1,019 research outputs found
Excitation of solar-like oscillations across the HR diagram
We extend semi-analytical computations of excitation rates for solar
oscillation modes to those of other solar-like oscillating stars to compare
them with recent observations. Numerical 3D simulations of surface convective
zones of several solar-type oscillating stars are used to characterize the
turbulent spectra as well as to constrain the convective velocities and
turbulent entropy fluctuations in the uppermost part of the convective zone of
such stars. These constraints, coupled with a theoretical model for stochastic
excitation, provide the rate 'P' at which energy is injected into the p-modes
by turbulent convection. These energy rates are compared with those derived
directly from the 3D simulations. The excitation rates obtained from the 3D
simulations are systematically lower than those computed from the
semi-analytical excitation model. We find that Pmax, the excitation rate
maximum, scales as (L/M)^s where s is the slope of the power law and L and M
are the mass and luminosity of the 1D stellar model built consistently with the
associated 3D simulation. The slope is found to depend significantly on the
adopted form of the eddy time-correlation ; using a Lorentzian form results in
s=2.6, whereas a Gaussian one gives s=3.1. Finally, values of Vmax, the maximum
in the mode velocity, are estimated from the computed power laws for Pmax and
we find that Vmax increases as (L/M)^sv. Comparisons with the currently
available ground-based observations show that the computations assuming a
Lorentzian eddy time-correlation yield a slope, sv, closer to the observed one
than the slope obtained when assuming a Gaussian. We show that the spatial
resolution of the 3D simulations must be high enough to obtain accurate
computed energy rates.Comment: 14 pages ; 7 figures ; accepted for publication in Astrophysics &
Astronom
Solar-like oscillation amplitudes and line-widths as a probe for turbulent convection in stars
Excitation of solar-like oscillations is attributed to turbulent convection
and takes place at the upper-most part of the outer convective zones.
Amplitudes of these oscillations depend on the efficiency of the excitation
processes as well as on the properties of turbulent convection. We present past
and recent improvements on the modeling of those processes. We show how the
mode amplitudes and mode line-widths can bring information about the turbulence
in the specific cases of the Sun and Alpha Cen A.Comment: 9 pages ; 3 figures ; invited talk given during the Symposium no. 239
"Convection in Astrophysics", International Astronomical Union., held 21-25
August, 2006 in Prague, Czech Republi
Numerical constraints on the model of stochastic excitation of solar-type oscillations
Analyses of a 3D simulation of the upper layers of a solar convective
envelope provide constraints on the physical quantities which enter the
theoretical formulation of a stochastic excitation model of solar p modes, for
instance the convective velocities and the turbulent kinetic energy spectrum.
These constraints are then used to compute the acoustic excitation rate for
solar p modes, P. The resulting values are found ~5 times larger than the
values resulting from a computation in which convective velocities and entropy
fluctuations are obtained with a 1D solar envelope model built with the
time-dependent, nonlocal Gough (1977) extension of the mixing length
formulation for convection (GMLT). This difference is mainly due to the assumed
mean anisotropy properties of the velocity field in the excitation region. The
3D simulation suggests much larger horizontal velocities compared to vertical
ones than in the 1D GMLT solar model. The values of P obtained with the 3D
simulation constraints however are still too small compared with the values
inferred from solar observations. Improvements in the description of the
turbulent kinetic energy spectrum and its depth dependence yield further
increased theoretical values of P which bring them closer to the observations.
It is also found that the source of excitation arising from the advection of
the turbulent fluctuations of entropy by the turbulent movements contributes ~
65-75 % to the excitation and therefore remains dominant over the Reynolds
stress contribution. The derived theoretical values of P obtained with the 3D
simulation constraints remain smaller by a factor ~3 compared with the solar
observations. This shows that the stochastic excitation model still needs to be
improved.Comment: 11 pages, 9 figures, accepted for publication in A&
Effect of local treatments of convection upon the solar p-mode excitation rates
We compute, for several solar models, the rates P at which the solar radial p
modes are expected to be excited. The solar models are computed with two
different local treatments of convection : the classical mixing-length theory
(MLT hereafter) and Canuto, Goldmann and Mazzitelli(1996, CGM hereafter)'s
formulation. For one set of solar models (EMLT and ECGM models), the atmosphere
is gray and assumes Eddington's approximation. For a second set of models (KMLT
and KCGM models), the atmosphere is built using a T(tau) law which has been
obtained from a Kurucz's model atmosphere computed with the same local
treatment of convection. The mixing-length parameter in the model atmosphere is
chosen so as to provide a good agreement between synthetic and observed Balmer
line profiles, while the mixing-length parameter in the interior model is
calibrated so that the model reproduces the solar radius at solar age. For the
MLT treatment, the rates P do depend significantly on the properties of the
atmosphere. On the other hand, for the CGM treatment, differences in P between
the ECGM and the KCGM models are very small compared to the error bars attached
to the seismic measurements. The excitation rates P for modes from the EMLT
model are significantly under-estimated compared with the solar seismic
constraints. The KMLT model results in intermediate values for P and shows also
an important discontinuity in the temperature gradient and the convective
velocity. On the other hand, the KCGM model and the ECGM model yield values for
P closer to the seismic data than the EMLT and KMLT models. We conclude that
the solar p-mode excitation rates provide valuable constraints and according to
the present investigation cleary favor the CGM treatment with respect to the
MLT.Comment: 4 pages, 3 figures, proceedings of the SOHO14/GONG 2004 workshop
"Helio- and Asteroseismology: Towards a Golden Future" from July 12-16 2004
at New Haven CT (USA
A closure model with plumes II. Application to the stochastic excitation of stellar p modes
Our goal is to improve the theoretical modelling of stochastic excitation of
p modes by turbulent convection. With the help of the closure model with plume
(CMP) developed in a companion paper, we refine the theoretical description of
the excitation by the turbulent Reynolds stress term. The CMP is generalized
for two-point correlation products so as to apply it to the formalism developed
by Samadi & Goupil (2001). The excitation source terms are then computed with
this improvement, and a comparison with solar data from the GOLF instrument is
performed. The present model provides a significant improvement when comparing
absolute values of theoretical ampplitudes with observational data. It gives
rise to a frequency dependence of the power supplied to solar p modes, which
agrees with GOLF observations. It is shown that the asymmetry of the turbulent
convection zone (up- and downflows) plays a major role in the excitation
processes. Despite an increase in the Reynolds stress term contribution due to
our improved description, an additional source of excitation, identified as the
entropy source term, is still necessary for reproducing the observational data.
Theoretical excitation rates in the frequency range [2.5 mHz, 4 mHz] now are in
agreement with the observational data from the GOLF instrument. However, at
lower frequencies, it exhibits small discrepancies at the maximum level of a
few per cent. Improvements are likely to come from a better physical
description of the excitation by entropy fluctuations in the superadiabatic
zone.Comment: 9 pages, accepted for publication in A&
Seismic evolution of low/intermediate mass PMS stars
This article presents a study of the evolution of the internal structure and
seismic properties expected for low/intermediate mass Pre-Main Sequence (PMS)
stars. Seismic and non-seismic properties of PMS stars were analysed. This was
done using 0.8 to 4.4M stellar models at stages ranging from the end of
the Hayashi track up to the Zero-Age Main-Sequence (ZAMS). This research
concludes that, for intermediate-mass stars (M1.3M), diagrams
comparing the effective temperature () against the small separation
can provide an alternative to Christensen-Dalsgaard (C-D) diagrams. The impact
of the metal abundance of intermediate mass stars (2.5-4.4M) has over
their seismic properties is also evaluated.Comment: 4 pages, 5 figures, accepted for publication on A&
Influence of local treatments of convection upon solar p mode excitation rates
We compute the rates P at which acoustic energy is injected into the solar
radial p modes for several solar models. The solar models are computed with two
different local treatments of convection: the classical mixing-length theory
(MLT hereafter) and Canuto et al (1996)'s formulation (CGM hereafter). Among
the models investigated here, our best models reproduce both the solar radius
and the solar luminosity at solar age and the observed Balmer line profiles.
For the MLT treatment, the rates P do depend significantly on the properties of
the atmosphere whereas for the CGM's treatment the dependence of P on the
properties of the atmosphere is found smaller than the error bars attached to
the seismic measurements. The excitation rates P for modes associated with the
MLT models are significantly underestimated compared with the solar seismic
constraints. The CGM models yield values for P closer to the seismic data than
the MLT models. We conclude that the solar p-mode excitation rates provide
valuable constraints and according to the present investigation clearly favor
the CGM treatment with respect to the MLT, although neither of them yields
values of P as close to the observations as recently found for 3D numerical
simulations.Comment: 11 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
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