Probing the parameter space of HD 49933: a comparison between global and local methods

We present two independent methods for studying the global stellar parameter space (mass M, age, initial chemical composition X_0, Z_0) of HD 49933 with seismic data. Using a local minimization and an MCMC algorithm, we obtain consistent results for the determination of the stellar properties: M = 1.1 - 1.2 M_solar, Age ~ 3.0 Gyr, Z_0 ~ 0.008. A description of the error ellipses can be defined using Singular Value Decomposition techniques, and this is validated by comparing the errors with those from the MCMC method.Comment: to be published in JPC

A study of binary constraints for seismology of delta Scuti stars

Seismology of single delta Scuti stars has mainly been inhibited by failing to detect many of the theoretically predicted pulsation modes, resulting in difficulties with mode identification. Theoretical and observational advances have, however, helped to overcome this problem, but the following questions then remain: do we know enough about the star to either use the (few) identified mode(s) to probe the structure of the star? or improve the determination of the stellar parameters? It is now generally accepted that for the observed frequencies to be used successfully as seismic probes for these objects, we need to concentrate on stars where we can constrain the number of free parameters in the problem, such as in binary systems or open clusters. The work presented here, investigates how much is gained in our understanding of the star, by comparing the information we obtain from a single star with that of an eclipsing binary system. Singular Value Decomposition is the technique used to explore the precision we expect in terms of stellar parameters (such as mass, age and chemical composition).Comment: v2: error in equation corrected. HELAS II Conference: Helioseismology, Asteroseismology and MHD Connections, August 2007 Goettingen, German

Asteroseismic modeling of 16 Cyg A & B using the complete Kepler data set

Asteroseismology of bright stars with well-determined properties from parallax measurements and interferometry can yield precise stellar ages and meaningful constraints on the composition. We substantiate this claim with an updated asteroseismic analysis of the solar-analog binary system 16 Cyg A & B using the complete 30-month data sets from the Kepler space telescope. An analysis with the Asteroseismic Modeling Portal (AMP), using all of the available constraints to model each star independently, yields the same age ($t=7.0 \pm 0.3$ Gyr) and composition ($Z=0.021 \pm 0.002$, $Y_i=0.25 \pm 0.01$) for both stars, as expected for a binary system. We quantify the accuracy of the derived stellar properties by conducting a similar analysis of a Kepler-like data set for the Sun, and we investigate how the reliability of asteroseismic inference changes when fewer observational constraints are available or when different fitting methods are employed. We find that our estimates of the initial helium mass fraction are probably biased low by 0.02-0.03 from neglecting diffusion and settling of heavy elements, and we identify changes to our fitting method as the likely source of small shifts from our initial results in 2012. We conclude that in the best cases reliable stellar properties can be determined from asteroseismic analysis even without independent constraints on the radius and luminosity.Comment: 5 emulateapj pages, 1 table, 1 figure. ApJ Letters, accepte

Variation in the frequency separations with activity and impact on stellar parameter determination

Frequency separations used to infer global properties of stars through asteroseismology can change depending on the strength and at what epoch of the stellar cycle the p-mode frequencies are measured. In the Sun these variations have been seen, even though the Sun is a low-activity star. In this paper, we discuss these variations and their impact on the determination of the stellar parameters (radius, mass and age) for the Sun. Using the data from maximum and minimum activity, we fitted an age for the Sun that differs on average by 0.2 Gyr: slightly older during minimum activity. The fitted radius is also lower by about 0.5% for the solar effective temperature during minimum.Comment: to be published in JPCS to be published in JPC