J. Southworth

Eclipsing binaries in open clusters. II. V453 Cyg in NGC 6871

We derive absolute dimensions of the early B-type detached eclipsing binary V453Cygni (B0.4IV + B0.7IV, P = 3.89d), a member of the open cluster NGC6871. From the analysis of new, high-resolution, spectroscopy and the UBV light curves of Cohen (1974) we find the masses to be 14.36 ± 0.20Mfl and 11.11 ± 0.13Mfl, the radii to be 8.55 ± 0.06Rfl and 5.49 ± 0.06Rfl, and the eective temperatures to be

Homogeneous studies of transiting extrasolar planets – I. Light-curve analyses

I present a homogeneous analysis of the transit light curves of 14 well-observed transiting extrasolar planets. The light curves are modelled using jktebop, random errors are measured using Monte Carlo simulations and the effects of correlated noise are included using a residual-permutation algorithm. The importance of stellar limb darkening on the light-curve solutions and parameter uncertainties is investigated using five different limb darkening laws and including different numbers of coefficients as fitted parameters. The linear limb darkening law cannot adequately fit the Hubble Space Telescope (HST) photometry of HD 209458, but the other four laws give very similar results to each other for all transit light curves. In most cases fixing the limb darkening coefficients at theoretically predicted values does not bias the results, but does cause the error estimates to be too small. The available theoretical limb darkening coefficients clearly disagree with empirical values measured from the HST light curves of HD 209458; limb darkening must be included as fitted parameters when analysing high-quality light curves. In most cases the results of my analysis agree with the values found by other authors, but the uncertainties I find can be significantly larger (by factors of up to 3). Despite these greater uncertainty estimates, the analyses of sets of independent light curves for both HD 189733 and HD 209458 lead to results which do not agree with each other. This discrepancy is worst for the ratio of the radii (6.7 sigma for HD 189733 and 3.7 sigma for HD 209458), which depends primarily on the depth of the transit. It is therefore not due to the analysis method but is present in the light curves. These underlying systematic errors cannot be detected from the reduced data alone unless at least three independent light curves are available for an individual planetary system. The surface gravities of transiting extrasolar planets are known to be correlated with their orbital periods. New surface gravity values, calculated from the light-curve results and the stellar spectroscopic orbits, show that this correlation is still present. New high-precision light curves are needed for HD 149026, OGLE-TR-10, OGLE-TR-56, OGLE-TR-132 and GJ 436, and new radial velocity curves for the XO-1, WASP-1, WASP-2 and the OGLE (Optical Gravitational Lensing Experiment) planetary systems.

A Gaseous Metal Disk Around a White Dwarf

The destiny of planetary systems through the late evolution of their host stars is very uncertain. We report a metal-rich gas disk around a moderately hot and young white dwarf. A dynamical model of the double-peaked emission lines constrains the outer disk radius to just 1.2 solar radii. The likely origin of the disk is a tidally disrupted asteroid, which has been destabilized from its initial orbit at a distance of more than 1000 solar radii by the interaction with a relatively massive planetesimal object or a planet. The white dwarf mass of 0.77 solar mass implies that planetary systems may form around high-mass stars.

Homogeneous studies of transiting extrasolar planets. IV. Thirty systems with space-based light curves

I calculate the physical properties of 32 transiting extrasolar planet and brown-dwarf systems from existing photometric observations and measured spectroscopic parameters. The systems studied include fifteen observed by the CoRoT satellite, ten by Kepler and five by the Deep Impact spacecraft. Inclusion of the objects studied in previous papers leads to a sample of 58 transiting systems with homogeneously measured properties. The Kepler data include observations from Quarter 2, and my analyses of several of the systems are the first to be based on short-cadence data from this satellite. The light curves are modelled using the JKTEBOP code, with attention paid to the treatment of limb darkening, contaminating light, orbital eccentricity, correlated noise, and numerical integration over long exposure times. The physical properties are derived from the light curve parameters, spectroscopic characteristics of the host star, and constraints from five sets of theoretical stellar model predictions. An alternative approach using a calibration from eclipsing binary star systems is explored and found to give comparable results whilst imposing a much smaller computational burden. My results are in good agreement with published properties for most of the transiting systems, but discrepancies are identified for CoRoT-5, CoRoT-8, CoRoT-13, Kepler-5 and Kepler-7. Many of the errorbars quoted in the literature are underestimated. Refined orbital ephemerides are given for CoRoT-8 and for the Kepler planets. Asteroseismic constraints on the density of the host stars are in good agreement with the photometric equivalents for HD17156 and TrES-2, but not for HAT-P-7 and HAT-P-11. Complete error budgets are generated for each transiting system, allowing identification of the observations best-suited to improve measurements of their physical properties. Whilst most systems would benefit from further photometry and spectroscopy, HD17156, HD80606, HAT-P-7 and TrES-2 are now extremely well characterised. HAT-P-11 is an exceptional candidate for studying starspots. The orbital ephemerides of some transiting systems are becoming uncertain and they should be re-observed in the near future. The primary results from the current work and from previous papers in the series have been placed in an online catalogue, from where they can be obtained in a range of formats for reference and further study. TEPCat is available at http://www.astro.keele.ac.uk/ jkt/tepcat/

Homogeneous studies of transiting extrasolar planets – III. Additional planets and stellar models

I derive the physical properties of 30 transiting extrasolar planetary systems using a homogeneous analysis of published data. The light curves are modelled with the jktebop code, with special attention paid to the treatment of limb darkening, orbital eccentricity and error analysis. The light from some systems is contaminated by faint nearby stars, which if ignored will systematically bias the results. I show that it is not realistically possible to account for this using only transit light curves: light-curve solutions must be constrained by measurements of the amount of contaminating light. A contamination of 5 per cent is enough to make the measurement of a planetary radius 2 per cent too low. The physical properties of the 30 transiting systems are obtained by interpolating in tabulated predictions from theoretical stellar models to find the best match to the light-curve parameters and the measured stellar velocity amplitude, temperature and metal abundance. Statistical errors are propagated by a perturbation analysis which constructs complete error budgets for each output parameter. These error budgets are used to compile a list of systems which would benefit from additional photometric or spectroscopic measurements. The systematic errors arising from the inclusion of stellar models are assessed by using five independent sets of theoretical predictions for low-mass stars. This model dependence sets a lower limit on the accuracy of measurements of the physical properties of the systems, ranging from 1 per cent for the stellar mass to 0.6 per cent for the mass of the planet and 0.3 per cent for other quantities. The stellar density and the planetary surface gravity and equilibrium temperature are not affected by this model dependence. An external test on these systematic errors is performed by comparing the two discovery papers of the WASP-11/HAT-P-10 system: these two studies differ in their assessment of the ratio of the radii of the components and the effective temperature of the star. I find that the correlations of planetary surface gravity and mass with orbital period have significance levels of only 3.1σ and 2.3σ, respectively. The significance of the latter has not increased with the addition of new data since Paper II. The division of planets into two classes based on Safronov number is increasingly blurred. Most of the objects studied here would benefit from improved photometric and spectroscopic observations, as well as improvements in our understanding of low-mass stars and their effective temperature scale.

Eclipsing binaries in open clusters – III. V621 Per in χ Persei

V621Persei is a detached eclipsing binary in the open clusterPersei which is com- posed of an early B-type giant star and a main sequence secondary component. From high-resolution spectroscopic observations and radial velocities from the literature, we determine the orbital period to be 25.5 days and the primary velocity semiamplitude to be K = 64.5 ± 0.4kms i1 . No trace of the secondary star has been found in the spectrum. We solve the discovery light curves of this totally-eclipsing binary and find that the surface gravity of the secondary star is loggB = 4.244 ± 0.054. We compare the absolute masses and radii of the two stars in the mass-radius diagram, for dier- ent possible values of the primary surface gravity, to the predictions of stellar models. We find that loggA … 3.55, in agreement with values found from fitting Balmer lines with synthetic profiles. The expected masses of the two stars are 12Mfl and 6Mfl and the expected radii are 10Rfl and 3Rfl. The primary component is near the blue loop stage in its evolution.

Homogeneous studies of transiting extrasolar planets – II. Physical properties

I present an homogeneous determination of the physical properties of 14 transiting extrasolar planetary systems for which good photometric and spectroscopic data are available. The input quantities for each system are the results of the light-curve analyses presented in Paper I, and published measurements of the stellar velocity amplitude, effective temperature and metal abundance. The physical properties are determined by interpolating within tabulated predictions from stellar theory to find the optimal match to these input data. Statistical uncertainties are found using a perturbation algorithm, which gives a detailed error budget for every output quantity. Systematic uncertainties are assessed for each quantity by comparing the values found using several independent sets of stellar models. As a theory-free alternative, physical properties are also calculated using an empirical mass–radius relation constructed from high-precision studies of low-mass eclipsing binary stars. I find that the properties of the planets depend mostly on parameters measured from the light and radial velocity curves, and have a relatively minor sensitivity to theoretical predictions. In contrast, the orbital semimajor axes and stellar masses have a strong dependence on theoretical predictions, and their systematic uncertainties can be substantially larger than the statistical ones. Using the empirical mass–radius relation instead, the semimajor axes and stellar masses are smaller by up to 15 per cent. Thus, our understanding of extrasolar planets is currently limited by our lack of understanding of low-mass stars. Using the properties of all known transiting extrasolar planets, I find that correlations between their orbital periods, masses and surface gravities are significant at the 2σ –3σ level. However, the separation of the known planets into two classes according to their Safronov number is weaker than previously found, and may not be statistically significant. Three systems, HAT-P-2, WASP-14 and XO-3, form their own little group of outliers, with eccentric orbits, massive planets and stars with masses ∼1.3 M� . The detailed error budgets calculated for each system show where further observations are needed. XO-1 and WASP-1 could do with new transit light curves. TrES-2 and WASP-2 would benefit from more precise stellar temperature and abundance measurements. Velocity measurements of the parent stars are vital for determining the planetary masses: TrES-1, XO-1, WASP-1, WASP-2 and the OGLEs need additional data. The homogeneous analysis presented here is a step towards large-scale statistical studies of transiting extrasolar planetary systems, in preparation for the expected deluge of new detections from CoRoT and Kepler.

Absolute dimensions of detached eclipsing binaries ¿ I. The metallic-lined system WW Aurigae

WWAurigae is a detached eclipsing binary composed of two metallic-lined A-type stars orbiting each other every 2.5 days. We have determined the masses and radii of both components to accuracies of 0.4% and 0.6%, respectively. From a cross-correlation analysis of high-resolution spectra we find masses of 1.964±0.007M⊙ for the primary star and 1.814±0.007M⊙ for the secondary star. From an analysis of photoelectric uvby and UBV light curves we find the radii of the stars to be 1.927±0.011R⊙ and 1.841± 0.011R⊙, where the uncertainties have been calculated using a Monte Carlo algorithm. Fundamental effective temperatures of the two stars have been derived, using the Hipparcos parallax of WWAur and published ultraviolet, optical and infrared fluxes, and are 7960±420 and 7670±410K. The masses, radii and effective temperatures of WWAur are only matched by theoretical evolutionary models for a fractional initial metal abundance, Z, of approximately 0.06 and an age of roughly 90Myr. This seems to be the highest metal abundance inferred for a well-studied detached eclipsing binary, but we find no evidence that it is related to the metallic-lined nature of the stars. The circular orbit of WWAur is in conflict with the circularization timescales of both the Tassoul and the Zahn tidal theories and we suggest that this is due to pre-mainsequence evolution or the presence of a circular orbit when the stars were formed.

On the evolutionary status of short-period cataclysmic variables

We present high-speed, three-colour photometry of seven short-period (P-orb <= 95 min) eclipsing cataclysmic variables (CVs) from the Sloan Digital Sky Survey. We determine the system parameters via a parametrized model of the eclipse fitted to the observed light curve by chi(2) minimization. Three out of seven of the systems possess brown dwarf donor stars and are believed to have evolved past the orbital period minimum. This is in line with the predictions that 40-70 per cent of CVs should have evolved past the orbital period minimum. Therefore, the main result of our study is that the missing population of post-period minimum CVs has finally been identified. The donor star masses and radii are, however, inconsistent with model predictions; the donor stars are approximately 10 per cent larger than expected across the mass range studied here. One explanation for the discrepancy is the enhanced angular momentum loss (e.g. from circumbinary discs); however, the mass-transfer rates, as deduced from white dwarf effective temperatures, are not consistent with enhanced angular momentum loss. We show that it is possible to explain the large donor radii without invoking enhanced angular momentum loss by a combination of geometrical deformation and the effects of starspots due to strong rotation and expected magnetic activity. Choosing unambiguously between these different solutions will require independent estimates of the mass-transfer rates in short-period CVs. The white dwarfs in our sample show a strong tendency towards high masses. We show that this is unlikely to be due to selection effects. The dominance of high-mass white dwarfs in our sample implies that erosion of the white dwarf during nova outbursts must be negligible, or even that white dwarfs grow in mass through the nova cycle. Amongst our sample, there are no helium-core white dwarfs, despite predictions that 30-80 per cent of short-period CVs should contain helium-core white dwarfs. We are unable to rule out selection effects as the cause of this discrepancy.

Eclipsing binaries in open clusters – I. V615 Per and V618 Per in h Persei

We have derived absolute dimensions for two early-type main sequence detached eclipsing binaries in the young open cluster hPersei (NGC869). V615Persei has a spectral type of B7V and a period of 13.7 days. V618Persei is A2V and has a period of 6.4 days. New ephemerides are calculated for both systems. The masses of the component stars have been derived using high-resolution spectroscopy and are 4.08 ± 0.06Mfl and 3.18±0.05Mfl for V615Per and 2.33±0.03Mfl and 1.56±0.02Mfl for V618Per. The radii have been measured by fitting the available light curves using ebop and are 2.29±0.14Rfl and 1.90±0.09Rfl for V615Per and 1.64±0.07Rfl and 1.32±0.07Rfl for V618Per. By comparing the observed spectra of V615Per to synthetic spectra from model atmospheres we find that the eective temperatures of the stars are 15000 ± 500K for the primary and 11000 ± 500K for the secondary. The equatorial rotational velocities of the primary and secondary components of V615Per are 28 ± 5kms i1 and 8 ± 5kms i1 , respectively. Both components of V618Per rotate at 10±5kms i1 . The equatorial rotational velocities for synchronous rotation are about 10kms i1 for all four stars. The timescales for orbital circularisation for both systems, and the timescale for rotational synchronisation of V615Per, are much greater than the age of hPer. Their negligible eccentricities and equatorial rotational velocities therefore support the hypothesis that they were formed by ‘delayed breakup’ (Tohline 2002). We have compared the radii of these stars to models by the Granada and the Padova groups for stars of the same masses but dierent compositions. We conclude that the metallicity of the stars is Z … 0.01. This appears to be the first estimate of the bulk metallicity of hPer. Recent photometric studies have assumed a solar metallicity so their results should be reviewed.