Modeling Convective Core Overshoot and Diffusion in Procyon Constrained by Asteroseismic Data

We compare evolved stellar models, which match Procyons mass and position in the HR diagram, to current ground-based asteroseismic observations. Diffusion of helium and metals along with two conventional core overshoot descriptions and the Kuhfuss nonlocal theory of convection are considered. We establish that one of the two published asteroseismic data reductions for Procyon, which mainly differ in their identification of even versus odd l-values, is a significantly more probable and self-consistent match to our models than the other. The most probable models according to our Bayesian analysis have evolved to just short of turnoff, still retaining a hydrogen convective core. Our most probable models include Y and Z diffusion and have conventional core overshoot between 0.9 and 1.5 pressure scale heights, which increases the outer radius of the convective core by between 22% to 28%, respectively. We discuss the significance of this comparatively higher than expected core overshoot amount in terms of internal mixing during evolution. The parameters of our most probable models are similar regardless of whether adiabatic or nonadiabatic model p-mode frequencies are compared to the observations, although, the Bayesian probabilities are greater when the nonadiabatic model frequencies are used. All the most probable models (with or without core overshoot, adiabatic or nonadiabatic model frequencies, diffusion or no diffusion, including priors for the observed HRD location and mass or not) have masses that are within one sigma of the observed mass 1.497+/-0.037 Msun

The Non-Homologous Nature of Solar Diameter Variations

We show in this paper that the changes of the solar diameter in response to variations of large scale magnetic fields and turbulence are not homologous. For the best current model, the variation at the photospheric level is over 1000 times larger than the variation at a depth of 5 Mm, which is about the level at which f-mode solar oscillations determine diameter variations. This model is supported by observations that indicate larger diameter changes for high degree f-modes than for low degree f-modes, since energy of the former are concentrated at shallower layers than the latter.Comment: 11 pages, 3 figures, aastex style, accepted for publication by ApJ

The Yale-Potsdam Stellar Isochrones (YaPSI)

We introduce the Yale-Potsdam Stellar Isochrones (YaPSI), a new grid of stellar evolution tracks and isochrones of solar-scaled composition. In an effort to improve the Yonsei-Yale database, special emphasis is placed on the construction of accurate low-mass models (Mstar < 0.6 Msun), and in particular of their mass-luminosity and mass-radius relations, both crucial in characterizing exoplanet-host stars and, in turn, their planetary systems. The YaPSI models cover the mass range 0.15 to 5.0 Msun, densely enough to permit detailed interpolation in mass, and the metallicity and helium abundance ranges [Fe/H] = -1.5 to +0.3, and Y = 0.25 to 0.37, specified independently of each other (i.e., no fixed Delta Y/Delta Z relation is assumed). The evolutionary tracks are calculated from the pre-main sequence up to the tip of the red giant branch. The isochrones, with ages between 1 Myr and 20 Gyr, provide UBVRI colors in the Johnson-Cousins system, and JHK colors in the homogeneized Bessell & Brett system, derived from two different semi-empirical Teff-color calibrations from the literature. We also provide utility codes, such as an isochrone interpolator in age, metallicity, and helium content, and an interface of the tracks with an open-source Monte Carlo Markov-Chain tool for the analysis of individual stars. Finally, we present comparisons of the YaPSI models with the best empirical mass- luminosity and mass-radius relations available to date, as well as isochrone fitting of well-studied steComment: 17 pages, 14 figures; accepted for publication in the Astrophysical Journa

Rotation, diffusion, and overshoot in the Sun: effects on the oscillation frequencies and the neutrino flux

We have studied the importance of the combined effects of rotation, diffusion, and convective overshoot on the p-mode oscillation spectrum and the neutrino flux of the standard solar model. To isolate the various physical affects included in the new rotation plus diffusion models we also constructed solar models to test the significance of diffusion and of overshoot by themselves. In previous studies, models that include helium diffusion during solar evolution were found to improve the predicted p-mode frequencies for some modes and worsen the agreement for others (Guenther \ea 1993). Here we verify this result for both the Bahcall and Loeb (1990) formulation of diffusion and the Proffitt and Michaud (1991) formulation of diffusion. We find that the effects of rotation on the Sun's structure in the outer layers perturbs the p-mode frequencies only slightly when compared to the more substantial effects due to diffusion. In the thin overshoot layer (taken here to be 0.1\, H_p), we have compared the effect of overmixing in a radiative layer versus convective (adiabatic) penetration. Neither radiative overmixing nor adiabatic penetration has any significant effect on the p-modes, probably in part because the overshoot layer is constrained to be thin. The predicted neutrino flux in our diffusion plus rotation model is 7.12 SNU for Cl detectors, 127 SNU for Ga detectors and 5.00\times 10^6\,{\rm erg \, cm^{-2} } for the ^8B neutrinos; this is approximately half-way between the standard solar model without diffusion, and the standard solar model with diffusion alone

Space and Ground Based Pulsation Data of Eta Bootis Explained with Stellar Models Including Turbulence

The space telescope MOST is now providing us with extremely accurate low frequency p-mode oscillation data for the star Eta Boo. We demonstrate in this paper that these data, when combined with ground based measurements of the high frequency p-mode spectrum, can be reproduced with stellar models that include the effects of turbulence in their outer layers. Without turbulence, the l=0 modes of our models deviate from either the ground based or the space data by about 1.5-4.0 micro Hz. This discrepancy can be completely removed by including turbulence in the models and we can exactly match 12 out of 13 MOST frequencies that we identified as l=0 modes in addition to 13 out of 21 ground based frequencies within their observational 2 sigma tolerances. The better agreement between model frequencies and observed ones depends for the most part on the turbulent kinetic energy which was taken from a 3D convection simulation for the Sun.Comment: 13 pages, 7 figures, ApJ in pres

Theoretical stellar models for old galactic clusters

We present new evolutionary stellar models suitable for old Population I clusters, discussing both the consequences of the most recent improvements in the input physics and the effect of element diffusion within the stellar structures. Theoretical cluster isochrones are presented, covering the range of ages from 1 to 9 Gyr for the four selected choices on the metallicity Z= 0.007, 0.010, 0.015 and 0.020. Theoretical uncertainties on the efficiency of superadiabatic convection are discussed in some details. Isochrone fitting to the CM diagrams of the two well observed galactic clusters NGC2420 and M67 indicates that a mixing length parameter alpha = 1.9 appears adequate for reproducing the observed color of cool giant stars. The problems in matching theoretical preditions to the observed slope of MS stars are discussed.Comment: 7 pages, 14 postscript figures, accepted for publication on MNRA

Inclusion of turbulence in solar modeling

The general consensus is that in order to reproduce the observed solar p-mode oscillation frequencies, turbulence should be included in solar models. However, until now there has not been any well-tested efficient method to incorporate turbulence into solar modeling. We present here two methods to include turbulence in solar modeling within the framework of the mixing length theory, using the turbulent velocity obtained from numerical simulations of the highly superadiabatic layer of the sun at three stages of its evolution. The first approach is to include the turbulent pressure alone, and the second is to include both the turbulent pressure and the turbulent kinetic energy. The latter is achieved by introducing two variables: the turbulent kinetic energy per unit mass, and the effective ratio of specific heats due to the turbulent perturbation. These are treated as additions to the standard thermodynamic coordinates (e.g. pressure and temperature). We investigate the effects of both treatments of turbulence on the structure variables, the adiabatic sound speed, the structure of the highly superadiabatic layer, and the p-mode frequencies. We find that the second method reproduces the SAL structure obtained in 3D simulations, and produces a p-mode frequency correction an order of magnitude better than the first method.Comment: 10 pages, 12 figure