arXiv subjects

B. Willems

Publications and source records attributed to B. Willems.

At least 37 records · Page 2Linked to original sources

arXiv2005

Angular momentum losses and the orbital period distribution of cataclysmic variables below the period gap: effects of circumbinary disks

The population synthesis of cataclysmic variables below the period is investigated. A grid of detailed binary evolutionary sequences has been calculated and included in the simulations to take account of additional angular momentum losses beyond that associated with gravitational radiation and mass loss, due to nova outbursts, from the system. As a specific example, we consider the effect of a circumbinary disk to gain insight into the ingredients necessary to reproduce the observed orbital period distribution. The resulting distributions show that the period minimum lies at about 80 minutes with the number of systems monotonically increasing with increasing orbital period to a maximum near 90 minutes. There is no evidence for an accumulation of systems at the period minimum which is a common feature of simulations in which only gravitational radiation losses are considered. The period distribution is found to be fairly flat for orbital periods ranging from about 85 to 120 minutes. The steepness of the lower edge of the period gap can be reproduced, for example, by an input of systems at periods near 2.25 hrs due to a flow of cataclysmic variable binary systems from orbital periods longer than 2.75 hrs. The good agreement with the cumulated distribution function of observed systems within the framework of our model indicates that the angular momentum loss by a circumbinary disk or a mechanism which mimics its features coupled with a weighting factor to account for selection effects in the discovery of such systems and a flow of systems from above the period gap to below the period gap are important ingredients for understanding the overall period distribution of cataclysmic variable binary systems.

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arXiv2005

Mass transfer and the period decrease in RXJ0806.3+1527

We examine the nature of RXJ0806.3+1527 and show that it is possible to reconcile the observed period decrease and X-ray luminosity with the transfer of mass between two white dwarfs provided that: either the system is (i) still in the early and short-lived (less than ~100yr) stages of mass transfer due to atmospheric Roche-lobe overflow, or (ii) in a standard, long-term, quasi-stationary mass-transfer phase that is significantly (~90%) non-conservative and the conversion of accretion energy to X-rays is quite inefficient. In either of the two cases and for a wide range of physical parameters, we find that orbital angular momentum is lost from the system at a rate that is a factor of a few (less than ~4) higher than the rate associated with the emission of gravitational waves. Although the physical origin of this extra angular momentum loss is not clear at present, it should be taken into account in the consideration of RXJ0806.3+1527 as a verification Galactic source for LISA.

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arXiv2005

Understanding Compact Object Formation and Natal Kicks I. Calculation Methods and the case of GRO J1655-40

In recent years proper motion measurements have been added to the set of observational constraints on the current properties of Galactic X-ray binaries. We develop an analysis that allows us to consider all this available information and reconstruct the full evolutionary history of X-ray binaries back to the time of compact object formation. This analysis accounts for mass transfer through the ongoing X-ray phase, tidal circularization before the onset of Roche-lobe overflow, motion through the Galactic potential after the formation of the compact object, and binary orbital dynamics and hydrodynamic modeling of the core collapse. We apply the analysis to the soft X-ray transient GRO J1655-40 and, for the first time, use its full 3D peculiar velocity constraints right after core collapse instead of lower limits on the current space velocity given by the present-day radial velocity. We find that the system has remained within 200pc from the Galactic plane throughout its entire life time and that the mass loss and a kick possibly associated with the black hole (BH) formation imparted a kick velocity of 45-115 km/s to the binary's center of mass. Right after BH formation, the system consists of a 3.5-6.3 Msun BH and a 2.3-4 Msun main-sequence star. At the onset of the X-ray phase the donor is still on the main sequence. We find that a symmetric BH formation event cannot be formally excluded, but that the associated system parameters are only marginally consistent with the currently observed binary properties. BH formation mechanisms involving an asymmetric supernova explosion with associated BH kick velocities of a few tens of km/s, on the other hand, satisfy the constraints much more comfortably. We also derive an upper limit on the BH kick magnitude of 210 km/s. (abridged)

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arXiv2004

Pulsar Kicks and Spin Tilts in the Close Double Neutron Stars PSR J0737-3039, PSR B1534+12 and PSR B1913+16

In view of the recent measurement of the scintillation velocity for PSR J0737-3039, we examine the complete set of constraints imposed on the pulsar B natal kicks (magnitude and orientation) and predict the most favorable pulsar kick velocity and spin tilt for both isotropic and polar kicks. Our analysis takes into account both currently unknown parameters: the orientation of the orbital plane on the sky (Omega) and the radial component of the systemic velocity (V_r). Assuming that the system's peculiar velocity is entirely due to the second supernova explosion, we find that the system may have crossed the Galactic plane multiple times since the birth of the second neutron star and that the post-supernova peculiar velocity could have been as high as 1200km/s. We also confirm the absolute lower and upper limits on the physical parameters derived in our earlier study. For specific combinations of the two unknown parameters Omega and V_r, however, we find much tighter constraints on the pre-supernova binary configuration and natal kicks imparted to pulsar B, as well as on the age of system. Once Omega is measured in the coming year, it will be straightforward to use the results presented here to further constrain the natal kicks and the spin-tilt predictions. We complete our comprehensive study and derive similar constraints and spin-tilt predictions for PSR B1534+12, where the only free parameter is V_r. Lastly, for PSR B1913+16, we update the progenitor and kick constraints using the measured pulsar spin tilt and allowing for Roche-lobe overflow from the progenitor of the pulsar companion.

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arXiv2004

Dynamic Tides and the Evolution of Stars in Close Binaries

In this talk, we review some recent advances in the theory of dynamic tides in close binaries. We particularly focus on the effects of resonances of dynamic tides with free oscillation modes and on the role of dynamic tides in the comparison of theoretically predicted and observationally inferred apsidal-motion rates.

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arXiv2004

The Formation of the Most Relativistic Pulsar PSR J0737-3039

We present an updated set of constraints for the progenitor of PSR J0737-3039 and for the natal kicks imparted to pulsar B taking into account both the evolutionary and kinematic history of the double neutron star. For this purpose, we use recently reported scintillation velocity measurements to trace the motion of the system in the Galaxy backwards in time as a function of the unknown orientation Omega of the systemic velocity projected on plane of the sky as well as the unknown radial velocity Vr. The absolute limits on the orbital separation and the mass of pulsar B's helium star progenitor just before its supernova explosion are 1.2Rsun < A0 < 1.7Rsun and 2.1Msun < M0 < 4.7Msun. The kick velocity is constrained to be between 60km/s and 1660km/s and to be misaligned from the pre-SN orbital angular axis (which could be associated with pulsar B's spin axis) by least 25 degrees. We also derive probability distribution functions for the kick velocity imparted to pulsar B and for the misalignment angle between pulsar A's spin and the post-supernova orbital angular momentum for both isotropic and polar kicks. The most probable values of both quantities depend sensitively on the unknown radial velocity. In particular, tilt angles lower than 30-50 degrees tend to be favored for current radial velocities of less than 500km/s in absolute value, while tilt angles higher than 120 degrees tend to be associated with radial velocities in excess of 1000km/s in absolute value.

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arXiv2004

The orbital period distribution of wide binary millisecond pulsars

We present results of a binary population synthesis study on the orbital period distribution of wide binary millisecond pulsars forming through four evolutionary channels. In three of the channels, the progenitor of the millisecond pulsar undergoes a common envelope phase prior to the supernova explosion which gives birth to the neutron star. In the fourth channel, the primary avoids the common-envelope phase and forms a neutron star when it ascends the asymptotic giant branch. The four formation channels yield an orbital period distribution which typically shows a short-period peak below 10 days, a long-period peak around 100 days, and a cut-off near 200 days. The agreement with the orbital period distribution of observed binary millisecond pulsars in the Galactic disk is best when the common-envelope ejection is efficient, the mass-transfer phase responsible for spinning up the pulsar is highly non-conservative, and no or moderate supernova kicks are imparted to neutron stars at birth.

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arXiv2004

Detached white dwarf main-sequence star binaries

We considered the formation of detached white dwarf main-sequence star (WDMS) binaries through seven evolutionary channels subdivided 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). We examine the sensitivity of the population to changes in the amount of mass lost from the system during 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 stems from wide systems in which both components 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. 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.

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arXiv2004

Constraints on the Formation of PSR J0737-3039: the most probable isotropic kick magnitude

A strongly relativistic binary pulsar has been recently discovered with the 64m Parkes telescope (Burgay et al. 2003). Here we use the measured properties of this binary (masses and orbital characteristics as well as age estimates), and we derive the complete set of constraints imposed on the physical properties of the binary pulsar progenitor right before the second supernova explosion. We find that: (i) according to our current understanding of neutron-star formation, the helium-rich progenitor of the second neutron star is most likely overflowing its Roche lobe; (ii) the neutron-star kick magnitude is constrained in the range 60-1560 km/s, with the most probable value being equal to 150 km/s. While the first conclusion is in agreement with Dewi & van den Heuvel (2003), our upper limit on the kick magnitude is significantly larger than that derived by these authors. We find that the difference arises because Dewi & van den Heuvel (2003) inadvertently neglected to consider kicks directed out of the pre-supernova orbital plane.

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arXiv2003

Excitation of oscillation modes by tides in close binaries: constraints on stellar and orbital parameters

The parameter space favourable for the resonant excitation of free oscillation modes by dynamic tides in close binary components is explored using qualitative considerations to estimate the order of magnitude of the tidal force and the frequency range covered by the tidally induced oscillations. The investigation is valid for slowly rotating stars with masses in the interval between 2 and 20 solar masses, and an evolutionary stage ranging from the beginning to the end of the main sequence. Oscillation modes with eigenfrequencies of the order of five times the inverse of the star's dynamical time scale - i.e. the lowest-order p-modes, the f-mode, and the lowest-order g-modes - are found to be outside the favourable parameter space since their resonant excitation requires orbital eccentricities that are too high for the binary to stay detached when the components pass through the periastron of their relative orbit. Resonances between dynamic tides and g-modes with frequencies of the order of half of the inverse of the star's dynamical time scale on the other hand are found to be favourable for orbital periods up to 200 times the star's dynamical time scale, provided that the binary mass ratio q is larger than 1/3 and the orbital eccentricity e is larger than approximately 0.25. This favourable range comes down to orbital periods of up to 5-12 days in the case of 2-20 solar mass zero-age main-sequence binary components, and orbital periods of up to 21-70 days in the case of terminal main-sequence binary components.

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arXiv2003

On the detection of pre-low-mass X-ray binaries

We explore the population of candidate pre-low-mass X-ray binaries in which a neutron star accretes mass from the wind of a low-mass companion (mass < 2Msun) in the framework of a binary population synthesis study. The simulated accretion-luminosity distribution shows a primary peak close to 1e31 erg/s and a secondary peak near 1e28 erg/s. The relative contribution of the two peaks depends primarily on the magnitude of the kick velocity imparted to the neutron star at birth. The secondary peak is negligible for average kick velocities larger than 200 km/s, but becomes dominant for average kick velocities smaller than 50 km/s. Regardless of the relative contributions of the two peaks, our calculations suggest that pre-low-mass X-ray binaries may provide a non-negligible contribution to the population of discrete low-luminosity X-ray sources in the Galaxy.

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arXiv2003

The mass of the neutron star in Vela X-1 and tidally induced non-radial oscillations in GP Vel

We report new radial velocity observations of GP Vel/HD77581, the optical companion to the eclipsing X-ray pulsar Vela X-1. Using data spanning more than two complete orbits of the system, we detect evidence for tidally induced non-radial oscillations on the surface of GP Vel, apparent as peaks in the power spectrum of the residuals to the radial velocity curve fit. By removing the effect of these oscillations (to first order) and binning the radial velocities, we have determined the semi-amplitude of the radial velocity curve of GP Vel to be K_o=22.6+/-1.5 km/s. Given the accurately measured semi-amplitude of the pulsar's orbit, the mass ratio of the system is 0.081+/-0.005. We are able to set upper and lower limits on the masses of the component stars as follows. Assuming GP Vel fills its Roche lobe then the inclination angle of the system, i=70.1+/-2.6 deg. In this case we obtain the masses of the two stars as M_x=2.27 +/-0.17 M_sun for the neutron star and M_o=27.9+/-1.3 M_sun for GP Vel. Conversely, assuming the inclination angle is i=90 deg, the ratio of the radius of GP Vel to the radius of its Roche lobe is beta=0.89+/-0.03 and the masses of the two stars are M_x=1.88+/-0.13 M_sun and M_o=23.1+/-0.2 M_sun. A range of solutions between these two sets of limits is also possible, corresponding to other combinations of i and beta. In addition, we note that if the zero phase of the radial velocity curve is allowed as a free parameter, rather than constrained by the X-ray ephemeris, a significantly improved fit is obtained with an amplitude of 21.2+/-0.7 km/s and a phase shift of 0.033+/-0.007 in true anomaly. The apparent shift in the zero phase of the radial velocity curve may indicate the presence of an additional radial velocity component at the orbital period.

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arXiv2002

The effect of stellar compressibility and non-resonant dynamic tides on the apsidal-motion rate in close binaries

The paper is devoted to the analysis of the systematic deviations between the classical formula for the rate of secular apsidal motion in close binaries and the formula established within the framework of the theory of dynamic tides. The basic behaviour is found to be governed by the dependency of the forcing frequencies on the orbital period and the rotational angular velocity, irrespective of the stellar model. For binaries with short orbital periods a good agreement between the classical apsidal-motion formula and the formula established within the framework of the theory of dynamic tides is only possible for certain values of the rotational angular velocity.

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arXiv2002

Nonadiabatic resonant dynamic tides and orbital evolution in close binaries

This investigation is devoted to the effects of nonadiabatic resonant dynamic tides generated in a uniformly rotating stellar component of a close binary. The companion is considered to move in a fixed Keplerian orbit, and the effects of the centrifugal force and the Coriolis force are neglected. Semi-analytical solutions for the linear, nonadiabatic resonant dynamic tides are derived by means of a two-time variable expansion procedure. The solution at the lowest order of approximation consists of the resonantly excited oscillation mode and displays a phase shift with respect to the tide-generating potential. Expressions are established for the secular variations of the semi-major axis, the orbital eccentricity, and the star's angular velocity of rotation caused by the phase shift. The orders of magnitude of these secular variations are considerably larger than those derived earlier by Zahn (1977) for the limiting case of dynamic tides with small frequencies. For a 5 solar mass ZAMS star, an orbital eccentricity e = 0.5, and orbital periods in the range from 2 to 5 days, numerous resonances of dynamic tides with second-degree lower-order gravity modes are seen to induce secular variations of the semi-major axis, the orbital eccentricity, and the star's angular velocity of rotation with time scales shorter than the star's nuclear life time.

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arXiv2002

Population synthesis of wide binary millisecond pulsars

Four evolutionary channels leading to the formation of wide binary millisecond pulsars are investigated. The majority of binary millisecond pulsars are found to descend from systems in which the most massive component undergoes a common-envelope phase prior to the supernova explosion leading to the birth of the neutron star. The orbital period distribution of simulated samples of wide binary millisecond pulsars is compared with the observed distribution of Galactic binary millisecond pulsars for a variety of parameters describing the formation and evolution of binaries. The distribution functions typically show a short-period peak below 10 days and a long-period peak around 100 days. The observed distribution is best reproduced by models with highly non-conservative mass transfer, common-envelope efficiencies equal to or larger than unity, a critical mass ratio for the delayed dynamical instability larger than 3, and no or moderate supernova kicks at the birth of the neutron star. Few systems are found with orbital periods longer than 200 days, irrespective of the accretion efficiency of neutron stars. This occurs as a result of the upper limit on the initial orbital periods beyond which the binary avoids the common-envelope phase prior to the supernova explosion of the primary.

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arXiv2002

Discovery and analysis of p-mode and g-mode oscillations in the A-type primary of the eccentric binary HD 209295

We have discovered both intermediate-order gravity mode and low-order pressure mode pulsation in the same star, HD 209295. It is therefore both a Gamma Doradus and a Delta Scuti star, which makes it the first pulsating star to be a member of two classes. The star is a single-lined spectroscopic binary with an orbital period of 3.10575 d and an eccentricity of 0.352. Weak pulsational signals are found in both the radial velocity and line-profile variations, allowing us to show that the two highest-amplitude Gamma Doradus pulsation modes are consistent with l=1 and |m|=1. In our 280 h of BVI multi-site photometry we detected ten frequencies in the light variations, one in the Delta Scuti regime and nine in the Gamma Doradus domain. Five of the Gamma Doradus frequencies are exact integer multiples of the orbital frequency. This observation leads us to suspect they are tidally excited. Results of theoretical modeling (stability analysis, tidal excitation) were consistent with the observations. We could not detect the secondary component of the system in infrared photometry, suggesting that it may not be a main-sequence star. Archival data of HD 209295 show a strong ultraviolet excess, the origin of which is not known. The orbit of the primary is consistent with a secondary mass of M > 1.04 Msun indicative of a neutron star or a white dwarf companion.

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arXiv2002

Tidally induced radial-velocity variations in close binaries

A theoretical framework for the determination of tidally induced radial-velocity variations in a component of a close binary is presented. Both the free and the forced oscillations of the component are treated as linear, isentropic perturbations of a spherically symmetric star. Resonances between dynamic tides and free oscillation modes are taken into account by means of the formalism developed by Smeyers et al. (1998). The amplitude of the tidally induced radial-velocity variations seen by the observer depends on the orbital eccentricity and on the orbital inclination. The amplitude increases with increasing orbital eccentricity and is most sensitive to the value of the orbital inclination when 20 < i < 70. In the case of a 5 solar mass ZAMS star with a 1.4 solar mass compact companion, it is shown that resonant dynamic tides can lead to radial-velocity variations with amplitudes large enough to be detected in observations. The shape of the tidally induced radial-velocity curves varies from very irregular for orbital periods away from any resonances with free oscillation modes to sinusoidal for orbital periods close to a resonance with a free oscillation mode. Our investigation is concluded with an application to the slowly pulsating B star HD 177863 showing the possibility of resonant excitation of a high-order second-degree g-mode in this star.

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arXiv2001

The influence of dynamic tides on the apsidal-motion rate in close binaries with an evolved main-sequence star

The validity of the classical formula for the rate of secular apsidal motion in close binaries is investigated for a sequence of models of a 5 solar mass star ranging from the last stages of the C12 -> N14 reaction to the phase where hydrogen is exhausted in the core. For binaries with short orbital periods, the apsidal-motion rates predicted by the classical formula deviate from the rates determined within the framework of the theory of dynamic tides due to the effects of the compressibility of the stellar fluid and due to resonances of dynamic tides with free oscillation modes of the component stars (Smeyers & Willems 2001). As the star evolves on the main sequence, the deviations caused by the compressibility of the stellar fluid increase with increasing radius of the star. The additional deviations caused by the resonances are largest near the end of the core-hydrogen burning phase. Both of these deviations increase with increasing values of the orbital eccentricity.

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