Eclipsing Binaries in Open Clusters

Detached eclipsing binaries are very useful objects for calibrating theoretical stellar models and checking their predictions. Detached eclipsing binaries in open clusters are particularly important because of the additional constraints on their age and chemical composition from their membership of the cluster. I compile a list containing absolute parameters of well-studied eclipsing binaries in open clusters, and present new observational data on the B-type systems V1481 Cyg and V2263 Cyg which are members of the young open cluster NGC 7128.Comment: 4 pages, 2 colour figures. Poster presentation for IAUS 240 (Binary Stars as Critical Tools and Tests in Contemporary Astrophysics), Prague, August 2006. The poster itself can be dowloaded in ppt and pdf versions from http://www.astro.keele.ac.uk/~jkt/pubs.htm

Probing the models: Abundances for high-mass stars in binaries

The complexity of composite spectra of close binary star system makes study of the spectra of their component stars extremely difficult. For this reason there exists very little information on the photospheric chemical composition of stars in close binaries, despite its importance for informing our understanding of the evolutionary processes of stars. In a long-term observational project we aim to fill this gap with systematic abundance studies for the variety of binary systems. The core of our analysis is the spectral disentangling technique, which allows isolation of the individual component star spectra from the time-series of observed spectra. We present new results for high-mass stars in close binaries. So far, we have measured detailed abundances for 22 stars in a dozen detached binary systems. The parameter space for the stars in our sample comprises masses in the range 8--22 M_sun, surface gravities of 3.1--4.2 (c.g.s.) and projected rotational velocities of 30--240 km/s. Whilst recent evolutionary models for rotating single stars predict changes in photospheric abundances even during the main sequence lifetime, no star in our sample shows signs of these predicted changes. It is clear that other effects prevail in the chemical evolution of components in binary stars even at the beginning of their evolution.Comment: 8 pages, 3 figures, to appear in the proceedings of the conference on 'Setting a New Standard in the Analysis of Binary Stars', K. Pavlovski, A. Tkachenko and G. Torres, eds., EAS Publication Serie

Absolute dimensions of eclipsing binaries. XXIV, The Be star system DW Carinae, a member of the open cluster Collinder 228

Context. The study of detached eclipsing binaries which are members of stellar clusters is is a powerful way of determining the properties of the cluster and of constraining the physical ingredients of theoretical stellar evolutionary models. Aims. DWCarinae is a close but detached early B-type eclipsing binary in the young open cluster Collinder 228. We have measured accurate physical properties of the components of DWCar (masses and radii to 1%, effective temperatures to 0.02 dex) and used these to derive the age, metallicity and distance of Collinder 228. Methods. The rotational velocities of both components of DWCar are high, so we have investigated the performance of double-Gaussian fitting, one- and two-dimensional cross-correlation and spectral disentangling for deriving spectroscopic radial velocites in the presence of strong line blending. Gaussian and cross-correlation analyses require substantial corrections for the effects of line blending, which are only partially successful for cross-correlation. Spectral disentangling is to be preferred because it does not assume anything about the shapes of spectral lines, and is not significantly affected by blending. However, it suffers from a proliferation of local minima in the least-squares fit. We show that the most reliable radial velocities are obtained using spectral disentangling constrained by the results of Gaussian fitting. Complete Strömgren uvby light curves have been obtained and accurate radii have been measured from them by modelling the light curves using the Wilson-Devinney program. This procedure also suffers from the presence of many local minima in parameter space, so we have constrained the solution using an accurate spectroscopic light ratio. The effective temperatures and reddening of the system have been found from Strömgren photometric calibrations. Results. The mass and radius of DWCarA are MA = 11.34 ± 0.12 M and RA = 4.558 ± 0.045 R. The values for DWCar B are MB =10.63 ± 0.14 M and RB = 4.297 ± 0.055 R. Strömgren photometric calibrations give effective temperatures of TeffA = 27 900 ±1000 K and TeffB = 26 500 ± 1000 K, and a reddening of Eb−y = 0.18 ± 0.02, where the quoted uncertainties include a contribution from the intrinsic uncertainty of the calibrations. The membership of DWCar in Cr 228 allows us to measure the distance, age and chemical composition of the cluster. We have used empirical bolometric corrections to calculate a distance modulus of 12.24 ±0.12 mag for DWCar, which is in agreement with, and more accurate than, literature values. A comparison between the properties of DWCar and the predictions of recent theoretical evolutionary models is undertaken in the mass-radius and mass-Teff diagrams. The model predictions match the measured properties of DWCar for an age of about 6 Myr and a fractional metal abundance of Z ≈ 0.01

High-precision photometry by telescope defocussing. III. The transiting planetary system WASP-2

We present high-precision photometry of three transits of the extrasolar planetary system WASP-2, obtained by defocussing the telescope, and achieving point-to-point scatters of between 0.42 and 0.73 mmag. These data are modelled using the JKTEBOP code, and taking into account the light from the recently-discovered faint star close to the system. The physical properties of the WASP-2 system are derived using tabulated predictions from five different sets of stellar evolutionary models, allowing both statistical and systematic errorbars to be specified. We find the mass and radius of the planet to be M_b = 0.847 +/- 0.038 +/- 0.024 Mjup and R_b = 1.044 +/- 0.029 +/- 0.015 Rjup. It has a low equilibrium temperature of 1280 +/- 21 K, in agreement with a recent finding that it does not have an atmospheric temperature inversion. The first of our transit datasets has a scatter of only 0.42 mmag with respect to the best-fitting light curve model, which to our knowledge is a record for ground-based observations of a transiting extrasolar planet.Comment: Accepted for publication in MNRAS. 9 pages, 3 figures, 10 table

Double riches: asteroseismology in eclipsing binaries

The study of eclipsing binaries is our primary source of measured properties of normal stars, achieved through analysis of light and radial velocity curves of eclipsing systems. The study of oscillations and pulsations is increasingly vital for determining the properties of single stars, and investigating the physical phenomena active in their interiors. Combining the two methods holds the promise of establishing stringent tests of stellar evolutionary theory, and of calibrating model-dependent asteroseismology with empirically measured stellar properties. I review recent advances and outline future work

Eclipsing binaries observed with the WIRE satellite. II, β Aurigae and non-linear limb darkening in light curves

Aims. We present the most precise light curve ever obtained of a detached eclipsing binary star and use it investigate the inclusion of non-linear limb darkening laws in light curve models of eclipsing binaries. This light curve, of the bright eclipsing system β Aurigae, was obtained using the star tracker aboard the wire satellite and contains 30 000 datapoints with a point-to-point scatter of 0.3mmag. Methods. We analyse the wire light curve using a version of the ebop code modified to include non-linear limb darkening laws and to directly incorporate observed times of minimum light and spectroscopic light ratios into the photometric solution as individual observations. We also analyse the dataset with the Wilson-Devinney code to ensure that the two models give consistent results. Results. ebop is able to provide an excellent fit to the high-precision wire data.Whilst the fractional radii of the stars are only defined to a precision of 5% by this light curve, including an accurate published spectroscopic light ratio improves this dramatically to 0.5%. Using non-linear limb darkening improves the quality of the fit significantly compared to the linear law and causes the measured radii to increase by 0.4%. It is possible to derive all of the limb darkening coefficients from the light curve, although they are strongly correlated with each other. The fitted coefficients agree with theoretical predictions to within their fairly large error estimates. We were able to obtain a reasonably good fit to the data using the Wilson-Devinney code, but only using the highest available integration accuracy and by iterating for a long time. Bolometric albedos of 0.6 were found, which are appropriate to convective rather than radiative envelopes. Conclusions. The radii and masses of the components of β Aur are RA = 2.762 ± 0.017 R, RB = 2.568 ± 0.017 R, MA = 2.376 ±0.027 M and MB = 2.291 ± 0.027 M, where A and B denote the primary and secondary star, respectively. Theoretical stellar evolutionary models can match these parameters for a solarmetal abundance and an age of 450−500 Myr. The Hipparcos trigonometric parallax and an interferometrically-derived orbital parallax give distances to β Aur which are in excellent agreement with each other and with distances derived using surface brightness relations and several sets of empirical and theoretical bolometric corrections