Analysis of alpha Centauri AB including seismic constraints

Detailed models of alpha Cen A and B based on new seismological data for alpha Cen B by Carrier & Bourban (2003) have been computed using the Geneva evolution code including atomic diffusion. Taking into account the numerous observational constraints now available for the alpha Cen system, we find a stellar model which is in good agreement with the astrometric, photometric, spectroscopic and asteroseismic data. The global parameters of the alpha Cen system are now firmly constrained to an age of t=6.52+-0.30 Gyr, an initial helium mass fraction Y_i=0.275+-0.010 and an initial metallicity (Z/X)_i=0.0434+-0.0020. Thanks to these numerous observational constraints, we confirm that the mixing-length parameter alpha of the B component is larger than the one of the A component, as already suggested by many authors (Noels et al. 1991, Fernandes & Neuforge 1995 and Guenther & Demarque 2000): alpha_B is about 8% larger than alpha_A (alpha_A=1.83+-0.10 and alpha_B=1.97+-0.10). Moreover, we show that asteroseismic measurements enable to determine the radii of both stars with a very high precision (errors smaller than 0.3%). The radii deduced from seismological data are compatible with the new interferometric results of Kervella et al. (2003) even if they are slightly larger than the interferometric radii (differences smaller than 1%).Comment: 13 pages, 9 figures, accepted for publication in A&

Constraining fundamental stellar parameters using seismology. Application to Alpha Centauri AB

We apply the Levenberg-Marquardt minimization algorithm to seismic and classical observables of the Alpha Cen binary system in order to derive the fundamental parameters of Alpha Cen A+B and to analyze the dependence of these parameters on the chosen observables, on their uncertainty and on the physics used in stellar modelling. The seismological data are those by Bouchy & Carrier (2002) for Alpha Cen A, and those by Carrier & Bourban (2003) for Alpha Cen B. We show that while the fundamental stellar parameters do not depend on the treatment of convection adopted (Mixing Length Theory -- MLT -- or ``Full Spectrum of Turbulence'' -- FST), the age of the system depends on the inclusion of gravitational settling, and is deeply biased by the small frequency separation of component B. We try to answer the question of the universality of the mixing length parameter, and we find a statistically reliable dependence of the alpha--parameter on the HR diagram location (with a trend similar to the one predicted by Ludwig et al.1999). We propose the frequency separation ratios introduced by Roxburgh & Voronstsov (2003) as better observables to determine the fundamental stellar parameters, and to use the large frequency separation and frequencies to extract information about the stellar structure. The effects of diffusion and equation of state on the oscillation frequencies are also studied, but present seismic data do not allow their detection.Comment: 15 pages, 8 figures, accepted by A&

X-Ray, FUV, and UV Observations of alpha Centauri B: Determination of Long-term Magnetic Activity Cycle and Rotation Period

We have been carrying out a study of stellar magnetic activity, dynamos, atmospheric physics, and spectral irradiances from a sample of solar-type G0-5 V stars with different ages. One of the major goals of this program is to study the evolution of the Sun's X-ray through NUV spectral irradiances with age. Of particular interest is the determination of the young Sun's elevated levels of high-energy fluxes because of the critical roles that X-ray through FUV emissions play on the photochemical and photoionization evolution of early, young planetary atmospheres and ionospheres. Motivated by the current exoplanetary search missions that are hunting for earth-size planets in the habitable zones of nearby main-sequence G-M stars, we are expanding our program to cooler, less luminous, but much more numerous main-sequence K-type stars, such as alpha Centauri B. The long life (2-3x longer than our Sun) and slow evolution of K stars provide nearly constant energy sources for possible hosted planets. Presented here are X-ray, UV, and recently acquired FUV observations of the K1 V star alpha Cen B. These combined high-energy measures provide a more complete look into the nature of alpha Cen B's magnetic activity and X-UV radiances. We find that alpha Cen B has exhibited significant long-term variability in X-ray through NUV emission fluxes, indicating a solar-like long-term activity cycle of P_cycle = 8.84 years. In addition, analysis of the short-term rotational modulation of mean light due to the effects of magnetically active regions has yielded a well-determined rotation period of P_rotation = 36.2 days. alpha Cen B is the only old main-sequence K star with a reliably determined age and rotation period, and for early K-stars, is an important calibrator for stellar age/rotation/activity relations

High-Resolution Spectroscopy of the Transiting Planet Host Star TrES-1

We report on a spectroscopic determination of the stellar parameters and chemical abundances for the parent star of the transiting planet TrES-1. Based on a detailed analysis of iron lines in our Keck and HET spectra we derive $T_\mathrm{eff} = 5250\pm 75$ K, $\log g = 4.6\pm 0.2$, and [Fe/H] $= 0.00\pm 0.09$. By measuring the \ion{Ca}{2} activity indicator and by putting useful upper limits on the Li abundance we constrain the age of TrES-1 to be $2.5\pm 1.5$ Gyr. By comparing theoretical stellar evolution models with the observational parameters we obtain $M_\star = 0.89\pm 0.05$ $M_\odot$, and $R_\star = 0.83\pm 0.05$ $R_\odot$. Our improved estimates of the stellar parameters are utilized in a new analysis of the transit photometry of TrES-1 to derive a mass $M_p = 0.76\pm 0.05$ $M_\mathrm{J}$, a radius $R_p = 1.04^{+0.08}_{-0.05}$ $R_\mathrm{J}$, and an inclination $i = 89^\circ.5^{+0.5}_{-1.3}$. The improved planetary mass and radius estimates provide the grounds for new crucial tests of theoretical models of evolution and evaporation of irradiated extrasolar giant planets.Comment: 14 pages, 3 figures, Astrophysical Journal Letters, accepte

How Good a Clock is Rotation? The Stellar Rotation-Mass-Age Relationship for Old Field Stars

The rotation-mass-age relationship offers a promising avenue for measuring the ages of field stars, assuming the attendant uncertainties to this technique can be well characterized. We model stellar angular momentum evolution starting with a rotation distribution from open cluster M37. Our predicted rotation-mass-age relationship shows significant zero-point offsets compared to an alternative angular momentum loss law and published gyrochronology relations. Systematic errors at the 30 percent level are permitted by current data, highlighting the need for empirical guidance. We identify two fundamental sources of uncertainty that limit the precision of rotation-based ages and quantify their impact. Stars are born with a range of rotation rates, which leads to an age range at fixed rotation period. We find that the inherent ambiguity from the initial conditions is important for all young stars, and remains large for old stars below 0.6 solar masses. Latitudinal surface differential rotation also introduces a minimum uncertainty into rotation period measurements and, by extension, rotation-based ages. Both models and the data from binary star systems 61 Cyg and alpha Cen demonstrate that latitudinal differential rotation is the limiting factor for rotation-based age precision among old field stars, inducing uncertainties at the ~2 Gyr level. We also examine the relationship between variability amplitude, rotation period, and age. Existing ground-based surveys can detect field populations with ages as old as 1-2 Gyr, while space missions can detect stars as old as the Galactic disk. In comparison with other techniques for measuring the ages of lower main sequence stars, including geometric parallax and asteroseismology, rotation-based ages have the potential to be the most precise chronometer for 0.6-1.0 solar mass stars.Comment: For a brief video explaining the key results of this paper, see http://www.youtube.com/user/OSUAstronom

A Bayesian approach to the modelling of alpha Cen A

Determining the physical characteristics of a star is an inverse problem consisting in estimating the parameters of models for the stellar structure and evolution, knowing certain observable quantities. We use a Bayesian approach to solve this problem for alpha Cen A, which allows us to incorporate prior information on the parameters to be estimated, in order to better constrain the problem. Our strategy is based on the use of a Markov Chain Monte Carlo (MCMC) algorithm to estimate the posterior probability densities of the stellar parameters: mass, age, initial chemical composition,... We use the stellar evolutionary code ASTEC to model the star. To constrain this model both seismic and non-seismic observations were considered. Several different strategies were tested to fit these values, either using two or five free parameters in ASTEC. We are thus able to show evidence that MCMC methods become efficient with respect to more classical grid-based strategies when the number of parameters increases. The results of our MCMC algorithm allow us to derive estimates for the stellar parameters and robust uncertainties thanks to the statistical analysis of the posterior probability densities. We are also able to compute odds for the presence of a convective core in alpha Cen A. When using core-sensitive seismic observational constraints, these can raise above ~40%. The comparison of results to previous studies also indicates that these seismic constraints are of critical importance for our knowledge of the structure of this star.Comment: 21 pages, 6 figures, to be published in MNRA

The Solar Heavy Element Abundances: II. Constraints from Stellar Atmospheres

Estimates of the bulk metal abundance of the Sun derived from the latest generation of model atmospheres are significantly lower than the earlier standard values. In Paper I we demonstrated that a low solar metallicity is inconsistent with helioseismology if the quoted errors in the atmospheres models (of order 0.05 dex) are correct. In this paper we undertake a critical analysis of the solar metallicity and its uncertainty from a model atmospheres perspective, focusing on CNO. We argue that the non-LTE corrections for abundances derived from atomic features are overestimated in the recent abundance studies, while systematic errors in the absolute abundances are underestimated. If we adopt the internal consistency between different indicators as a measure of goodness of fit, we obtain intermediate abundances [C/H] = 8.44 +/- 0.06, [N/H] = 7.96 +/- 0.10 and [O/H] = 8.75 +/- 0.08. The errors are too large to conclude that there is a solar abundance problem, and permit both the high and low scales. However, the center-to-limb continuum flux variations predicted in the simulations appear to be inconsistent with solar data, which would favor the traditional thermal structure and lead to high CNO abundances of (8.52, 7.96, 8.80) close to the seismic scale. We argue that further empirical tests of non-LTE corrections and the thermal structure are required for precise absolute abundances. The implications for beryllium depletion and possible sources of error in the numerical simulations are discussed.Comment: 36 pages, 4 figures, submitted Ap

Modeling Mid-Ultraviolet Spectra. I. Temperatures of Metal-Poor Stars

Determining the properties of old stellar systems using evolutionary population synthesis requires a library of reliable model stellar fluxes. Empirical libraries are limited to spectra of stars in the solar neighborhood, with nearly solar abundances and abundance ratios. We report here a first step towards providing a flux library that includes nonsolar abundances, based on calculations from first principles that are calibrated empirically. We have started with main-sequence stars, whose light dominates the mid-ultraviolet spectrum of an old stellar system. We have calculated mid-ultraviolet spectra for the Sun and nine nearby, near-main-sequence stars spanning metallicities from less than 1/100 solar to greater than solar, encompassing a range of light-element abundance enhancements. We first determined temperatures of eight of the stars by analyzing optical echelle spectra together with the mid-ultraviolet. Both could be matched at the same time only when models with no convective overshoot were adopted, and only when an approximate chromosphere was incorporated near the surface of relatively metal-rich models. Extensive modifications to mid-UV line parameters were also required, notably the manual assignment of approximate identifications for mid-UV lines missing from laboratory linelists. Without recourse to additional missing opacity, these measures suffice to reproduce in detail almost the entire mid-UV spectrum of solar-temperature stars up to one-tenth solar metallicity, and the region from 2900A to 3100A throughout the entire metallicity range. Ramifications for abundance determinations in individual metal-poor stars and for age-metallicity determinations of old stellar systems are briefly discussed, with emphasis on the predictive power of the calculations.Comment: Proof revision -Minor changes to revised version submitted to Astrophysical Journal May 1, 2001. 29 pages, 4 figures (Fig. 3 with 5 panels and Fig. 4 with 6 panels). Figures 1 and 2 are .gif; postscript versions of Figures 1 and 2 are available from http://www.astro.virginia.edu/~rtr/uv/index.htm

Characterization of the HD 17156 planetary system

AIMS : To improve the parameters of the HD 17156 system (peculiar due to the eccentric and long orbital period of its transiting planet) and constrain the presence of stellar companions. METHODS : Photometric data were acquired for 4 transits, and high precision radial velocity measurements were simultaneously acquired with SARG@TNG for one transit. The template spectra of HD 17156 was used to derive effective temperature, gravity, and metallicity. A fit of the photometric and spectroscopic data was performed to measure the stellar and planetary radii, and the spin-orbit alignment. Planet orbital elements and ephemeris were derived from the fit. Near infrared adaptive optic images was acquired with ADOPT@TNG. RESULTS: We have found that the star has a radius of R_S = 1.43+/-0.03 R_sun and the planet R_P =1.02+/-0.08 R_jup. The transit ephemeris is T_c = 2454\756.73134+/-0.00020+N*21.21663+/-0.00045 BJD. The analysis of the Rossiter-Mclaughlin effect shows that the system is spin orbit aligned with an angle lambda = 4.8 +/- 5.3 deg. The analysis of high resolution images has not revealed any stellar companion with projected separation between 150 and 1000 AU from HD 17156.Comment: submitted to A&

Accurate fundamental parameters for 23 bright solar-type stars

We combine results from interferometry, asteroseismology and spectroscopy to determine accurate fundamental parameters of 23 bright solar-type stars, from spectral type F5 to K2 and luminosity classes III to V. For some stars we can use direct techniques to determine the mass, radius, luminosity and effective temperature, and we compare with indirect methods that rely on photometric calibrations or spectroscopic analyses. We use the asteroseismic information available in the literature to infer an indirect mass with an accuracy of 4-15 percent. From indirect methods we determine luminosity and radius to 3 percent. For Teff we find a slight offset of -40+-20 K between the spectroscopic method and the direct method, meaning the spectroscopic temperatures are too high. From the spectroscopic analysis we determine the detailed chemical composition for 13 elements, including Li, C and O. We find no significant offset between the spectroscopic surface gravity and the value from combining asteroseismology with radius estimates. From the spectroscopy we also determine vsini and we present a new calibration of macro- and microturbulence. From the comparison between the results from the direct and spectroscopic methods we claim that we can determine Teff, log g, and [Fe/H] with absolute accuracies of 80 K, 0.08 dex, and 0.07 dex. The indirect methods are important to obtain reliable estimates of the fundamental parameters of relatively faint stars when interferometry cannot be used. Our study is the first to compare direct and indirect methods for a large sample of stars, and we conclude that indirect methods are valid, although slight corrections may be needed.Comment: Accepted by MNRAS. Abstract abridge