4,067 research outputs found
Stable higher order finite-difference schemes for stellar pulsation calculations
Context: Calculating stellar pulsations requires a sufficient accuracy to
match the quality of the observations. Many current pulsation codes apply a
second order finite-difference scheme, combined with Richardson extrapolation
to reach fourth order accuracy on eigenfunctions. Although this is a simple and
robust approach, a number of drawbacks exist thus making fourth order schemes
desirable. A robust and simple finite-difference scheme, which can easily be
implemented in either 1D or 2D stellar pulsation codes is therefore required.
Aims: One of the difficulties in setting up higher order finite-difference
schemes for stellar pulsations is the so-called mesh-drift instability. Current
ways of dealing with this defect include introducing artificial viscosity or
applying a staggered grids approach. However these remedies are not well-suited
to eigenvalue problems, especially those involving non-dissipative systems,
because they unduly change the spectrum of the operator, introduce
supplementary free parameters, or lead to complications when applying boundary
conditions.
Methods: We propose here a new method, inspired from the staggered grids
strategy, which removes this instability while bypassing the above
difficulties. Furthermore, this approach lends itself to superconvergence, a
process in which the accuracy of the finite differences is boosted by one
order.
Results: This new approach is shown to be accurate, flexible with respect to
the underlying grid, and able to remove mesh-drift.Comment: 15 pages, 11 figures, accepted for publication in A&
Inequalities on stellar rotational splittings derived from assumptions on the rotation profile
Context: A number of pulsating stars with rotational splittings have been
observed thanks to the CoRoT and Kepler missions. This is particularly true of
evolved (sub-giant and giant) stars, and has led various groups to investigate
their rotation profiles via different methods.
Aims: We would like to set up some criteria which will help us to know
whether a decreasing rotation profile, or one which satisfies Rayleigh's
stability criterion, is compatible with a set of observed rotational splittings
for a given reference model.
Methods: We derive inequalities on the rotational splittings using a
reformulated version of the equation which relates the splittings to the
rotation profile and kernels.
Results: These inequalities are tested out on some simple examples. The first
examples show how they are able to reveal when a rotation profile is increasing
somewhere or inconsistent with Rayleigh's criterion in a main sequence star,
depending on the profile and the values of the splittings. The next
example illustrates how a slight mismatch between an observed evolved star and
a reference model can lead to erroneous conclusions about the rotation profile.
We also show how frequency differences between the star and the model, which
should normally reveal this mismatch, can be masked by frequency corrections
for near-surface effects.Comment: 15 pages, 19 figures, accepted for publication in A&
Using seismic inversions to obtain an internal mixing processes indicator for main-sequence solar-like stars
Determining accurate and precise stellar ages is a major problem in
astrophysics. These determinations are either obtained through empirical
relations or model-dependent approaches. Currently, seismic modelling is one of
the best ways of providing accurate ages. However, current methods are affected
by simplifying assumptions concerning mixing processes. In this context,
providing new structural indicators which are less model-dependent and more
sensitive to such processes is crucial. We build a new indicator for core
conditions on the main sequence, which should be more sensitive to structural
differences and applicable to older stars than the indicator t presented in a
previous paper. We also wish to analyse the importance of the number and type
of modes for the inversion, as well as the impact of various constraints and
levels of accuracy in the forward modelling process that is used to obtain
reference models for the inversion. First, we present a method to obtain new
structural kernels and use them to build an indicator of central conditions in
stars and test it for various effects including atomic diffusion, various
initial helium abundances and metallicities, following the seismic inversion
method presented in our previous paper. We then study its accuracy for 7
different pulsation spectra including those of 16CygA and 16CygB and analyse
its dependence on the reference model by using different constraints and levels
of accuracy for its selection We observe that the inversion of the new
indicator using the SOLA method provides a good diagnostic for additional
mixing processes in central regions of stars. Its sensitivity allows us to test
for diffusive processes and chemical composition mismatch. We also observe that
octupole modes can improve the accuracy of the results, as well as modes of low
radial order.Comment: Accepted for publication in Astronomy and Astrophysic
Constraints on the structure of 16 Cyg A and 16 Cyg B using inversion techniques
Constraining mixing processes and chemical composition is a central problem
in stellar physics as their impact on stellar age determinations leads to
biases in our studies of stellar evolution, galactic history and exoplanetary
systems. In two previous papers, we showed how seismic inversion techniques
could offer strong constraints on such processes by pointing out weaknesses in
theoretical models. We now apply our technique to the solar analogues 16CygA
and 16CygB, being amongst the best targets in the Kepler field to test the
diagnostic potential of seismic inversions. The combination of various seismic
indicators helps to provide more constrained and accurate fundamendal
parameters for these stars. We use the latest seismic, spectroscopic and
interferometric observational constraints in the litterature for this system to
determine reference models independently for both stars. We carry out seismic
inversions of the acoustic radius, the mean density and a core conditions
indicator. We note that a degeneracy exists for the reference models. Namely,
changing the diffusion coefficient or the chemical composition within the
observational values leads to 5% changes in mass, 3% changes in radius and up
to 8% changes in age. We use acoustic radius and mean density inversions to
improve our reference models then carry out inversions for a core conditions
indicator. Thanks to its sensitivity to microscopic diffusion and chemical
composition mismatches, we are able to reduce the mass dispersion to 2%, namely
[0.96, 1.0] M_sun, the radius dispersion to 1%, namely [1.188, 1.200] R_sun and
the age dispersion to 3%, namely [7.0, 7.4] Gy, for 16CygA. For 16CygB, we can
check the consistency of the models but not reduce independently the age
dispersion. Nonetheless, assuming consistency with the age of 16CygA helps to
further constrain its mass and radius.Comment: Submitted to Astronomy and Astrophysic
Regular Oscillation Sub-spectrum of Rapidly Rotating Stars
We present an asymptotic theory that describes regular frequency spacings of
pressure modes in rapidly rotating stars. We use an asymptotic method based on
an approximate solution of the pressure wave equation constructed from a stable
periodic solution of the ray limit. The approximate solution has a Gaussian
envelope around the stable ray, and its quantization yields the frequency
spectrum. We construct semi-analytical formulas for regular frequency spacings
and mode spatial distributions of a subclass of pressure modes in rapidly
rotating stars. The results of these formulas are in good agreement with
numerical data for oscillations in polytropic stellar models. The regular
frequency spacings depend explicitly on internal properties of the star, and
their computation for different rotation rates gives new insights on the
evolution of mode frequencies with rotation.Comment: 14 pages, 10 figure
Mode identification in rapidly rotating stars
Context: Recent calculations of pulsation modes in rapidly rotating polytropic models and models based on the Self-Consistent Field method have shown that the frequency spectrum of low degree pulsation modes can be described by an empirical formula similar to Tassoul's asymptotic formula, provided that the underlying rotation profile is not too differential.
Aims: Given the simplicity of this asymptotic formula, we investigate whether it can provide a means by which to identify pulsation modes in rapidly rotating stars.
Methods: We develop a new mode identification scheme which consists in scanning a multidimensional parameter space for the formula coefficients which yield the best-fitting asymptotic spectra. This mode identification scheme is then tested on artificial spectra based on the asymptotic formula, on random frequencies and on spectra based on full numerical eigenmode calculations for which the mode identification is known beforehand. We also investigate the effects of adding random frequencies to mimic the effects of chaotic modes which are also expected to show up in such stars.
Results: In the absence of chaotic modes, it is possible to accurately find a correct mode identification for most of the observed frequencies provided these frequencies are sufficiently close to their asymptotic values. The addition of random frequencies can very quickly become problematic and hinder correct mode identification. Modifying the mode identification scheme to reject the worst fitting modes can bring some improvement but the results still remain poorer than in the case without chaotic modes
Insect (Arthropoda: Insecta) Composition in the Diet of Ornate Box Turtles (Terrapene ornata ornata) in Two Western Illinois Sand Prairies, with a New State Record for Cyclocephala (Coleoptera: Scarabaeidae)
A study of fecal samples collected over a two-year period from juvenile ornate box turtles (Terrapene ornata ornata Agassiz) revealed diets consisting of six orders of insects representing 19 families. Turtles were reared in captivity from eggs harvested from local, wild populations, and released at two remnant prairies. Identifiable insect fragments were found in 94% of samples in 2013 (n=33) and 96% in 2014 (n=25). Frequency of occurrence of insects in turtle feces is similar to results reported in previous studies of midwestern Terrapene species. A comparison of insect composition presented no significant difference between release sites. There is no significant difference in consumed insect species between turtles released into or outside of a fenced enclosure at the same site. Specimens of Cyclocephala longula LeConte collected during this study represent a new state record for Illinois
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