A. Gimenez

Accurate masses and radii of normal stars: modern results and applications

This article presents and discusses a critical compilation of accurate, fundamental determinations of stellar masses and radii. We have identified 95 detached binary systems containing 190 stars (94 eclipsing systems, and α Centauri) that satisfy our criterion that the mass and radius of both stars be known within errors of ±3% accuracy or better. All of them are non-interacting systems, and so the stars should have evolved as if they were single. This sample more than doubles that of the earlier similar review by Andersen (Astron Astrophys Rev 3:91–126, 1991), extends the mass range at both ends and, for the first time, includes an extragalactic binary. In every case, we have examined the original data and recomputed the stellar parameters with a consistent set of assumptions and physical constants. To these we add interstellar reddening, effective temperature, metal abundance, rotational velocity and apsidal motion determinations when available, and we compute a number of other physical parameters, notably luminosity and distance. These accurate physical parameters reveal the effects of stellar evolution with unprecedented clarity, and we discuss the use of the data in observational tests of stellar evolution models in some detail. Earlier findings of significant structural differences between moderately fast-rotating, mildly active stars and single stars, ascribed to the presence of strong magnetic and spot activity, are confirmed beyond doubt. We also show how the best data can be used to test prescriptions for the subtle interplay between convection, diffusion, and other non-classical effects in stellar models. The amount and quality of the data also allow us to analyse the tidal evolution of the systems in considerable depth, testing prescriptions of rotational synchronisation and orbital circularisation in greater detail than possible before. We show that the formulae for pseudo-synchronisation of stars in eccentric orbits predict the observed rotations quite well, except for very young and/or widely separated stars. Deviations do occur, however, especially for stars with convective envelopes. The superior data set finally demonstrates that apsidal motion rates as predicted from General Relativity plus tidal theory are in good agreement with the best observational data. No reliable binary data exist, which challenge General Relativity to any significant extent. The new data also enable us to derive empirical calibrations of M and R for single (post-) main-sequence stars above

The apsidal-motion test of stellar structure and evolution: an update

{0.6\,M_{\odot}}

Accurate Masses and Radii as Test Data for Stellar Models: Status and Progress on the Copenhagen Project

. Simple, polynomial functions of Teff, log g and [Fe/H] yield M and R within errors of 6 and 3%, respectively. Excellent agreement is found with independent determinations for host stars of transiting extrasolar planets, and good agreement with determinations of M and R from stellar models as constrained by trigonometric parallaxes and spectroscopic values of Teff and [Fe/H]. Finally, we list a set of 23 interferometric binaries with masses known to be better than 3%, but without fundamental radius determinations (except α Aur). We discuss the prospects for improving these and other stellar parameters in the near future.

THE BURST OBSERVER AND OPTICAL TRANSIENT EXPLORING SYSTEM (BOOTES)

Context. The influence of stellar activity on the fundamental properties of stars around and below 1 Mis not well understood. Accurate mass, radius, and abundance determinations from solar-type binaries exhibiting various levels of activity are needed for a better insight into the structure and evolution of these stars. Aims. We aim to determine absolute dimensions and abundances for the solar-type detached eclipsing binary V636 Cen, and to perform a detailed comparison with results from recent stellar evolutionary models. Methods. uvby light curves and uvbyβ standard photometry were obtained with the Stromgren Automatic Telescope, radial velocity observations with the CORAVEL spectrometer, and high-resolution spectra with the FEROS spectrograph, all at ESO, La Silla. State- of-the-art methods were applied for the photometric and spectroscopic analyses. Results. Masses and radii that are precise to 0.5% have been established for the components of V636 Cen. The 0.85 Msecondary component is moderately active with starspots and Ca ii H and K emission, and the 1.05 Mprimary shows signs of activity as well, but at a much lower level. We derive a (Fe/H) abundance of −0.20 ± 0.08 and similar abundances for Si, Ca, Ti, V, Cr, Co, and Ni. Corresponding solar-scaled stellar models are unable to reproduce V636 Cen, especially its secondary component, which is ∼10% larger and ∼400 K cooler than predicted. Models adopting significantly lower mixing-length parameters l/Hp remove these discrepancies, seen also for other solar-type binary components. For the observed (Fe/H), Claret models for l/Hp = 1.4 (primary) and 1.0 (secondary) reproduce the components of V636 Cen at a common age of 1.35 Gyr. The orbit is eccentric (e = 0.135 ± 0.001), and apsidal motion with a 40% relativistic contribution has been detected. The period is U = 5 270 ± 335 yr, and the inferred mean central density concentration coefficient, log(k2) = −1.61 ± 0.05, agrees marginally with model predictions. The measured rotational velocities, 13.0 ± 0.2 (primary) and 11.2 ± 0.5 (secondary) km s −1 , are in remarkable agreement with the theoretically predicted pseudo-synchronous velocities, but are about 15% lower than the periastron values. Conclusions. V636 Cen and 10 other well-studied inactive and active solar-type binaries suggest that chromospheric activity, and its effect on envelope convection, is likely to cause radius and temperature discrepancies, which can be removed by adjusting the model mixing length parameters downwards. Noting this, the sample may also lend support to theoretical 2D radiation hydrodynamics studies, which predict a slight decrease of the mixing length parameter with increasing temperature/mass for inactive main sequence stars. More binaries are, however, needed for a description/calibration in terms of physical parameters and level of activity.

Stellar models for a wide range of initial chemical compositions until helium burning - IV. From X = 0.65 to X = 0.80, for Z = 0.004

Aims. The recent revision of a sample of 94 double-lined eclipsing binaries with accurate absolute dimensions provides a set of 18 systems with eccentric orbits that are precise enough to carry out an updated apsidal motion test of stellar structure. The availability of these new high-quality observational data, together with improved stellar models, that include the latest opacities, stellar compressibility, resonances, and rotation, has triggered the comparison presented in this paper between theoretical and observed apsidal motion rates. Methods. The applicability of the adopted stellar models to the sample of eclipsing binaries with accurate absolute dimensions and eccentric orbits was checked with two classical tests of the good agreement between the predicted and observed effective temperatures of the component stars and a fit to a common isochrone of their position in the H-R diagram. For this comparison, stellar models were computed for the observed individual masses and incorporated the most recent input physics available, including the effects of rotation and convective core overshooting. Results. The final comparison of the theoretical and observed weighted internal-structure constants k2 shows good agreement and no systematic effects. Observational evidence does not show more massive cores than predicted by the models as has been suggested, and this result is independent of the evolutionary status.

Four-colour photometry of eclipsing binaries. XLI.

The Burst Observer and Optical Transient Exploring System (BOOTES) is considered as a part of the preparations for ESA’s INTEGRAL satellite, and is currently being developed in Spain, in collaboration with two Czech institutions. It makes use of two sets of wide-field cameras, 240 km apart, and two robotic 0.3-m telescopes. The first observing station (BOOTES-1) is located in Huelva (Spain) and the first light was obtained in July 1998. During the test phase, it has provided rapid follow-up observations with the wide-field cameras for 19 GRBs detected by BATSE aboard CGRO, and narrow-field imaging for 6 bursts. Limiting magnitudes for any GRB optical afterglow are I∼13 and R∼16.5, a few minutes after the events.

Four-colour photometry of eclipsing binaries. XXIA: Photometric analysis and apsidal motion study of V346 Centauri

The most important ingredient of a stellar model is its initial mass . Hence, real stars with known masses are important test objects for stellar models. Initial chemical composition is the other important starting parameter and should therefore also be known (Clausen, 1992). Finally, the most direct indicator of the evolution of a star is its radius . Together, these parameters determine the effective temperature and luminosity of the model, to be tested against a real star.