Absolute dimensions of eclipsing binaries

Abstract

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. DW Carinae 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 DW Car (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 DW Car are high, so we have investigated the performance of doubleGaussian 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 DW Car A are M A = 11.34 ± 0.12 M and R A = 4.558 ± 0.045 R . The values for DW Car B are M B = 10.63 ± 0.14 M and R B = 4.297 ± 0.055 R . Strömgren photometric calibrations give effective temperatures of T eff A = 27 900 ± 1000 K and T eff B = 26 500 ± 1000 K, and a reddening of E b−y = 0.18 ± 0.02, where the quoted uncertainties include a contribution from the intrinsic uncertainty of the calibrations. The membership of DW Car 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 DW Car, which is in agreement with, and more accurate than, literature values. A comparison between the properties of DW Car and the predictions of recent theoretical evolutionary models is undertaken in the mass-radius and mass-T eff diagrams. The model predictions match the measured properties of DW Car for an age of about 6 Myr and a fractional metal abundance of Z ≈ 0.01.

HD 174884: a strongly eccentric, short-period early-type binary system discovered by CoRoT

“…To obtain reliable orbital elements of HD 174884 based on all 45 spectra we applied the disentangling in the Fourier domain developed by Hadrava (1995Hadrava ( , 1997Hadrava ( , 2004, using the latest In general, our experience with KOREL fully confirms both the findings by Southworth & Clausen (2007) and also that the result is sensitive to the choice of starting values of the elements and initial values of the simplex steps, i.e. that the simplex solution can easily be trapped in a number of various local minima.…”

Physical elements of the eclipsing binaryδ Orionis

“…This is understandable since the sum of squares of residuals using all data points of individual spectra is a very complicated one, and it is easy to end up in a local minimum. This has been nicely illustrated in an excellent study of DW Car by Southworth & Clausen (2007) -see their Fig. 3.…”

A VLT/FLAMES survey for massive binaries in Westerlund 1

“…Finally, measurements of blended hydrogen lines with Gaussian fits tend to yield systematically lower values of K 1 and K 2 than methods such as spectral disentangling (Simon & Sturm 1994) that are less affected by blending (Andersen 1975;Southworth & Clausen 2007). The paucity of strong lines free from significant interstellar, telluric and wind contamination in the R-and I-band spectra of ∼B0 supergiants makes the extent of this effect on our determination of K em and K abs hard to quantify, and we therefore note that our method may underestimate the masses of the two components of W13.…”