878 research outputs found
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
Theoretical power spectra of mixed modes in low mass red giant stars
CoRoT and Kepler observations of red giant stars revealed very rich spectra
of non-radial solar-like oscillations. Of particular interest was the detection
of mixed modes that exhibit significant amplitude, both in the core and at the
surface of the stars. It opens the possibility of probing the internal
structure from their inner-most layers up to their surface along their
evolution on the red giant branch as well as on the red-clump. Our objective is
primarily to provide physical insight into the physical mechanism responsible
for mixed-modes amplitudes and lifetimes. Subsequently, we aim at understanding
the evolution and structure of red giants spectra along with their evolution.
The study of energetic aspects of these oscillations is also of great
importance to predict the mode parameters in the power spectrum. Non-adiabatic
computations, including a time-dependent treatment of convection, are performed
and provide the lifetimes of radial and non-radial mixed modes. We then combine
these mode lifetimes and inertias with a stochastic excitation model that gives
us their heights in the power spectra. For stars representative of CoRoT and
Kepler observations, we show under which circumstances mixed modes have heights
comparable to radial ones. We stress the importance of the radiative damping in
the determination of the height of mixed modes. Finally, we derive an estimate
for the height ratio between a g-type and a p-type mode. This can thus be used
as a first estimate of the detectability of mixed-modes
Surface-effect corrections for solar-like oscillations using 3D hydrodynamical simulations
The space-borne missions have provided us with a wealth of high-quality
observational data that allows for seismic inferences of stellar interiors.
This requires the computation of precise and accurate theoretical frequencies,
but imperfect modeling of the uppermost stellar layers introduces systematic
errors. To overcome this problem, an empirical correction has been introduced
by Kjeldsen et al. (2008, ApJ, 683, L175) and is now commonly used for seismic
inferences. Nevertheless, we still lack a physical justification allowing for
the quantification of the surface-effect corrections. We used a grid of these
simulations computed with the COBOLD code to model the outer layers of
solar-like stars. Upper layers of the corresponding 1D standard models were
then replaced by the layers obtained from the horizontally averaged 3D models.
The frequency differences between these patched models and the 1D standard
models were then calculated using the adiabatic approximation and allowed us to
constrain the Kjeldsen et al. power law, as well as a Lorentzian formulation.
We find that the surface effects on modal frequencies depend significantly on
both the effective temperature and the surface gravity. We further provide the
variation in the parameters related to the surface-effect corrections using
their power law as well as a Lorentzian formulation. Scaling relations between
these parameters and the elevation (related to the Mach number) is also
provided. The Lorentzian formulation is shown to be more robust for the whole
frequency spectrum, while the power law is not suitable for the frequency
shifts in the frequency range above .Comment: 11 pages, 14 figures, 4 tables; accepted for publication in Astronomy
& Astrophysic
Reduced-rank Envelope Vector Autoregressive Models
The standard vector autoregressive (VAR) models suffer from
overparameterization which is a serious issue for high-dimensional time series
data as it restricts the number of variables and lags that can be incorporated
into the model. Several statistical methods, such as the reduced-rank model for
multivariate (multiple) time series (Velu et al., 1986; Reinsel and Velu, 1998;
Reinsel et al., 2022) and the Envelope VAR model (Wang and Ding, 2018), provide
solutions for achieving dimension reduction of the parameter space of the VAR
model. However, these methods can be inefficient in extracting relevant
information from complex data, as they fail to distinguish between relevant and
irrelevant information, or they are inefficient in addressing the rank
deficiency problem. We put together the idea of envelope models into the
reduced-rank VAR model to simultaneously tackle these challenges, and propose a
new parsimonious version of the classical VAR model called the reduced-rank
envelope VAR (REVAR) model. Our proposed REVAR model incorporates the strengths
of both reduced-rank VAR and envelope VAR models and leads to significant gains
in efficiency and accuracy. The asymptotic properties of the proposed
estimators are established under different error assumptions. Simulation
studies and real data analysis are conducted to evaluate and illustrate the
proposed method
Seismic diagnostics for transport of angular momentum in stars 2. Interpreting observed rotational splittings of slowly-rotating red giant stars
Asteroseismology with the space-borne missions CoRoT and Kepler provides a
powerful mean of testing the modeling of transport processes in stars.
Rotational splittings are currently measured for a large number of red giant
stars and can provide stringent constraints on the rotation profiles. The aim
of this paper is to obtain a theoretical framework for understanding the
properties of the observed rotational splittings of red giant stars with slowly
rotating cores. This allows us to establish appropriate seismic diagnostics for
rotation of these evolved stars. Rotational splittings for stochastically
excited dipolar modes are computed adopting a first-order perturbative approach
for two benchmark models assuming slowly rotating cores. For red
giant stars with slowly rotating cores, we show that the variation of the
rotational splittings of modes with frequency depends only on the
large frequency separation, the g-mode period spacing, and the ratio of the
average envelope to core rotation rates (). This leds us to propose a
way to infer directly from the observations. This method is
validated using the Kepler red giant star KIC 5356201. Finally, we provide a
theoretical support for the use of a Lorentzian profile to measure the observed
splittings for red giant stars.Comment: 15 pages, 15 figures, accepted for publication in A&
Density assessment of muddy shore crabs in the eastern part of Bandar Abbas
Crabs of intertidal zone of a muddy shore in Bandar Abbas were studied using monthly examination of Goharshad and Terminal stations during August 2005 to July 2006. Sampling was done by Corer and 14 species from 6 families were identified. Crabs were from Ocypodidae, Pinnotheridae, Pilumnidae, Trapeziidae, Leucosiidae and Varunidae families. Results showed Ocypodidae, the most abundant and diverse with 1196 individuals/m^2, was more abundant but less diverse in Goharshad station compared to Terminal which could be attributed to the urban pollution in Terminal station. Pinnotheridae with 64 individuals/m 2 showed an adaptation to organic materials but was not resistant to being out of water. Pilumnidae and Trapeziidae were found in small numbers in the stattions. Genus Ebalia from Leucosiidae with 27.18 individuals/m^2 showed positive reaction to organic materials and was resistant to being out of water. Varunidae were present with a few specimens probably because of human disturbance or speed in burrowing and escaping from sampling. Because civic pollution enter Terminal station, The results of this research showed that crabs react to urban pollution and hence are suitable as environmental indicators
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