439 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 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
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
Stellar Models with Microscopic Diffusion and Rotational Mixing II: Application to Open Clusters
Stellar models with masses ranging from 0.5 to were constructed
in order to compare to young cluster observations of Li and of rotation
velocities. The amount of Li depletion in cool stars is sensitive to the amount
of overshoot at the base of the surface convection zone, and the exact
metallicity of the models. Even when this uncertainty is taken into account,
the Li observations are a severe constraint for the models and rule out
standard models, and pure diffusion models. Stellar models which include
diffusion and rotational mixing in the radiative regions of stars are able to
simultaneously match the Li abundances observed in the Pleiades, UMaG, Hyades,
Praesepe, NGC 752 and M67. They also match the observed rotation periods in the
Hyades. However, these models are unable to simultaneously explain the presence
of the rapidly rotating late G and K stars in the Pleiades and the absence of
rapidly rotating late F and early G stars.Comment: 15 pages, uuencoded postscript, also available by anonymous ftp to
ftp.cita.utoronto.ca in: /cita/brian/papers/cluster.ps submitted to ApJ,
CITA-94-3
Stellar Models with Microscopic Diffusion and Rotational Mixing I: Application to the Sun
The Yale stellar evolution code has been modified to include the combined
effects of diffusion and rotational mixing on H, He and the trace
elements He, Li, Li, and Be. The interaction between rotational
mixing and diffusion is studied by calculating a number of calibrated solar
models. The rotational mixing inhibits the diffusion in the outer parts of the
models, leading to a decrease in the envelope diffusion by 25 -- 50\%.
Conversely, diffusion leads to gradients in mean molecular weight which can
inhibit the rotational mixing. The degree to which gradients in mean molecular
weight inhibits the rotational mixing is somewhat uncertain. A comparison to
the observed solar oblateness suggests that gradients in the mean molecular
weight play a smaller role in inhibiting the rotational mixing previously
believed. This is reinforced by the fact that the model with the standard value
for the inhibiting effect of mean molecular weight on the rotational mixing
depletes no Li on the main sequence. This is in clear contrast to the
observations. A reduction in the inhibiting effect of mean molecular weight
gradients by a factor of ten leads to noticeable main sequence Li depletion.Comment: 16 pages, uuencoded postscript, also available by anonymous ftp to
ftp.cita.utoronto.ca in: /cita/brian/papers/sun.ps submitted to ApJ,
CITA-94-3
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
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
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