M. V. Van Der Sluys

The chemical composition of donors in AM CVn stars and ultracompact X-ray binaries: observational tests of their formation

We study the formation of ultra-compact binaries (AM CVn stars and ultra-compact X-ray binaries) with emphasis on the surface chemical abundances of the donors in these systems. Hydrogen is not convincingly detected in the spectra of any these systems. Three different proposed formation scenarios involve different donor stars, white dwarfs, helium stars or evolved main-sequence stars. Using detailed evolutionary calculations we show that the abundances of helium white dwarf donors and evolved main-sequence stars are close to equilibrium CNOprocessed material, and the detailed abundances correlate with the core temperature and thus mass of the main-sequenceprogenitors. Evolved main-sequence donors typically have traces of H left. For hybrid or carbon/oxygen white dwarf donors, the carbon and oxygen abundances depend on the temperature of the helium burning and thus on the helium core mass of the progenitors. For helium star donors in addition to their mass, t he abundances depend strongly on the amount of helium burnt before mass transfer starts and can range from unprocessed and thus almost equal to CNO-processed matter, to strongly processed and thus C/O rich and N-deficient. We briefly discuss the relative frequency of the se cases for helium star donors, based on population synthesis results. Finally we give diagnostics for applying our results to observed systems and find that the most important test is the N /C ratio, which can indicate the formation scenario as well as, in some cases, the mass of the progenitor of the donor. In addition, if observed, the N/O, O/He and O/C ratios can distinguish between helium star and white dwarf donors. Applied to the known systems we find evide nce for white dwarf donors in the AM CVn systems GP Com, CE 315 and SDSS J0804+16 and evide nce for hybrid white dwarf or very evolved helium star donors in the ultra-compact X-ray binaries 4U 1626-67 and 4U 0614+09.

Analytical Expressions for the Envelope Binding Energy of Giants as a Function of Basic Stellar Parameters

The common-envelope (CE) phase is an important stage in the evolution of binary stellar populations. The most common way to compute the change in orbital period during a CE is to relate the binding energy of the envelope of the Roche-lobe filling giant to the change in orbital energy. Especially in population-synthesis codes, where the evolution of millions of stars must be computed and detailed evolutionary models are too expensive computationally, simple approximations are made for the envelope binding energy. In this study, we present accurate analytic prescriptions based on detailed stellar-evolution models that provide the envelope binding energy for giants with metallicities between Z = 10 i4 and Z = 0:03 and masses between 0:8Mfl and 100Mfl, as a function of the metallicity, mass, radius and evolutionary phase of the star. Our results are also presented in the form of electronic data tables and Fortran routines that use them. We find that the accuracy of our fits is better than 15% for 90% of our model data points in all cases, and better than 10% for 90% of our data points in all cases except the asymptotic giant branches for three of the six metallicities we consider. For very massive stars (M & 50Mfl), when stars lose more than » 20% of their initial mass due to stellar winds, our fits do not describe the models as accurately. Our results are more widely applicable — covering wider ranges of metallicity and mass — and are of higher accuracy than those of previous studies. Subject headings: stars: evolution, stars: fundamental parameters, stars: mass loss, binaries: close

Modelling the formation of double white dwarfs

We investigate the formation of the ten double-lined double white dwarfs that have been observed so far. A detailed stellar evolution code is used to calculate grids of single-star and binary models and we use these to reconstruct possible evolutionary scenarios. We apply various criteria to select the acceptable solutions from these scenarios. We confirm the conclusion of Nelemans et al. (2000) that formation via conservative mass transfer and a common envelope with spiral-in based on energy balance or via two such spiralins cannot explain the formation of all observed systems. We investigate three different prescriptions of envelope ejection due to dynamical mass loss with angular-momentum balance and show that they can explain the observed masses and orbital periods well. Next, we demand that the age difference of our model is comparable to the observed cooling-age difference and show that this puts a strong constraint on the model solutions. However, the scenario in which the primary loses its envelope in an isotropic wind and the secondary transfers its envelope, which is then re-emitted isotropically, can explain the observed age differences as well. One of these solutions explains the DB-nature of the oldest white dwarf in PG 1115+116 along the evolutionary scenario proposed by Maxted et al. (2002a), in which the helium core of the primary becomes exposed due to envelope ejection, evolves into a giant phase and loses its hydrogen-rich outer layers.

Degeneracies in sky localization determination from a spinning coalescing binary through gravitational wave observations: a Markov-chain Monte Carlo analysis for two detectors

Gravitational-wave signals from inspirals of binary compact objects (black holes and neutron stars) are primary targets of the ongoing searches by groundbased gravitational-wave interferometers (LIGO, Virgo and GEO-600). We present parameter-estimation simulations for inspirals of black-hole–neutronstar binaries using Markov-chain Monte Carlo methods. As a specific example of the power of these methods, we consider source localization in the sky and analyze the degeneracy in it when data from only two detectors are used. We focus on the effect that the black-hole spin has on the localization estimation. We also report on a comparative Markov-chain Monte Carlo analysis with two different waveform families, at 1.5 and 3.5 post-Newtonian orders.

Gravitational-wave astronomy with inspiral signals of spinning compact-object binaries

Inspiral signals from binary compact objects (black holes and neutron stars) are primary targets of the ongoing searches by ground-based gravitational-wave interferometers (LIGO, Virgo, and GEO-600). We present parameter-estimation simulations for inspirals of black hole-neutron star binaries using Markov Chain Monte Carlo methods. For the first time, we both estimated the parameters of a binary inspiral source with a spinning, precessing component and determined the accuracy of the parameter estimation, for simulated observations with ground-based gravitational-wave detectors. We demonstrate that we can obtain the distance, sky position, and binary orientation at a higher accuracy than previously suggested in the literature. For an observation of an inspiral with sufficient spin and two or three detectors we find an accuracy in the determination of the sky position of the order of tens of square degrees.

Reduced magnetic braking and the magnetic capture model for the formation of ultra-compact binaries

A binary in which a slightly evolved star starts mass transfer to a neutron star can evolve towards ultra-short orbital periods under the influence of magnetic braking. This is called magnetic capture. In a previous paper we showed that ultra-short periods are only reached for an extremely small range of initial binary parameters, in particular orbital period and donor mass. Our conclusion was based on one specific choice for the law of magnetic braking, and for the loss of mass and angular momentum during mass transfer. In this paper we show that for less efficient magnetic braking it is impossible to evolve to ultra-short periods, independent of the amount of mass and associated angular momentum lost from the binary.

On the stability of thermonuclear shell sources in stars

We present a quantitative criterion for the thermal stability of thermonuclear shell sources. We find that the thermal stability of shell sources depends on exactly three factors: they are more stable when they are geometrically thicker, less degenerate and hotter. This confirms and unifies previously obtained results in terms of the geometry, temperature and density of the shell source, by a simplified but quantitative approach to the physics of shell nuclear burning. We present instability diagrams in the temperature-density plane for hydrogen and helium shell burning, which allow a simple evaluation of the stability conditions of such shell sources in stellar models. The performance of our stability criterion is demonstrated in various numerical models: in a 3 MAGB star, in helium accreting CO white dwarfs, in a helium white dwarf which is covered by a thin hydrogen envelope, and in a 1.0 Mgiant.

Population synthesis of ultracompact X-ray binaries in the Galactic Bulge

Aims. We model the present-day number and properties of ultracompact X-ray binaries (UCXBs) in the Galactic bulge. The main objective is to compare the results to the known UCXB population as well as to data from the Galactic Bulge Survey, in order to learn about the formation of UCXBs and their evolution, such as the onset of mass transfer and late-time behavior. Methods. The binary population synthesis code SeBa and detailed stellar evolutionary tracks have been used to model the UCXB population in the Bulge. The luminosity behavior of UCXBs has been predicted using long-term X-ray observations of the known UCXBs as well as the thermal-viscous disk instability model. Results. In our model, the majority of UCXBs initially have a helium burning star donor. Of the white dwarf donors, most have helium composition. In the absence of a mechanism that destroys old UCXBs, we predict (0.2−1.9) × 10 5 UCXBs in the Galactic bulge, depending on assumptions, mostly at orbital periods longer than 60 min (a large number of long-period systems also follows from the observed short-period UCXB population). About 5−50 UCXBs should be brighter than 10 35 erg s −1 , mostly persistent sources with orbital periods shorter than about 30 min and with degenerate helium and carbon-oxygen donors. This is about one order of magnitude more than the observed number of (probably) three. Conclusions. This overprediction of short-period UCXBs by roughly one order of magnitude implies that fewer systems are formed, or that a super-Eddington mass transfer rate is more difficult to survive than we assumed. The very small number of observed longperiod UCXBs with respect to short-period UCXBs, the surprisingly high luminosity of the observed UCXBs with orbital periods around 50 min, and the properties of the PSR J1719−1438 system all point to much faster UCXB evolution than expected from angular momentum loss via gravitational wave radiation alone. Old UCXBs, if they still exist, probably have orbital periods longer than 2 h and have become very faint due to either reduced accretion or quiescence, or have become detached. UCXBs are promising candidate progenitors of isolated millisecond radio pulsars.

The dynamics of the nebula M1-67 around the run-away Wolf-Rayet star WR 124

A new point of view on the dynamics of the circumstellar nebula M1-67 around the run-away Wolf-Rayet (WR) star WR 124 is presented. We simulated the outbursts of nebulae with different morphologies, to compare the results to the observed dynamical spectra of M1-67. We found that it has been interacting with the surrounding ISM and has formed a bow shock due to its high velocity of about 180 kms −1 relative to the local ISM. The star is about 1.3 parsec away from the front of this bow shock. The outbursts that are responsible for the nebula are assumed to be discrete outbursts that occurred inside this bow shock. The ejecta collide with this bow shock shortly after the outburst. After the collision, they are dragged away by the pressure of the ISM, along the surface of the bow shock. The bow shock is oriented in such way that we are looking from the rear into this paraboloid, almost along the main axis. Evidence for this is given firstly by the fact that the far hemisphere is much brighter than the near hemisphere, secondly by the fact that there is hardly any emission found with radial velocities higher than the stars radial velocity, thirdly by the fact that the star looks to be in the centre of the nebula, as seen from Earth, and finally by the asymmetric overall velocity distribution of the nebula, which indicates higher radial velocities in the centre of the nebula, and lower velocities near the edges. We find evidence for at least two discrete outbursts that occurred inside this bow shock. For these outbursts, we find expansion velocities of vexp ≈ 150 kms −1 and dynamical timescales of about 0.8 and 2 ×10 4 yr, which are typical values for LBV outbursts. We therefore conclude that M1-67 originates from several outbursts that occurred inside the bow shock around WR 124, during an LBV phase that preceded the current WR phase of the star.

Using electromagnetic observations to aid gravitational-wave parameter estimation of compact binaries observed with LISA

We present a first-stage study of the e ect of using knowledge from electromagnetic (EM) observations in the gravitational wave (GW) data analysis of Galactic binaries that are predicted to be observed by the new Laser Interferometer Space Antenna in the lowfrequency range, 10 4 Hz < f < 1Hz. In particular, we examine the extent to which the accuracy of GW parameter estimation improves if we use available information from EM data. We do this by investigating whether correlations exist between the GW parameters that describe these binaries and whether some of these parameters are also available from EM observations. We used verification binaries, which are known as the guaranteed sources for eLISA and will test the functioning of the instrument. We find that of the seven parameters that characterise such a binary, only a few are correlated. The most useful result is the strong correlation between amplitude and inclination, which can be used to constrain the parameter uncertainty in amplitude by making use of the constraint of inclination from EM measurements. The improvement can be up to a factor of 6:5, but depends on the signal-to-noise ratio of the source data. Moreover, we find that this strong correlation depends on the inclination. For mildly face-on binaries ( . 45 ), EM data on inclination can improve the estimate of the GW amplitude by a significant factor. However, for edge-on binaries ( 90 ), the inclination can be determined accurately from GW data alone, thus GW data can be used to select systems that will likely be eclipsing binaries for EM follow-up.