409 research outputs found
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 Pulsation Properties of Procyon A
A grid of stellar evolution models for Procyon A has been calculated. These
models include the best physics available to us (including the latest opacities
and equation of state) and are based on the revised astrometric mass of Girard
et al (1996). Models were calculated with helium diffusion and with the
combined effects of helium and heavy element diffusion. Oscillation frequencies
for l=0,1,2 and 3 p-modes and the characteristic period spacing for the g-modes
were calculated for these models. We find that g-modes are sensitive to model
parameters which effect the structure of the core, such as convective core
overshoot, the heavy element abundance and the evolutionary state (main
sequence or shell hydrogen burning) of Procyon A. The p-modes are relatively
insensitive to the details of the physics used to model Procyon A, and only
depend on the evolutionary state of Procyon A. Hence, observations of p-mode
frequencies on Procyon A will serve as a robust test of stellar evolution
models.Comment: 4 pages, to appear in ApJ
The theoretical calculation of the Rossby number and the "non-local" convective overturn time for pre-main sequence and early post-main sequence stars
This paper provides estimates of convective turnover time scales for Sun-like stars in the pre-main sequence and early post-main sequence phases of evolution, based on up-to-date physical input for the stellar models. In this first study, all models have solar abundances, which is typical of the stars in the Galactic disk where most of the available data have been collected. A new feature of these models is the inclusion of rotation in the evolutionary sequences, thus making it possible to derive theoretically the Rossby number for each star along its evolutionary track, based on its calculated rotation rate and its local convective turnover time near the base of the convection zone. Global turnover times are also calculated for the complete convection zone. This information should make possible a new class of observational tests of stellar theory which were previously impossible with semi-empirical models, particularly in the study of stellar activity and in research related to angular momentum transfer in stellar interiors during the course of stellar evolution
Improved calibration of the radii of cool stars based on 3D simulations of convection: implications for the solar model
Main sequence, solar-like stars (M < 1.5 Msun) have outer convective
envelopes that are sufficiently thick to affect significantly their overall
structure. The radii of these stars, in particular, are sensitive to the
details of inefficient, super-adiabatic convection occurring in their outermost
layers. The standard treatment of convection in stellar evolution models, based
on the Mixing-Length Theory (MLT), provides only a very approximate description
of convection in the super-adiabatic regime. Moreover, it contains a free
parameter, alpha_MLT, whose standard calibration is based on the Sun, and is
routinely applied to other stars ignoring the differences in their global
parameters (e.g., effective temperature, gravity, chemical composition) and
previous evolutionary history. In this paper, we present a calibration of
alpha_MLT based on three-dimensional radiation-hydrodynamics (3D RHD)
simulations of convection. The value of alpha_MLT is adjusted to match the
specific entropy in the deep, adiabatic layers of the convective envelope to
the corresponding value obtained from the 3D RHD simulations, as a function of
the position of the star in the (log g, log T_eff) plane and its chemical
composition. We have constructed a model of the present-day Sun using such
entropy-based calibration. We find that its past luminosity evolution is not
affected by the entropy calibration. The predicted solar radius, however,
exceeds that of the standard model during the past several billion years,
resulting in a lower surface temperature. This illustrative calculation also
demonstrates the viability of the entropy approach for calibrating the radii of
other late-type stars.Comment: 16 pages, 14 figures, accepted for publication in the Astrophysical
Journa
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