380 research outputs found
Angular momentum transport efficiency in post-main sequence low-mass stars
Context. Using asteroseismic techniques, it has recently become possible to
probe the internal rotation profile of low-mass (~1.1-1.5 Msun) subgiant and
red giant stars. Under the assumption of local angular momentum conservation,
the core contraction and envelope expansion occurring at the end of the main
sequence would result in a much larger internal differential rotation than
observed. This suggests that angular momentum redistribution must be taking
place in the interior of these stars. Aims. We investigate the physical nature
of the angular momentum redistribution mechanisms operating in stellar
interiors by constraining the efficiency of post-main sequence rotational
coupling. Methods. We model the rotational evolution of a 1.25 Msun star using
the Yale Rotational stellar Evolution Code. Our models take into account the
magnetic wind braking occurring at the surface of the star and the angular
momentum transport in the interior, with an efficiency dependent on the degree
of internal differential rotation. Results. We find that models including a
dependence of the angular momentum transport efficiency on the radial
rotational shear reproduce very well the observations. The best fit of the data
is obtained with an angular momentum transport coefficient scaling with the
ratio of the rotation rate of the radiative interior over that of the
convective envelope of the star as a power law of exponent ~3. This scaling is
consistent with the predictions of recent numerical simulations of the
Azimuthal Magneto-Rotational Instability. Conclusions. We show that an angular
momentum transport process whose efficiency varies during the stellar evolution
through a dependence on the level of internal differential rotation is required
to explain the observed post-main sequence rotational evolution of low-mass
stars.Comment: 8 pages, 6 figures; accepted for publication in Astronomy &
Astrophysic
Seismic evidence for a weak radial differential rotation in intermediate-mass core helium burning stars
The detection of mixed modes that are split by rotation in Kepler red giants
has made it possible to probe the internal rotation profiles of these stars,
which brings new constraints on the transport of angular momentum in stars.
Mosser et al. (2012) have measured the rotation rates in the central regions of
intermediate-mass core helium burning stars (secondary clump stars). Our aim
was to exploit& the rotational splittings of mixed modes to estimate the amount
of radial differential rotation in the interior of secondary clump stars using
Kepler data, in order to place constraints on angular momentum transport in
intermediate-mass stars. We selected a subsample of Kepler secondary clump
stars with mixed modes that are clearly rotationally split. By applying a
thorough statistical analysis, we showed that the splittings of both
gravity-dominated modes (trapped in central regions) and p-dominated modes
(trapped in the envelope) can be measured. We then used these splittings to
estimate the amount of differential rotation by using inversion techniques and
by applying a simplified approach based on asymptotic theory (Goupil et al.
2013). We obtained evidence for a weak radial differential rotation for six of
the seven targets that were selected, with the central regions rotating
to times faster than the envelope. The last target was
found to be consistent with a solid-body rotation. This demonstrates that an
efficient redistribution of angular momentum occurs after the end of the main
sequence in the interior of intermediate-mass stars, either during the
short-lived subgiant phase, or once He-burning has started in the core. In
either case, this should bring constraints on the angular momentum transport
mechanisms that are at work.Comment: 16 pages, 8 figures, accepted in A&
Strong Approximation of Empirical Copula Processes by Gaussian Processes
We provide the strong approximation of empirical copula processes by a
Gaussian process. In addition we establish a strong approximation of the
smoothed empirical copula processes and a law of iterated logarithm
A transmission problem across a fractal self-similar interface
We consider a transmission problem in which the interior domain has
infinitely ramified structures. Transmission between the interior and exterior
domains occurs only at the fractal component of the interface between the
interior and exterior domains. We also consider the sequence of the
transmission problems in which the interior domain is obtained by stopping the
self-similar construction after a finite number of steps; the transmission
condition is then posed on a prefractal approximation of the fractal interface.
We prove the convergence in the sense of Mosco of the energy forms associated
with these problems to the energy form of the limit problem. In particular,
this implies the convergence of the solutions of the approximated problems to
the solution of the problem with fractal interface. The proof relies in
particular on an extension property. Emphasis is put on the geometry of the
ramified domain. The convergence result is obtained when the fractal interface
has no self-contact, and in a particular geometry with self-contacts, for which
an extension result is proved
Properties of oscillation modes in subgiant stars observed by Kepler
Mixed modes seen in evolved stars carry information on their deeper layers
that can place stringent constraints on their physics and on their global
properties (mass, age, etc...). In this study, we present a method to identify
and measure all oscillatory mode characteristics (frequency, height, width).
Analyzing four subgiants stars, we present the first measure of the effect of
the degree of mixture on the l=1 mixed modes characteristics. We also show that
some stars have measurable l=2 mixed modes and discuss the interest of their
measure to constrain the deeper layers of stars.Comment: Accepted to Ap
Estimating stellar mean density through seismic inversions
Determining the mass of stars is crucial both to improving stellar evolution
theory and to characterising exoplanetary systems. Asteroseismology offers a
promising way to estimate stellar mean density. When combined with accurate
radii determinations, such as is expected from GAIA, this yields accurate
stellar masses. The main difficulty is finding the best way to extract the mean
density from a set of observed frequencies.
We seek to establish a new method for estimating stellar mean density, which
combines the simplicity of a scaling law while providing the accuracy of an
inversion technique.
We provide a framework in which to construct and evaluate kernel-based linear
inversions which yield directly the mean density of a star. We then describe
three different inversion techniques (SOLA and two scaling laws) and apply them
to the sun, several test cases and three stars.
The SOLA approach and the scaling law based on the surface correcting
technique described by Kjeldsen et al. (2008) yield comparable results which
can reach an accuracy of 0.5 % and are better than scaling the large frequency
separation. The reason for this is that the averaging kernels from the two
first methods are comparable in quality and are better than what is obtained
with the large frequency separation. It is also shown that scaling the large
frequency separation is more sensitive to near-surface effects, but is much
less affected by an incorrect mode identification. As a result, one can
identify pulsation modes by looking for an l and n assignment which provides
the best agreement between the results from the large frequency separation and
those from one of the two other methods. Non-linear effects are also discussed
as is the effects of mixed modes. In particular, it is shown that mixed modes
bring little improvement as a result of their poorly adapted kernels.Comment: Accepted for publication in A&A, 20 pages, 19 figure
Open issues in probing interiors of solar-like oscillating main sequence stars: 2. Diversity in the HR diagram
We review some major open issues in the current modelling of low and
intermediate mass, main sequence stars based on seismological studies. The
solar case was discussed in a companion paper, here several issues specific to
other stars than the Sun are illustrated with a few stars observed with CoRoT
and expectations from Kepler data.Comment: GONG 2010 - SoHO 24, A new era of seismology of the Sun and
solar-like stars, To be published in the Journal of Physics: Conference
Series (JPCS
Asteroseismic measurement of slow, nearly uniform surface-to-core rotation in the main-sequence F star KIC 9244992
We have found a rotationally split series of core g-mode triplets and surface p-mode multiplets in a main-sequence F star, KIC 9244992. Comparison with models shows that the star has a mass of about 1.45 M�, and is at an advanced stage of main-sequence evolution in which the central hydrogen abundance mass fraction is reduced to about 0.1. This is the second case, following KIC 11145123, of an asteroseismic determination of the rotation of the deep core and surface of an A-F main-sequence star. We have found, essentially model independently, that the rotation near the surface, obtained from p-mode splittings, is 66 d, slightly slower than the rotation of 64 d in the core, measured by g-mode splittings. KIC 9244992 is similar to KIC 11145123 in that both are near the end of main-sequence stage with very slow and nearly uniform rotation. This indicates the angular momentum transport in the interior of an A-F star during the main-sequence stage is much stronger than that expected from standard theoretical formulations
Amplitudes and lifetimes of solar-like oscillations observed by CoRoT* Red-giant versus main-sequence stars
Context. The advent of space-borne missions such as CoRoT or Kepler providing
photometric data has brought new possibilities for asteroseismology across the
H-R diagram. Solar-like oscillations are now observed in many stars, including
red giants and main- sequence stars. Aims. Based on several hundred identified
pulsating red giants, we aim to characterize their oscillation amplitudes and
widths. These observables are compared with those of main-sequence stars in
order to test trends and scaling laws for these parameters for both
main-sequence stars and red giants. Methods. An automated fitting procedure is
used to analyze several hundred Fourier spectra. For each star, a modeled
spectrum is fitted to the observed oscillation spectrum, and mode parameters
are derived. Results. Amplitudes and widths of red-giant solar-like
oscillations are estimated for several hundred modes of oscillation. Amplitudes
are relatively high (several hundred ppm) and widths relatively small (very few
tenths of a {\mu}Hz). Conclusions. Widths measured in main-sequence stars show
a different variation with the effective temperature than red giants. A single
scaling law is derived for mode amplitudes of both red giants and main-sequence
stars versus their luminosity to mass ratio. However, our results suggest that
two regimes may also be compatible with the observations.Comment: Accepted in A&A on 2011 February 8th, now includes corrections
(results now more precise on \Gamma and A_max in Section 4.3 and 4.4, fig. 7
corrected consequently
Nonparametric Inference for Copulas and Measures of Dependence Under Length-Biased Sampling and Informative Censoring
Length-biased data are often encountered in cross-sectional surveys and prevalent-cohort studies on disease durations. Under length-biased sampling subjects with longer disease durations have greater chance to be observed. As a result, covariate values linked to the longer survivors are favored by the sampling mechanism. When the sampled durations are also subject to right censoring, the censoring is informative. Modeling dependence structure without adjusting for these issues leads to biased results. In this article, we consider copulas for modeling dependence when the collected data are length-biased and account for both informative censoring and covariate bias that are naturally linked to length-biased sampling. We address nonparametric estimation of the bivariate distribution, copula function and its density, and Kendall and Spearman measures for right-censored length-biased data. The proposed estimator for the bivariate cdf is a Hadamard-differentiable functional of two MLEs (Kaplan–Meier and empirical cdf) and inherits their efficiency. Based on this estimator, we devise two estimators for copula function and a local-polynomial estimator for copula density that accounts for boundary bias. The limiting processes of the estimators are established by deriving their iid representations. As a by-product, we establish the oscillation behavior of the bivariate cdf estimator. In addition, we introduce estimators for Kendall and Spearman measures and study their weak convergence. The proposed method is applied to analyze a set of right-censored length-biased data on survival with dementia, collected as part of a nationwide study in Canada
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