Initial Secondary Mass Research Articles

On the origin of W UMa type contact binaries – a new method for computation of initial masses

W UMa type binaries have two defining characteristics. These are (i) the effective temperatures of both components are very similar, and (ii) the secondary (currently less massive) component is overluminous for its current mass. We consider the latter to be an indication of its mass before the mass-transfer event. For these stars we define a mass difference ($\delta M$) between the mass determined from its luminosity and the present mass determined from fitting the binary orbit. We compare the observed values of the mass difference to stellar models with mass loss. The range of initial secondary masses that we find for observed W UMa type binaries is 1.3-2.6 M$_{\odot}$. We discover that the A- and the W-subtype contact binaries have different ranges of initial secondary masses. Binary systems with an initial mass higher than $1.8 \pm 0.1$ M$_{\odot}$ become A-subtype while systems with initial masses lower than this become W-subtype. Only 6 per cent of systems violate this behavior. We also obtain the initial masses of the primaries using the following constraint for the reciprocal of initial mass ratio: $0 < 1/q_i < 1$. The range of initial masses we find for the primaries is 0.2-1.5 M$_{\odot}$, except for two systems. Finally in comparing our models to observed systems we find evidence that the mass transfer process is not conservative. We find that only 34 per cent of the mass from the secondary is transferred to the primary. The remainder is lost from the system.

Eclipsing binaries in extrasolar planet transit surveys: the case of SuperWASP

Extrasolar planet transit surveys will also detect eclipsing binaries. Using a comprehensive binary population synthesis scheme, we investigate the statistical properties of a sample of eclipsing binaries that is detectable by an idealized extrasolar planet transit survey with specifications broadly similar to those of the Wide Angle Search for Planets (SuperWASP) project. In this idealized survey, the total number of detectable single stars in the Galactic disc is of the order of 106-107, while, for a flat initial mass ratio distribution, the total number of detectable eclipsing binaries is of the order of 104-105. The majority of the population of detectable single stars is made up of main-sequence stars (~60 per cent), horizontal-branch stars (~20 per cent), and giant-branch stars (~10 per cent). The largest contributions to the population of detectable eclipsing binaries stem from detached double main-sequence star binaries (~60 per cent), detached giant-branch main-sequence star binaries (~20 per cent), and detached horizontal-branch main-sequence star binaries (~10 per cent). White dwarf main-sequence star binaries make up approximately 0.3 per cent of the sample. The ratio of the number of eclipsing binaries to the number of single stars detectable by the idealized SuperWASP survey varies by less than a factor of 2.5 across the sky, and decreases with increasing Galactic latitude. It is found to be largest in the direction of the Galactic longitude l=-7 b 5 and the Galactic latitude b=-22 b 5. We also show that the fractions of systems in different subgroups of eclipsing binaries are sensitive to the adopted initial mass ratio or initial secondary mass distribution, which is one of the poorest constrained input parameters in present-day binary population synthesis calculations. This suggests that once statistically meaningful results from transit surveys are available, they will be able to significantly improve the predictive power of population synthesis studies of interacting binaries and related objects.

Detached white dwarf main-sequence star binaries

We initiated a comprehensive state of the art binary population synthesis study of white dwarf main-sequence star (WDMS) binaries to serve as a foundation for subsequent studies on pre-cataclysmic variables, double white dwarfs, and white dwarf + B-star binaries. We considered seven distinct formation channels subdivided into three main groups according to the evolutionary process that gives rise to the formation of the white dwarf or its helium-star progenitor: dynamically stable Roche- lobe overflow (Algol-type evolution), dynamically unstable Roche-lobe overflow (common-envelope evolution), or stellar winds (single star evolution). For each formation channel, we examine the sensitivity of the population to changes in the amount of mass lost from the system during dynamically stable Roche-lobe overflow, the common-envelope ejection efficiency, and the initial mass ratio or initial secondary mass distribution. In the case of a flat initial mass ratio distribution, the local space density of WDMS binaries is of the order of ∼10 −3 pc −3 . This number decreases to ∼10 −4 pc −3 when the initial mass ratio distribution is approximately proportional to the inverse of the initial mass ratio. More than 75% of the WDMS binary population originates from wide systems in which both components essentially evolve as if they were single stars. The remaining part of the population is dominated by systems in which the white dwarf is formed in a common-envelope phase when the primary ascends the first giant branch or the asymptotic giant branch. When dynamically stable mass transfer proceeds highly conservative and the common-envelope ejection process is very efficient, the birthrate of WDMS binaries forming through a common-envelope phase is about 10 times larger than the birthrate of WDMS binaries forming through a stable Roche-lobe overflow phase. The ratio of the number of helium white dwarf systems to the number of carbon/oxygen or oxygen/neon/magnesium white dwarf systems derived from large samples of observed WDMS binaries by, e.g., future planet-search missions such as SuperWASP, COROT, and Kepler may furthermore constrain the common-envelope ejection efficiency.

Formation of contact in massive close binaries

We present evolutionary calculations for 74 close binaries systems with initial primary masses in the range 12...25 M_sun, and initial secondary masses between 6 and 24 M_sun. The initial periods were chosen such that mass overflow starts during the core hydrogen burning phase of the primary (Case A), or shortly thereafter (Case B). We assume conservative evolution for contact-free systems, i.e., no mass or angular momentum loss from those system except due to stellar winds. We investigate the borderline between contact-free evolution and contact, as a function of the initial system parameters. We also investigate the effect of the treatment of convection, and found it relevant for contact and supernova order in Case A systems, particularly for the highest considered masses. For Case B systems we find contact for initial periods above approximate 10 days and below. However, in that case (and for not too large periods) contact occurs only after the mass ratio has been reversed, due to the increased fraction of the donor's convective envelope. As most In all Cases we find contact for mass ratios below approximate 0.65. We derive the observable properties of our systems after the major mass transfer event, where the mass gainer is a main sequence or supergiant O or early B type star, and the mass loser is a helium star. We point out that the assumption of conservative evolution for contact-free systems could be tested by finding helium star companions to O stars.

Cooling curves and initial models for low-mass white dwarfs (&lt;0.25 M ) with helium cores

We present a detailed calculation of the evolution of low-mass (<0.25 M⊙) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 and 0.06 M⊙. As a result of mixing between the core and outer envelope, the surface hydrogen content (Xsurf) is 0.5–0.35, depending on the initial value of the heavy element Z and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.213 M⊙ (Z=0.003), 0.198 M⊙ (Z=0.01), 0.192 M⊙ (Z=0.02) or 0.183 M⊙ (Z=0.03). The duration of the flashes (independent of chemical composition) is between a few ×106 and a few ×107 yr. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporarily stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche lobe overflow) occurs between 0.183 and 0.213 M⊙ (depending on the heavy element value).

The Formation of Low‐Mass Transient X‐Ray Binaries

We consider constraints on the formation of low-mass X-ray binaries containing neutron stars (NLMXBs) arising from the presence of soft X-ray transients among these systems. For a neutron star of mass M1 1.4 M☉ at formation, we show that in short-period (1-2 day) systems driven by angular momentum loss these constraints require the secondary at the beginning of mass transfer to have a mass of 1.3 M☉ M2 1.5 M☉ and to be significantly nuclear evolved, provided that supernova (SN) kick velocities are generally small compared with the pre-SN orbital velocity. As a consequence, a comparatively large fraction of such systems appear as soft X-ray transients even at short periods, as observed. Moreover, the large initial secondary masses account for the rarity of NLMXBs at periods P 3 hr. In contrast, NLMXB populations forming with large kick velocities would not have these properties, suggesting that the kick velocity is generally small compared with the pre-SN orbital velocity in a large fraction of systems, consistent with a recent reevaluation of pulsar proper motions. The results also place tight constraints on the strength of magnetic braking: if magnetic braking is significantly stronger than the standard form, too many unevolved NLMXBs would form; if it is slower by only a factor of 4, no short-period NLMXBs would form at all in the absence of a kick velocity. The narrow range for M2 found for negligible kick velocity implies restricted ranges near 4 M☉ for the helium star antecedent of the neutron star and near 18 M☉ for the original main-sequence progenitor. The pre-common-envelope period must lie near 4 yr, and we estimate the short-period NLMXB formation rate in the disk of the Galaxy as ~2 × 10-7 yr-1. Our results show that the neutron star mass at short-period NLMXB formation cannot be significantly larger than 1.4 M☉. Systems with formation masses of M1 1.2 M☉ would have disrupted, so observations implying M1 ~ 1.4 M☉ in some NLMXBs suggest that much of the transferred mass is lost from these systems.