and orbital period changes of the orbit and each binary component across the entire parameter space of direct impact double white dwarf binary systems. We find a significant decrease in the amount of angular momentum removed from the orbit during mass transfer, as well as cases where this process increases the angular momentum and orbital period of the orbit at the expense of the spin angular momentum of the donor and accretor. We find that our analysis yields an increase in the predicted number of stable systems compared to that in the previous studies, survive the onset of mass transfer, even if this mass transfer is initially unstable. In addition, as a consequence of the tidal coupling, systems that come into contact near the mass transfer instability boundary undergo a phase of mass transfer with their orbital period.

Orbital periods and component masses of three double white dwarfs

The merger of close double white dwarfs (CDWDs) is one of the favourite evolutionary channels for producing Type Ia supernovae (SN Ia). Unfortunately, current theories of the evolution and formation of CDWDs are still poorly constrained and have several serious uncertainties, which affect the predicted SN Ia rates. Moreover, current observational constraints on this evolutionary pathway for SN Ia mainly rely on only 18 double-lined and/or eclipsing CDWDs with measured orbital and stellar parameters for both white dwarfs. In this paper we present the orbital periods and the individual masses of three new double-lined CDWDs, derived using a new method. This method employs mass ratios, the Halpha core ratios and spectral model-fitting to constrain the masses of the components of the pair. The three CDWDs are WD0028-474 (Porb=9.350 +- 0.007 hours, M1=0.60 +- 0.06 Msun, M2=0.45 +- 0.04 Msun), HE0410-1137 (Porb = 12.208 +- 0.008 hours, M1= 0.51 +- 0.04 Msun, M2= 0.39 +- 0.03 Msun) and SDSSJ031813.25-010711.7 (Porb = 45.908 +- 0.006 hours, among the longest period systems, M1= 0.40 +- 0.05 Msun, M2= 0.49 +- 0.05 Msun). While the three systems studied here will merge in timescales longer than the Hubble time and are expected to become single massive (>~0.9 Msun) white dwarfs rather than exploding as SN Ia, increasing the small sample of CDWDs with determined stellar parameters is crucial for a better overall understanding of their evolution.

A binary population synthesis study on gravitational wave sources

AbstractGravitational waves (GW) are a natural consequence of Einstein's theory of gravity (general relativity), and minute distortions of space-time. Gravitational Wave Astronomy is an emerging branch of observational astronomy which aims to use GWs to collect observational data about objects such as neutron stars and black holes, about events such as supernovae and about the early universe shortly after the big bang.This field will evolve to become an established component of 21st century multi-messenger astronomy, and will stand shoulder-to-shoulder with gamma-ray, x-ray, optical, infrared and radio astronomers in exploring the cosmos. In this paper, we state a recent theoretical study on GW sources, and present the results of our studies on the field using a binary population synthesis (BPS) approach, which was designed to investigate the formation of many interesting binary-related objects, including close double white dwarfs, AM CVn stars, ultra-compact X-ray binaries(UCXBs), double neutron stars, double stellar black holes. Here we report how BPS can be used to determine the GW radiation from double compact objects.

Gravitational-wave radiation from double compact objects with eLISA in the Galaxy

The phase of inspiral of double compact objects (DCOs: NS + WD, NS + NS, BH + NS, and BH + BH binaries) in the disk field population of the Galaxy provides a potential source in the frequency range from 10-4 to 0.1 Hz, which can be detected by the European New Gravitational Observatory (NGO: eLISA is derived from the previous LISA proposal) project. In this frequency range, much stronger gravitational wave (GW) radiation can be obtained from DCO sources because they possess more mass than other compact binaries (e.g., close double white dwarfs). In this study, we aim to calculate the gravitational wave signals from the resolvable DCO sources in the Galaxy using a binary population synthesis approach, and determine physical properties of these binaries using Monte Carlo simulations. Combining the sensitivity curve of the eLISA detector and a confusion-limited noise floor of close double white dwarfs, we find that only a handful of DCO sources can be detected by the eLISA detector. The detectable number of DCO sources reaches 160; in the context of low-frequency eLISA observations we find that the number of NS + WD, NS + NS, BH + NS, and BH + BH objects are 132, 16, 3, and 6, respectively.

Supernova Type Ia progenitors from merging double white dwarfs

The study of Type Ia supernovae (SNIa) has lead to greatly improved insights into many fields in astrophysics, however a theoretical explanation of the origin of these events is still lacking. We investigate the potential contribution to the SNIa rate from the population of merging double carbon-oxygen white dwarfs. We aim to develope a model that fits the observed SNIa progenitors as well as the observed close double white dwarf population. We differentiate between two scenarios for the common envelope (CE) evolution; the alpha-formalism based on the energy equation and the gamma-formalism that is based on the angular momentum equation. In one model we apply the alpha-formalism always. In the second model the gamma-formalism is applied, unless the binary contains a compact object or the CE is triggered by a tidal instability for which the alpha-formalism is used. The binary population synthesis code SeBa was used to evolve binary systems from the zero-age main sequence to the formation of double white dwarfs and subsequent mergers. SeBa has been thoroughly updated since the last publication of the content of the code. The limited sample of observed double white dwarfs is better represented by the simulated population using the gamma-formalism than the alpha-formalism. For both CE formalisms, we find that although the morphology of the simulated delay time distribution matches that of the observations within the errors, the normalisation and time-integrated rate per stellar mass are a factor 7-12 lower than observed. Furthermore, the characteristics of the simulated populations of merging double carbon-oxygen white dwarfs are discussed and put in the context of alternative SNIa models for merging double white dwarfs.

SOME CONSTRAINTS ON THE LOWER MASS LIMIT FOR DOUBLE-DEGENERATE PROGENITORS OF TYPE Ia SUPERNOVAE

Recent theoretical and observational studies both argue that the merging of double carbon-oxygen white dwarfs (WDs) is responsible for at least some Type Ia supernovae (SNe Ia). Previous (standard) studies of the anticipated SN birthrate from this channel have assumed that the merger process is conservative and that the primary criterion for explosion is that the merged mass exceeds the Chandrasekhar mass. Han & Webbink (1999) demonstrated that mass transfer and merger in close double WDs will in many cases be non-conservative. Pakmor et al. (2011) further suggested that the merger process should be violent in order to initiate an explosion. We have therefore investigated how the SN Ia birthrate from the double-degenerate (DD) channel is affected by these constraints. Using the binary-star population-synthesis method, we have calculated the DD SN Ia birthrate under conservative and non-conservative approximations, and including lower mass and mass-ratio limits indicated by recent smoothed-particle-hydrodynamic calculations. The predicted DD SN Ia rate is significantly reduced by all of these constraints. With dynamical mass loss alone (violent merger) the birthrate is reduced to 56% of the conservative rate. Requiring that the mass ratio $q>2/3$ further reduces the birthrate to 18% that of the standard assumption. An upper limit of 0.0061 SNuM, or a Galactic rate of $4.6 \times 10^{-4}{\rm yr}^{-1}$, might be realistic.

A COMPREHENSIVE STUDY OF CLOSE DOUBLE WHITE DWARFS AS GRAVITATIONAL WAVE SOURCES: EVOLUTIONARY CHANNELS, BIRTH RATES, AND PHYSICAL PROPERTIES

Close double white dwarfs (CDWDs) are believed to dominate the Galactic gravitational wave (GW) radiation in the frequency range 10(-4) to 0.1 Hz, which will be detected by the Laser Interferometer Space Antenna (LISA) detector. The aim of this detector is to detect GW radiation from astrophysical sources in the universe and to help improve our understanding of the origin of the sources and their physical properties (masses and orbital periods). In this paper, we study the probable candidate sources in the Galaxy for the LISA detector: CDWDs. We use the binary population synthesis approach of CDWDs together with the latest findings of the synthesis models from Han, who proposed three evolutionary channels: (1) stable Roche lobe overflow plus common envelope (RLOF+CE), (2) CE+CE, and (3) exposed core plus CE. As a result, we systematically investigate the detailed physical properties (the distributions of masses, orbital periods, and chirp masses) of the CDWD sources for the LISA detector, examine the importance of the three evolutionary channels for the formation of CDWDs, and carry out Monte Carlo simulations. Our results show that RLOF+CE and CE+CE are the main evolutionary scenarios leading to the formation of CDWDs. For the LISA detectable sources, we also explore and discuss the importance of these three evolutionary channels. Using the calculated birth rate, we compare our results to the LISA sensitivity curve and the foreground noise floor of CDWDs. We find that our estimate for the number of CDWD sources that can be detected by the LISA detector is greater than 10,000. We also find that the detectable CDWDs are produced via the CE+CE channel and we analyze the fraction of the detectable CDWDs that are double helium (He+He), or carbon-oxygen plus helium (CO+He) WD binary systems.

Contribution of close double white dwarfs to the Galactic gravitational wave foreground

This paper provides new detailed specifics and characteristics of the close double white dwarfs (CDWDs) LISA foreground noise floor using the newest information on the astrophysics with binary population algorithms, which were judged to satisfy the observations. We use the binary population synthesis approach of CDWDs in combination with the latest findings of synthesis models from Han (Mon. Not. R. Astron. Soc. 296:1019, 1998), here, specifically formation by the three evolutionary channels of (a) stable Roche lobe overflow plus common envelope (RLOF + CE), (b) CE plus CE, (c) exposed core plus CE. We collect resulting CDWDs in the frequency range of 10(-8) Hz to 10(-2) Hz and compare strength and number statistics per resolvable frequency bin to derive the properties of the noise floor and the nature of possible outliers that may become individually resolvable. We find resolvable outliers to be solely produced by CE + CE evolution; the other two channels contribute only to the unresolvable foreground noise. Within the CE + CE evolution a detailed look into the chemical buildup of the stars reveals dominantly double helium (He + He) WDs or carbon-oxygen CO + He WDs.

Gravitational wave radiation from close double white dwarfs in the Galaxy

Close double white dwarfs (CDWDs) are believed to dominate the Galactic gravitational wave (GW) foreground radiation in the frequency range 10(-4) to 0.1 Hz, which will be detected by the Laser Interferometer Space Antenna (LISA). In this frequency range, the foreground radiation is predicted to be larger than both instrumental noise and the GW radiation from other sources, such as extragalactic low-mass X-ray binaries. We study the production of CDWDs using a binary population synthesis approach, and calculate their contribution to the Galactic GW foreground radiation. From a sample of 10(7) binaries, we select CDWD systems that would radiate at frequencies between 10(-8) and 10(-2) Hz. We calculate the evolutionary processes of the selected objects and conclude that the CDWD systems radiating at lower frequencies than 10(-3) Hz are LISA foreground sources, while those radiating at higher frequencies are possibly resolved sources. The CDWD systems which make up the LISA GW foreground are produced through three evolutionary channels: stable Roche lobe overflow (RLOF) plus common envelope (CE); CE plus CE; and exposed core plus CE, as proposed by Han (1998). We find that the resolved CDWDs are produced by the CE plus CE channel. We analyse the proportion of the resolved CDWDs that are compositionally double helium, or carbon-oxygen plus helium WD binary systems.

Binaries discovered by the SPY project

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 WD0326-273. We briefly discuss the consequences of our findings for our understanding of the formation and evolution of double white dwarfs.

Gravitational waves from double white dwarfs and AM CVn binaries

I give a brief overview of our model for the galactic population of compact binaries that is used to predict the low-frequency gravitational wave signal from the galaxy, and discuss recent observational developments that will enable us to test and improve this model. The SPY project will discover some 150 new close double white dwarfs and, recently, two ROSAT sources turned out to be new AM CVn candidates, one with an orbital period of only 5 min. I give an update on the expected binaries that will be resolved by LISA and discuss what we can learn about the galactic population of compact binaries once LISA gives her first results.

Population synthesis for double white dwarfs

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.

Double white dwarfs as progenitors of R Coronae Borealis stars and Type I supernovae

Close double white dwarfs should arise from the second phase of mass exchagne in close binaries which first encountered mass exchange while the more massive star was crossing the Hertzprung gap. Tidal mass transfer in these double degenerate systems is explored. The sequence of double white dwarf divides naturally into three segments. (1) Low-mass helium/helium pairs are unstable to dynamical time-scale mass transfer and probably coalesce to form helium-burning sdO stars. (2) In helium/carbon-oxygen pairs, mass transfer occurs on the time scale for gravitational radiation losses (approx.10/sup -4/ M/sub sun/ yr/sup -1/); the accreted helium is quickly ignited, and the accretor expands to dimensions characteristic of R CrB stars, engulfing its companion star. (3) Carbon-oxygen/carbon-oxygen pairs are again unstable to dynamical time-scale mass transfer and, since their total masses exceed the Chandrasekhar limit, are destined to become supernovae. Inactive lifetimes in these latter systems between creation and interaction can exceed 10/sup 10/ years. Birthrates of R CrB stars and Type I supernovae by evolution of double white dwarfs are in reasonable agreement with observational estimates.