251 research outputs found
Rotation of the solar convection zone from helioseismology
Helioseismology has provided very detailed inferences about rotation of the
solar interior. Within the convection zone the rotation rate roughly shares the
latitudinal variation seen in the surface differential rotation. The transition
to the nearly uniformly rotating radiative interior takes place in a narrow
tachocline, which is likely important to the operation of the solar magnetic
cycle.The convection-zone rotation displays zonal flows, regions of slightly
more rapid and slow rotation, extending over much of the depth of the
convection zone and converging towards the equator as the solar cycle
progresses. In addition, there is some evidence for a quasi-periodic variation
in rotation, with a period of around 1.3 yr, at the equator near the bottom of
the convection zone.Comment: 12 pages, 8 figures. To appear in Proc. IAU Symposium 239: Convection
in Astrophysics,eds F. Kupka, I. W. Roxburgh & K. L. Chan, Cambridge
University Pres
Correcting stellar oscillation frequencies for near-surface effects
In helioseismology, there is a well-known offset between observed and
computed oscillation frequencies. This offset is known to arise from improper
modeling of the near-surface layers of the Sun, and a similar effect must occur
for models of other stars. Such an effect impedes progress in asteroseismology,
which involves comparing observed oscillation frequencies with those calculated
from theoretical models. Here, we use data for the Sun to derive an empirical
correction for the near-surface offset, which we then apply three other stars
(alpha Cen A, alpha Cen B and beta Hyi). The method appears to give good
results, in particular providing an accurate estimate of the mean density of
each star.Comment: accepted for publication in ApJ Letter
Measurements of Stellar Properties through Asteroseismology: A Tool for Planet Transit Studies
Oscillations occur in stars of most masses and essentially all stages of
evolution. Asteroseismology is the study of the frequencies and other
properties of stellar oscillations, from which we can extract fundamental
parameters such as density, mass, radius, age and rotation period. We present
an overview of asteroseismic analysis methods, focusing on how this technique
may be used as a tool to measure stellar properties relevant to planet transit
studies. We also discuss details of the Kepler Asteroseismic Investigation --
the use of asteroseismology on the Kepler mission in order to measure basic
stellar parameters. We estimate that applying asteroseismology to stars
observed by Kepler will allow the determination of stellar mean densities to an
accuracy of 1%, radii to 2-3%, masses to 5%, and ages to 5-10% of the
main-sequence lifetime. For rotating stars, the angle of inclination can also
be determined.Comment: To appear in the Proceedings of the 253rd IAU Symposium: "Transiting
Planets", May 2008, Cambridge, M
Improvements to stellar structure models, based on a grid of 3D convection simulations. II. Calibrating the mixing-length formulation
We perform a calibration of the mixing length of convection in stellar
structure models against realistic 3D radiation-coupled hydrodynamics (RHD)
simulations of convection in stellar surface layers, determining the adiabat
deep in convective stellar envelopes.
The mixing-length parameter is calibrated by matching averages of
the 3D simulations to 1D stellar envelope models, ensuring identical atomic
physics in the two cases. This is done for a previously published grid of
solar-metallicity convection simulations, covering from 4200 K to 6900 K on the
main sequence, and 4300-5000 K for giants with logg=2.2.
Our calibration results in an varying from 1.6 for the warmest
dwarf, which is just cool enough to admit a convective envelope, and up to 2.05
for the coolest dwarfs in our grid. In between these is a triangular plateau of
~ 1.76. The Sun is located on this plateau and has seen little change
during its evolution so far. When stars ascend the giant branch, they largely
do so along tracks of constant , with decreasing with
increasing mass.Comment: 22 pages, 15 figures, accepted for publication in MNRA
Improvements to Stellar Structure Models, Based on a Grid of 3D Convection Simulations. I. -Relations
Relations between temperature, T, and optical depth, tau, are often used for
describing the photospheric transition from optically thick to optically thin
in stellar structure models. We show that this is well justified, but also that
currently used T(tau) relations are often inconsistent with their
implementation. As an outer boundary condition on the system of stellar
structure equations, T(tau) relations have an undue effect on the overall
structure of stars. In this age of precision asteroseismology, we need to
re-assess both the method for computing and for implementing T(tau) relations,
and the assumptions they rest on. We develop a formulation for proper and
consistent evaluation of T(tau) relations from arbitrary 1D or 3D stellar
atmospheres, and for their implementation in stellar structure and evolution
models. We extract radiative T(tau) relations, as described by our new
formulation, from 3D simulations of convection in deep stellar atmospheres of
late-type stars from dwarfs to giants. These simulations employ realistic
opacities and equation of state, and account for line-blanketing. For
comparison, we also extract T(tau) relations from 1D MARCS model atmospheres
using the same formulation. T(tau)-relations from our grid of 3D convection
simulations display a larger range of behaviours with surface gravity, compared
with those of conventional theoretical 1D hydrostatic atmosphere models. Based
on this, we recommend no longer to use scaled solar T(tau) relations. Files
with T(tau) relations for our grid of simulations are made available to the
community, together with routines for interpolating in this irregular grid. We
also provide matching tables of atmospheric opacity, for consistent
implementation in stellar structure models.Comment: 18 pages, 7 figures, 2 tables. Accepted for publication in MNRAS,
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