2,431 research outputs found
Solar-like oscillations in the metal-poor subgiant nu Indi: II. Acoustic spectrum and mode lifetime
Convection in stars excites resonant acoustic waves which depend on the sound
speed inside the star, which in turn depends on properties of the stellar
interior. Therefore, asteroseismology is an unrivaled method to probe the
internal structure of a star. We made a seismic study of the metal-poor
subgiant star nu Indi with the goal of constraining its interior structure. Our
study is based on a time series of 1201 radial velocity measurements spread
over 14 nights obtained from two sites, Siding Spring Observatory in Australia
and ESO La Silla Observatory in Chile. The power spectrum of the high precision
velocity time series clearly presents several identifiable peaks between 200
and 500 uHz showing regularity with a large and small spacing of 25.14 +- 0.09
uHz and 2.96 +- 0.22 uHz at 330 uHz. Thirteen individual modes have been
identified with amplitudes in the range 53 to 173 cm/s. The mode damping time
is estimated to be about 16 days (1-sigma range between 9 and 50 days),
substantially longer than in other stars like the Sun, the alpha Cen system or
the giant xi Hya.Comment: 5 pages, 7 figures, A&A accepte
Shark Research: emerging technologies and applications for the field and laboratory
Over the last decade, the study of shark biology has benefited from the development, refinement, and rapid expansion of novel techniques and advances in technology. These have given new insight into the fields of shark genetics, feeding, foraging, bioenergetics, imaging, age and growth, movement, migration, habitat preference, and habitat use. This pioneering book, written by experts in shark biology, examines technologies such as autonomous vehicle tracking, underwater video approaches, molecular genetics techniques, and accelerometry, among many others. Each detailed chapter offers new insights and promises for future studies of elasmobranch biology, provides an overview of appropriate uses of each technique, and can be readily extended to other aquatic fish and marine mammals and reptiles
An observational asteroseismic study of the pulsating B-type stars in the open cluster NGC 884
Recent progress in the seismic interpretation of field β Cep stars has resulted in improvements of the physical description in the stellar structure and evolution model computations of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition. We present an observational asteroseismic study based on the discovery of numerous multi-periodic and mono-periodic B-type stars in the open cluster NGC 884 (χ Persei). Our study illustrates the current status of ensemble asteroseismology of this young open cluste
Analysis of 70 Ophiuchi AB including seismic constraints
The analysis of solar-like oscillations for stars belonging to a binary
system provides a unique opportunity to probe the internal stellar structure
and to test our knowledge of stellar physics. Such oscillations have been
recently observed and characterized for the A component of the 70 Ophiuchi
system. A model of 70 Ophiuchi AB that correctly reproduces all observational
constraints available for both stars is determined. An age of 6.2 +- 1.0 Gyr is
found with an initial helium mass fraction Y_i=0.266 +- 0.015 and an initial
metallicity (Z/X)_i=0.0300 +- 0.0025 when atomic diffusion is included and a
solar value of the mixing-length parameter assumed. A precise and independent
determination of the value of the mixing-length parameter needed to model 70
Oph A requires accurate measurement of the mean small separation, which is not
available yet. Current asteroseismic observations, however, suggest that the
value of the mixing-length parameter of 70 Oph A is lower or equal to the solar
calibrated value. The effects of atomic diffusion and of the choice of the
adopted solar mixture were also studied. We also tested and compared the
theoretical tools used for the modeling of stars for which p-modes frequencies
are detected by performing this analysis with three different stellar evolution
codes and two different calibration methods. We found that the different
evolution codes and calibration methods we used led to perfectly coherent
results.Comment: 9 pages, 8 figures, accepted for publication in A&
Non-radial oscillations in the red giant HR7349 measured by CoRoT
Convection in red giant stars excites resonant acoustic waves whose
frequencies depend on the sound speed inside the star, which in turn depends on
the properties of the stellar interior. Therefore, asteroseismology is the most
robust available method for probing the internal structure of red giant stars.
Solar-like oscillations in the red giant HR7349 are investigated. Our study is
based on a time series of 380760 photometric measurements spread over 5 months
obtained with the CoRoT satellite. Mode parameters were estimated using maximum
likelihood estimation of the power spectrum. The power spectrum of the
high-precision time series clearly exhibits several identifiable peaks between
19 and 40 uHz showing regularity with a mean large and small spacing of Dnu =
3.47+-0.12 uHz and dnu_02 = 0.65+-0.10 uHz. Nineteen individual modes are
identified with amplitudes in the range from 35 to 115 ppm. The mode damping
time is estimated to be 14.7+4.7-2.9 days.Comment: 8 pages, A&A accepte
Oscillating red giants in the CoRoT exo-field: Asteroseismic mass and radius determination
Context. Observations and analysis of solar-type oscillations in red-giant
stars is an emerging aspect of asteroseismic analysis with a number of open
questions yet to be explored. Although stochastic oscillations have previously
been detected in red giants from both radial velocity and photometric
measurements, those data were either too short or had sampling that was not
complete enough to perform a detailed data analysis of the variability. The
quality and quantity of photometric data as provided by the CoRoT satellite is
necessary to provide a breakthrough in observing p-mode oscillations in red
giants. We have analyzed continuous photometric time-series of about 11 400
relatively faint stars obtained in the exofield of CoRoT during the first 150
days long-run campaign from May to October 2007. We find several hundred stars
showing a clear power excess in a frequency and amplitude range expected for
red-giant pulsators. In this paper we present first results on a sub-sample of
these stars. Aims. Knowing reliable fundamental parameters like mass and radius
is essential for detailed asteroseismic studies of red-giant stars. As the
CoRoT exofield targets are relatively faint (11-16 mag) there are no (or only
weak) constraints on the star's location in the H-R diagram. We therefore aim
to extract information about such fundamental parameters solely from the
available time series. Methods. We model the convective background noise and
the power excess hump due to pulsation with a global model fit and deduce
reliable estimates for the stellar mass and radius from scaling relations for
the frequency of maximum oscillation power and the characteristic frequency
separation.Comment: 10 pages, 7 figures, accepted for publication in A&
An Arbitrary Curvilinear Coordinate Method for Particle-In-Cell Modeling
A new approach to the kinetic simulation of plasmas in complex geometries,
based on the Particle-in- Cell (PIC) simulation method, is explored. In the two
dimensional (2d) electrostatic version of our method, called the Arbitrary
Curvilinear Coordinate PIC (ACC-PIC) method, all essential PIC operations are
carried out in 2d on a uniform grid on the unit square logical domain, and
mapped to a nonuniform boundary-fitted grid on the physical domain. As the
resulting logical grid equations of motion are not separable, we have developed
an extension of the semi-implicit Modified Leapfrog (ML) integration technique
to preserve the symplectic nature of the logical grid particle mover. A
generalized, curvilinear coordinate formulation of Poisson's equations to solve
for the electrostatic fields on the uniform logical grid is also developed. By
our formulation, we compute the plasma charge density on the logical grid based
on the particles' positions on the logical domain. That is, the plasma
particles are weighted to the uniform logical grid and the self-consistent mean
electrostatic fields obtained from the solution of the logical grid Poisson
equation are interpolated to the particle positions on the logical grid. This
process eliminates the complexity associated with the weighting and
interpolation processes on the nonuniform physical grid and allows us to run
the PIC method on arbitrary boundary-fitted meshes.Comment: Submitted to Computational Science & Discovery December 201
Effects of rotational mixing on the asteroseismic properties of solar-type stars
The influence of rotational mixing on the evolution and asteroseismic
properties of solar-type stars is studied. Rotational mixing changes the global
properties of a solar-type star with a significant increase of the effective
temperature resulting in a shift of the evolutionary track to the blue part of
the HR diagram. These differences are related to changes of the chemical
composition, because rotational mixing counteracts the effects of atomic
diffusion leading to larger helium surface abundances for rotating models than
for non-rotating ones. Higher values of the large frequency separation are then
found for rotating models than for non-rotating ones at the same evolutionary
stage, because the increase of the effective temperature leads to a smaller
radius and hence to an increase of the stellar mean density. Rotational mixing
also has a considerable impact on the structure and chemical composition of the
central stellar layers by bringing fresh hydrogen fuel to the core, thereby
enhancing the main-sequence lifetime. The increase of the central hydrogen
abundance together with the change of the chemical profiles in the central
layers result in a significant increase of the values of the small frequency
separations and of the ratio of the small to large separations for models
including shellular rotation. This increase is clearly seen for models with the
same age sharing the same initial parameters except for the inclusion of
rotation as well as for models with the same global stellar parameters and in
particular the same location in the HR diagram. By computing rotating models of
solar-type stars including the effects of a dynamo that possibly occurs in the
radiative zone, we find that the efficiency of rotational mixing is strongly
reduced when the effects of magnetic fields are taken into account, in contrast
to what happens in massive stars.Comment: 11 pages, 15 figures, accepted for publication in A&
The analytic structure of 2D Euler flow at short times
Using a very high precision spectral calculation applied to the
incompressible and inviscid flow with initial condition , we find that the width of its analyticity
strip follows a law at short times over eight decades. The
asymptotic equation governing the structure of spatial complex-space
singularities at short times (Frisch, Matsumoto and Bec 2003, J.Stat.Phys. 113,
761--781) is solved by a high-precision expansion method. Strong numerical
evidence is obtained that singularities have infinite vorticity and lie on a
complex manifold which is constructed explicitly as an envelope of analyticity
disks.Comment: 19 pages, 14 figures, published versio
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