1,330 research outputs found
Excitation of stellar p-modes by turbulent convection: 1. Theoretical formulation
Stochatic excitation of stellar oscillations by turbulent convection is
investigated and an expression for the power injected into the oscillations by
the turbulent convection of the outer layers is derived which takes into
account excitation through turbulent Reynolds stresses and turbulent entropy
fluctuations. This formulation generalizes results from previous works and is
built so as to enable investigations of various possible spatial and temporal
spectra of stellar turbulent convection. For the Reynolds stress contribution
and assuming the Kolmogorov spectrum we obtain a similar formulation than those
derived by previous authors. The entropy contribution to excitation is found to
originate from the advection of the Eulerian entropy fluctuations by the
turbulent velocity field. Numerical computations in the solar case in a
companion paper indicate that the entropy source term is dominant over Reynold
stress contribution to mode excitation, except at high frequencies.Comment: 14 pages, accepted for publication in A&
Period spacings in red giants II. Automated measurement
The space missions CoRoT and Kepler have provided photometric data of
unprecedented quality for asteroseismology. A very rich oscillation pattern has
been discovered for red giants, including mixed modes that are used to decipher
the red giants interiors. They carry information on the radiative core of red
giant stars and bring strong constraints on stellar evolution. Since more than
15,000 red giant light curves have been observed by Kepler, we have developed a
simple and efficient method for automatically characterizing the mixed-mode
pattern and measuring the asymptotic period spacing. With the asymptotic
expansion of the mixed modes, we have revealed the regularity of the
gravity-mode pattern. The stretched periods were used to study the evenly space
periods with a Fourier analysis and to measure the gravity period spacing, even
when rotation severely complicates the oscillation spectra. We automatically
measured gravity period spacing for more than 6,100 Kepler red giants. The
results confirm and extend previous measurements made by semi-automated
methods. We also unveil the mass and metallicity dependence of the relation
between the frequency spacings and the period spacings for stars on the red
giant branch. The delivery of thousands of period spacings combined with all
other seismic and non-seismic information provides a new basis for detailed
ensemble asteroseismology.Comment: 13 pages, 13 figure
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
Stellar granulation as seen in disk-integrated intensity. I. Simplified theoretical modeling
The solar granulation is known for a long time to be a surface manifestation
of convection. Thanks to the current space-borne missions CoRoT and Kepler, it
is now possible to observe in disk-integrated intensity the signature of this
phenomena in a growing number of stars. The space-based photometric
measurements show that the global brightness fluctuations and the lifetime
associated with granulation obeys characteristic scaling relations. We thus aim
at providing a simple theoretical modeling to reproduce these scaling relations
and subsequently at inferring the physical properties of granulation properties
across the HR diagram.
We develop a simple 1D theoretical model that enable us to test any
prescription concerning the time-correlation between granules. The input
parameters of the model are extracted from 3D hydrodynamical models of the
surface layers of stars, and the free parameters involved in the model are
calibrated with solar observations. Two different prescriptions for
representing the eddy time-correlation in the Fourier space are compared: a
Lorentzian and an exponential form. Finally, we compare our theoretical
prediction with a 3D radiative hydrodynamical (RHD) numerical modeling of
stellar granulation (ab-initio approach). Provided that the free parameters are
appropriately adjusted, our theoretical model satisfactorily reproduces the
shape and the amplitude of the observed solar granulation spectrum. The best
agreement is obtained with an exponential form. Furthermore, our theoretical
model results in granulation spectra that consistently agree with the these
calculated on the basis of the ab-initio approach with two 3D RHD models.
Comparison between theoretical granulation spectra calculated with the present
model and high precision photometry measurements of stellar granulation is
undertaken in a companion paper.Comment: 10 pages, 2 figures, accepted for publication in A&
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&
Stochastic excitation of non-radial modes I. High-angular-degree p modes
Turbulent motions in stellar convection zones generate acoustic energy, part
of which is then supplied to normal modes of the star. Their amplitudes result
from a balance between the efficiencies of excitation and damping processes in
the convection zones. We develop a formalism that provides the excitation rates
of non-radial global modes excited by turbulent convection. As a first
application, we estimate the impact of non-radial effects on excitation rates
and amplitudes of high-angular-degree modes which are observed on the Sun. A
model of stochastic excitation by turbulent convection has been developed to
compute the excitation rates, and it has been successfully applied to solar
radial modes (Samadi & Goupil 2001, Belkacem et al. 2006b). We generalize this
approach to the case of non-radial global modes. This enables us to estimate
the energy supplied to high-() acoustic modes. Qualitative arguments as
well as numerical calculations are used to illustrate the results. We find that
non-radial effects for modes are non-negligible:
- for high- modes (i.e. typically ) and for high values of ;
the power supplied to the oscillations depends on the mode inertia.
- for low- modes, independent of the value of , the excitation is
dominated by the non-diagonal components of the Reynolds stress term. We
carried out a numerical investigation of high- modes and we find that
the validity of the present formalism is limited to due to the
spatial separation of scale assumption. Thus, a model for very high-
-mode excitation rates calls for further theoretical developments, however
the formalism is valid for solar modes, which will be investigated in a
paper in preparation.Comment: 12 pages, accepted for publication in A&
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