J. Fernandes

The helium content and age of the Hyades: - Constraints from five binary systems and Hipparcos parallaxes

We compare the accurate empirical mass-luminosity (M-L) relation based on five Hyades binary systems to predictions of stellar models calculated with various input parameters (helium, metallicity, age) or physics (mixing-length ratio, model atmosphere, equation of state, microscopic diffusion). Models based on a helium content Ysim0.28 inferred from the dydz enrichment law are more than 3sigma beyond the observations, suggesting that the initial helium abundance is lower than expected from its supersolar metallicity. With the photometric metallicity (FeH=0.144pm0.013 dex, Grenon (2000) we derive Y=0.255\pm0.009. Because of the (Y,FeH) degeneracy in the M-L plane, the uncertainty grows to Delta Y=0.013 if the metallicity from spectroscopy is adopted (FeH=0.14pm0.05 dex, Cayrel de Strobel et al 1997). We use these results to discuss the Hertzsprung-Russell (HR) diagram of the Hyades, in the (Mv,B-V) plane, based on the very precise Hipparcos dynamical parallaxes. Present models fit the tight observed sequence very well except at low temperatures. In the low mass region of the HR diagram sensitive to the mixing-length parameter (aMLT), the slope of the main sequence (MS) suggests that aMLT could decrease from a solar (or even supersolar) value at higher mass to subsolar values at low mass, which is also supported by the modeling of the vB22 M-L relation. We find that the discrepancy at low temperatures (B-V\gtrsim 1.2) remains, even if an improved equation of state or better model atmospheres are used. Finally, we discuss the positions of the stars at turn-off in the light of their observed rotation rates and we deduce that the maximum age of the Hyades predicted by the present models is sim650 Myr.

New and updated stellar parameters for 90 transit hosts - The effect of the surface gravity

Context. Precise stellar parameters are crucial in exoplanet research for correctly determining the planetary parameters. For stars hosting a transiting planet, determining the planetary mass and radius depends on the stellar mass and radius, which in turn depend on the atmospheric stellar parameters. Different methods can provide different results, which leads to different planet characteristics. Aims. In this paper, we use a uniform method to spectroscopically derive stellar atmospheric parameters, chemical abundances, stellar masses, and stellar radii for a sample of 90 transit hosts. Surface gravities are also derived photometrically using the stellar density as derived from the light curve. We study the effect of using these different surface gravities on the determination of the chemical abundances and the stellar mass and radius. Methods. A spectroscopic analysis based on Kurucz models in local thermodynamical equilibrium was performed through the MOOG code to derive the atmospheric parameters and the chemical abundances. The photometric surface gravity was determined through isochrone fitting and the use of the stellar density, directly determined from the light curve. Stellar masses and radii are determined through calibration formulae. Results. Spectroscopic and photometric surface gravities differ, but this has very little effect on the precise determination of the stellar mass in our spectroscopic analysis. The stellar radius, and hence the planetary radius, is most affected by the surface gravity discrepancies. For the chemical abundances, the difference is, as expected, only noticable for the abundances derived from analyzing lines of ionized species.

Li depletion in solar analogues with exoplanets - Extending the sample

Aims. We want to study the effects of the formation of planets and planetary systems on the atmospheric Li abundance of planet host stars. Methods. In this work we present new determinations of lithium abundances for 326 main sequence stars with and without planets in the Teff range 5600–5900 K. The 277 stars come from the HARPS sample, the remaining targets were observed with a variety of high-resolution spectrographs. Results. We confirm significant differences in the Li distribution of solar twins (Teff = T� ± 80 K, log g = log g� ± 0. 2a nd [Fe/H] = [Fe/H]� ± 0.2): the full sample of planet host stars (22) shows Li average values lower than “single” stars with no detected planets (60). If we focus on subsamples with narrower ranges in metallicity and age, we observe indications of a similar result though it is not so clear for some of the subsamples. Furthermore, we compare the observed spectra of several couples of stars with very similar parameters that show differences in Li abundances up to 1.6 dex. Therefore we show that neither age, mass, nor metallicity of a parent star is the only cause for enhanced Li depletion in solar analogues. Conclusions. We conclude that another variable must account for that difference and suggest that this could be the presence of planets that causes additional rotationally induced mixing in the external layers of planet host stars. Moreover, we find indications that the amount of depletion of Li in planet-host solar-type stars is higher when the planets are more massive than Jupiter.

A homogeneous spectroscopic analysis of host stars of transiting planets

Context. The analysis of transiting extra-solar planets provides an enormous amount of information about the formation and evolution of planetary systems. A precise knowledge of the host stars is necessary to derive the planetary properties accurately. The properties of the host stars, especially their chemical composition, are also of interest in their own right. Aims. Information about planet formation is inferred by, among others, correlations between different parameters such as the orbital period and the metallicity of the host stars. The stellar properties studied should be derived as homogeneously as possible. The present work provides new, uniformly derived parameters for 13 host stars of transiting planets. Methods. Effective temperature, surface gravity, microturbulence parameter, and iron abundance were derived from spectra of both high signal-to-noise ratio and high resolution by assuming iron excitation and ionization equilibria. Results. For some stars, the new parameters differ from previous determinations, which is indicative of changes in the planetary radii. A systematic offset in the abundance scale with respect to previous assessments is found for the TrES and HAT objects. Our abundance measurements are remarkably robust in terms of the uncertainties in surface gravities. The iron abundances measured in the present work are supplemented by all previous determinations using the same analysis technique. The distribution of iron abundance then agrees well with the known metal-rich distribution of planet host stars. To facilitate future studies, the spectroscopic results of the current work are supplemented by the findings for other host stars of transiting planets, for a total dataset of 50 objects.

Analysis of 70 Ophiuchi AB including seismic constraints

Context. The analysis of solar-like oscillations for stars belonging to a binary system provides a unique opportunity to probe the internal stellar structure and to test our knowledge of stellar physics. Such oscillations have been recently observed and characterized for the A component of the 70 Ophiuchi system. Aims. We determined the global parameters of 70 Ophiuchi AB using the new asteroseismic measurements now available for 70 Oph A and tested the input physics introduced in stellar evolution codes. Methods. Three different stellar evolution codes and two different calibration methods were used to perform a comprehensive analysis of the 70 Ophiuchi system. Results. A model of 70 Ophiuchi AB that correctly reproduces all observational constraints available for both stars is determined. An age of 6.2± 1.0 Gyr is found with an initial helium mass fraction Yi = 0.266± 0.015 and an initial metallicity (Z/X)i = 0.0300± 0.0025 when atomic diffusion is included and a solar value of the mixing-length parameter assumed. A precise and independent determination of the value of the mixing-length parameter needed to model 70 Oph A requires accurate measurement of the mean small separation, which is not available yet. Current asteroseismic observations, however, suggest that the value of the mixing-length parameter of 70 Oph A is lower or equal to the solar calibrated value. The effects of atomic diffusion and of the choice of the adopted solar mixture were also studied. We finally found that the different evolution codes and calibration methods we used led to perfectly coherent results.

HR diagram and asteroseismic analysis of models for β Hydri

A detailed review of the models for the star β Hydri is done by adjusting the evolutionary sequences to the most recent bolometric, spectroscopic and astrometric data. The dependence of the solution on some of the relevant modelling parameters is analysed and the degeneracy of the solution using the HR diagram analysis discussed. Recent ground-based observations, and the forthcoming asteroseismic missions, can provide detailed oscillation spectra for this star. The seismic data can be used as a complementary constraint on the models. We analyse the seismic properties of the star and discuss the viability of using such complementary tests in order to clarify some of the degeneracy of the model solutions. Preliminary results on the fundamental parameters of β Hydri are presented and the expected seismic behaviour of the star is described.

Higher depletion of lithium in planet host stars: no age and mass effect

Recent observational work by Israelian et al. has shown that sun-like planet host stars in the temperature range 5700 K < Teff < 5850 K have lithium abundances that are significantly lower than those observed for “single” field stars. In this letter we use stellar evolutionary models to show that differences in stellar mass and age are not responsible for the observed correlation. This result, along with the finding of Israelian et al., strongly suggest that the observed lithium difference is likely linked to some process related to the formation and evolution of planetary systems.

The '666' collaboration on OGLE transits: I. Accurate radius of the planets OGLE-TR-10b and OGLE-TR-56b with VLT deconvolution photometry ⋆

Transiting planets are essential to study the structure and evolution of extra-solar planets. For that purpose, it is important to measure precisely the radius of these planets. Here we report new high-accuracy photometry of the transits of OGLE-TR-10 and OGLE-TR-56 with VLT/FORS1. One transit of each object was covered in Bessel V and R filters, and treated with the deconvolution-based photometry algorithm DECPHOT, to ensure accurate millimagnitude light curves. Together with earlier spectroscopic measurements, the data imply a radius of 1.22 +0.12 −0.07 RJ for OGLE-TR-10b and 1.30±0.05 RJ for OGLE- TR-56b. A re-analysis of the original OGLE photometry resolves an earlier discrepancy about the radius of OGLE-TR-10. The transit of OGLE-TR-56 is almost grazing, so that small systematics in the photometry can cause large changes in the derived radius. Our study confirms both planets as inflated hot Jupite rs, with large radii comparable to that of HD 209458b and at least two other recently discovered transiting gas giants.

The role of the time step and overshooting in the modelling of PMS evolution: The case of EK Cephei

EK Cephei (HD 206821) is a unique candidate to test predictions based on stellar evolutionary models. It is a double-lined detached eclipsing binary system with accurate absolute dimensions available and a precise determination of the metallicity. Most importantly for our work, its low mass (1.12 M� ) component appears to be in the pre-main sequence (PMS) phase. We have produced detailed evolutionary models of the binary EK Cep using the CESAM stellar evolution code (Morel 1997). A χ 2 -minimisation was performed to derive the most reliable set of modelling parameters (age, αA, αB and Yi). We have found that an evolutionary age of about 26.8 Myr fits both components in the same isochrone. The positions of EK Cep A and B in the HR diagram are consistent (within the observational uncertainties) with our results. Our revised calibration shows clearly that EK Cep A is in the beginning of the main sequence, while EK Cep B is indeed a PMS star. Such a combination allows for a precise age determination of the binary, and provides a strict test of the modelling. In particular we have found that the definition of the time step in calculating the PMS evolution is crucial to reproduce the observations. A discussion of the optimal time step for calculating PMS evolution is presented. The fitting to the radii of both components is a more difficult task; although we managed to do it for EK Cep B, EK Cep A has a lower radius than our best models. We further studied the effect of the inclusion of a moderate convective overshooting; the calibration of the binary is not signifi- cantly altered, but the effect of the inclusion of overshooting can be dramatic in the approach to the main sequence of stars with masses high enough to burn hydrogen through the CNO cycle on the main sequence.

Disentangling discrepancies between stellar evolution theory and sub-solar mass stars The influence of the mixing length parameter for the UV Psc binary

Serious discrepancies have recently been observed between predictions of stellar evolution models in the 0.7-1.1 M� mass range and accurately measured properties of binary stars with components in this mass range. We study one of these objects, the eclipsing binary UV Piscium, which is particularly interesting because Popper (1997) derived age estimates for each component that differed by more than a factor of two. In an attempt to solve this significant discrepancy (a difference in age of 11 Gyr), we compute a large grid of stellar evolution models with the CESAM code for each component. By fixing the masses to their accurately determined values (relative error smaller than 1% for both stars), we consider a wide range of possible metallicities Z (0.01 to 0.05), and helium content Y (0.25 to 0.34) uncorrelated to Z. In addition, the mixing length parameter αMLT is left as another free parameter. We obtain a best fit in the Teff-radius diagram for a common chemical composition (Z, Y) = (0.012, 0.31), but a different MLT parameter αMLT,A = 0.95 ± 0.12(statistical)+0.30(systematic) and αMLT,B = 0.65 ± 0.07(stat)+0.10(syst). The apparent age discrepancy found by Popper (1997) disappears with this solution, the components being coeval to within 1%. This suggests that fixing αMLT to its solar value (∼1.6), a common hypothesis assumed in most stellar evolutionary models, may not be correct. Secondly, since αMLT is smaller for the less massive component, this suggests that the αMLT parameter may decrease with stellar mass, showing yet another shortcoming of the mixing length theory to explain stellar convection. This trend needs further confirmation with other binary stars with accurate data.