827 research outputs found
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&
Simulations of Oscillation Modes of the Solar Convection Zone
We use the three-dimensional hydrodynamic code of Stein and Nordlund to
realistically simulate the upper layers of the solar convection zone in order
to study physical characteristics of solar oscillations. Our first result is
that the properties of oscillation modes in the simulation closely match the
observed properties. Recent observations from SOHO/MDI and GONG have confirmed
the asymmetry of solar oscillation line profiles, initially discovered by
Duvall et al. In this paper we compare the line profiles in the power spectra
of the Doppler velocity and continuum intensity oscillations from the SOHO/MDI
observations with the simulation. We also compare the phase differences between
the velocity and intensity data. We have found that the simulated line profiles
are asymmetric and have the same asymmetry reversal between velocity and
intensity as observed. The phase difference between the velocity and intensity
signals is negative at low frequencies and jumps in the vicinity of modes as is
also observed. Thus, our numerical model reproduces the basic observed
properties of solar oscillations, and allows us to study the physical
properties which are not observed.Comment: Accepted for publication in ApJ Letter
The CoRoT target HD175726: an active star with weak solar-like oscillations
Context. The CoRoT short runs give us the opportunity to observe a large
variety of late-type stars through their solar-like oscillations. We report
observations of the star HD175726 that lasted for 27 days during the first
short run of the mission. The time series reveals a high-activity signal and
the power spectrum presents an excess due to solar-like oscillations with a low
signal-to-noise ratio. Aims. Our aim is to identify the most efficient tools to
extract as much information as possible from the power density spectrum.
Methods. The most productive method appears to be the autocorrelation of the
time series, calculated as the spectrum of the filtered spectrum. This method
is efficient, very rapid computationally, and will be useful for the analysis
of other targets, observed with CoRoT or with forthcoming missions such as
Kepler and Plato. Results. The mean large separation has been measured to be
97.2+-0.5 microHz, slightly below the expected value determined from solar
scaling laws.We also show strong evidence for variation of the large separation
with frequency. The bolometric mode amplitude is only 1.7+-0.25 ppm for radial
modes, which is 1.7 times less than expected. Due to the low signal-to-noise
ratio, mode identification is not possible for the available data set of
HD175726. Conclusions. This study shows the possibility of extracting a seismic
signal despite a signal-to-noise ratio of only 0.37. The observation of such a
target shows the efficiency of the CoRoT data, and the potential benefit of
longer observing runs.Comment: 8 pages. Accepted in A&
What Causes P-mode Asymmetry Reversal?
The solar acoustic p-mode line profiles are asymmetric. Velocity spectra have
more power on the low-frequency sides, whereas intensity profiles show the
opposite sense of asymmetry. Numerical simulations of the upper convection zone
have resonant p-modes with the same asymmetries and asymmetry reversal as the
observed modes. The temperature and velocity power spectra at optical depth
have the opposite asymmetry as is observed for the
intensity and velocity spectra. At a fixed geometrical depth, corresponding to
, however, the temperature and velocity spectra have the
same asymmetry. This indicates that the asymmetry reversal is produced by
radiative transfer effects and not by correlated noise.Comment: 16 pages, 10 figures, submitted to Astrophysical Journa
Comparison of High-degree Solar Acoustic Frequencies and Asymmetry between Velocity and Intensity Data
Using the local helioseismic technique of ring diagram we analyze the
frequencies of high--degree f- and p-modes derived from both velocity and
continuum intensity data observed by MDI. Fitting the spectra with asymmetric
peak profiles, we find that the asymmetry associated with velocity line
profiles is negative for all frequency ranges agreeing with previous
observations while the asymmetry of the intensity profiles shows a complex and
frequency dependent behavior. We also observe systematic frequency differences
between intensity and velocity spectra at the high end of the frequency range,
mostly above 4 mHz. We infer that this difference arises from the fitting of
the intensity rather than the velocity spectra. We also show that the frequency
differences between intensity and velocity do not vary significantly from the
disk center to the limb when the spectra are fitted with the asymmetric profile
and conclude that only a part of the background is correlated with the
intensity oscillations.Comment: Accepted for publication in Astrophysical Journa
Numerical 3D constraints on convective eddy time-correlations : consequences for stochastic excitation of solar p modes
A 3D simulation of the upper part of the solar convective zone is used to
obtain information on the frequency component, chi_k, of the correlation
product of the turbulent velocity field. This component plays an important role
in the stochastic excitation of acoustic oscillations. A time analysis of the
solar simulation shows that a gaussian function does not correctly reproduce
the nu-dependency of chi_k inferred from the 3D simuation in the frequency
range where the acoustic energy injected into the solar p modes is important
(nu ~ 2 - 4 mHz). The nu-dependency of chi_k is fitted with different
analytical functions which can then conveniently be used to compute the
acoustic energy supply rate P injected into the solar radial oscillations. With
constraints from a 3D simulation, adjustment of free parameters to solar data
is no longer necessary and is not performed here. The result is compared with
solar seismic data. Computed values of P obtained with the analytical function
which fits best chi_k are found ~ 2.7 times larger than those obtained with the
gaussian model and reproduce better the solar seismic observations. This
non-gaussian description also leads to a Reynolds stress contribution of the
same order as the one arising from the advection of the turbulent fluctuations
of entropy by the turbulent motions. Some discrepancy between observed and
computed P values still exist at high frequency and possible causes for this
discrepancy are discussed.Comment: 11 pages; 4 figures, 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&
Asteroseismology across the HR diagram
High precision spectroscopy provides essential information necessary to fully
exploit the opportunity of probing the internal structure of stars using
Asteroseismology. In this work we discuss how Asteroseismology combined with
High Precision Spectroscopy can establish a detailed view on stellar structure
and evolution of stars across the HR diagramme.Comment: 6 pages, 2 figures - to appear in Precision Spectroscopy in
Astrophysics, (Eds) L. Pasquini, M. Romaniello, N.C. Santos, and A. Correia,
ESO Astrophysics Symposia, 200
The Relation between Physical and Gravitational Geometry
The appearance of two geometries in one and the same gravitational theory is
familiar. Usually, as in the Brans-Dicke theory or in string theory, these are
conformally related Riemannian geometries. Is this the most general relation
between the two geometries allowed by physics ? We study this question by
supposing that the physical geometry on which matter dynamics take place could
be Finslerian rather than just Riemannian. An appeal to the weak equivalence
principle and causality then leads us the conclusion that the Finsler geometry
has to reduce to a Riemann geometry whose metric - the physical metric - is
related to the gravitational metric by a generalization of the conformal
transformation.Comment: 15 pages, Te
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