421 research outputs found
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
Apsidal motion in the massive binary HD152218
Massive binary systems are important laboratories in which to probe the
properties of massive stars and stellar physics in general. In this context, we
analysed optical spectroscopy and photometry of the eccentric short-period
early-type binary HD 152218 in the young open cluster NGC 6231. We
reconstructed the spectra of the individual stars using a separating code. The
individual spectra were then compared with synthetic spectra obtained with the
CMFGEN model atmosphere code. We furthermore analysed the light curve of the
binary and used it to constrain the orbital inclination and to derive absolute
masses of 19.8 +/- 1.5 and 15.0 +/- 1.1 solar masses. Combining radial velocity
measurements from over 60 years, we show that the system displays apsidal
motion at a rate of (2.04^{+.23}_{-.24}) degree/year. Solving the
Clairaut-Radau equation, we used stellar evolution models, obtained with the
CLES code, to compute the internal structure constants and to evaluate the
theoretically predicted rate of apsidal motion as a function of stellar age and
primary mass. In this way, we determine an age of 5.8 +/- 0.6 Myr for HD
152218, which is towards the higher end of, but compatible with, the range of
ages of the massive star population of NGC 6231 as determined from isochrone
fitting.Comment: Accepted for publication in Astronomy & Astrophysic
Volume 3: Ethnographies of Islam : Ritual Performances and Everyday Practices
This comparative approach to the various uses of the ethnographic method in research about Islam in anthropology and other social sciences is particularly relevant in the current climate. Political discourses and stereotypical media portrayals of Islam as a monolithic civilisation have prevented the emergence of cultural pluralism and individual freedom.
This book counters such discourses by showing the diversity and plurality of Muslim societies and by promoting reflection on how the ethnographic method allows the description, representation and analysis of the social and cultural complexity of Muslim societies in the discourse of anthropology.https://ecommons.aku.edu/uk_ismc_series_emc/1006/thumbnail.jp
The CoRoT target HD 49933: 2- Comparison of theoretical mode amplitudes with observations
From the seismic data obtained by CoRoT for the star HD 49933 it is possible,
as for the Sun, to constrain models of the excitation of acoustic modes by
turbulent convection. We compare a stochastic excitation model described in
Paper I (arXiv:0910.4027) with the asteroseismology data for HD 49933, a star
that is rather metal poor and significantly hotter than the Sun. Using the mode
linewidths measured by CoRoT for HD 49933 and the theoretical mode excitation
rates computed in Paper I, we derive the expected surface velocity amplitudes
of the acoustic modes detected in HD 49933. Using a calibrated quasi-adiabatic
approximation relating the mode amplitudes in intensity to those in velocity,
we derive the expected values of the mode amplitude in intensity. Our amplitude
calculations are within 1-sigma error bars of the mode surface velocity
spectrum derived with the HARPS spectrograph. The same is found with the mode
amplitudes in intensity derived for HD 49933 from the CoRoT data. On the other
hand, at high frequency, our calculations significantly depart from the CoRoT
and HARPS measurements. We show that assuming a solar metal abundance rather
than the actual metal abundance of the star would result in a larger
discrepancy with the seismic data. Furthermore, calculations that assume the
``new'' solar chemical mixture are in better agreement with the seismic data
than those that assume the ``old'' solar chemical mixture. These results
validate, in the case of a star significantly hotter than the Sun and Alpha Cen
A, the main assumptions in the model of stochastic excitation. However, the
discrepancies seen at high frequency highlight some deficiencies of the
modelling, whose origin remains to be understood.Comment: 8 pages, 3 figures (B-W and color), accepted for publication in
Astronomy & Astrophysics. Corrected typo in Eq. (4). Updated references.
Language improvement
Influence of external flows on crystal growth: numerical investigation
We use a combined phase-field/lattice-Boltzmann scheme [D. Medvedev, K.
Kassner, Phys. Rev. E {\bf 72}, 056703 (2005)] to simulate non-facetted crystal
growth from an undercooled melt in external flows. Selected growth parameters
are determined numerically.
For growth patterns at moderate to high undercooling and relatively large
anisotropy, the values of the tip radius and selection parameter plotted as a
function of the Peclet number fall approximately on single curves. Hence, it
may be argued that a parallel flow changes the selected tip radius and growth
velocity solely by modifying (increasing) the Peclet number. This has
interesting implications for the availability of current selection theories as
predictors of growth characteristics under flow.
At smaller anisotropy, a modification of the morphology diagram in the plane
undercooling versus anisotropy is observed. The transition line from dendrites
to doublons is shifted in favour of dendritic patterns, which become faster
than doublons as the flow speed is increased, thus rendering the basin of
attraction of dendritic structures larger.
For small anisotropy and Prandtl number, we find oscillations of the tip
velocity in the presence of flow. On increasing the fluid viscosity or
decreasing the flow velocity, we observe a reduction in the amplitude of these
oscillations.Comment: 10 pages, 7 figures, accepted for Physical Review E; size of some
images had to be substantially reduced in comparison to original, resulting
in low qualit
The CoRoT target HD 49933 . I. Effect of the metal abundance on the mode excitation rates
Context. Solar-like oscillations are stochastically excited by turbulent convection at the surface layers of the stars. Aims: We study the role of the surface metal abundance on the efficiency of the stochastic driving in the case of the CoRoT target HD 49933. Methods: We compute two 3D hydrodynamical simulations representative - in effective temperature and gravity - of the surface layers of the CoRoT target HD 49933, a star that is rather metal poor and significantly hotter than the Sun. One 3D simulation has a solar metal abundance, and the other has a surface iron-to-hydrogen, [Fe/H], abundance ten times smaller. For each 3D simulation we match an associated global 1D model, and we compute the associated acoustic modes using a theoretical model of stochastic excitation validated in the case of the Sun and α Cen A. Results: The rate at which energy is supplied per unit time into the acoustic modes associated with the 3D simulation with [Fe/H] = -1 is found to be about three times smaller than those associated with the 3D simulation with [Fe/H] = 0. As shown here, these differences are related to the fact that low metallicity implies surface layers with a higher mean density. In turn, a higher mean density favors smaller convective velocities and hence less efficient driving of the acoustic modes. Conclusions: Our result shows the importance of taking the surface metal abundance into account in the modeling of the mode driving by turbulent convection. A comparison with observational data is presented in a companion paper using seismic data obtained for the CoRoT target HD 49933. The CoRoT space mission, launched on December 27, 2006, has been developped and is operated by CNES, with the contribution of Austria, Belgium, Brasil, ESA, Germany and Spain
Apsidal motion in the massive binary HD 152248 -- Constraining the internal structure of the stars
Apsidal motion in massive eccentric binaries offers precious information
about the internal structure of the stars. This is especially true for twin
binaries consisting of two nearly identical stars. We make use of the tidally
induced apsidal motion in the twin binary HD152248 to infer constraints on the
internal structure of the O7.5 III-II stars composing this system. We build
stellar evolution models with the code Cl\'es assuming different prescriptions
for the internal mixing occurring inside the stars. We identify the models that
best reproduce the observationally determined present-day properties of the
components of HD152248, as well as their , and the apsidal motion rate of
the system. We analyse the impact of some poorly constrained input parameters,
including overshooting, turbulent diffusion, and metallicity. We further build
'single' and 'binary' GENEC models that account for stellar rotation to
investigate the impacts of binarity and rotation. We discuss some effects that
could bias our interpretation of the apsidal motion in terms of the internal
structure constant. Reproducing the observed value and rate of apsidal
motion simultaneously with the other stellar parameters requires a significant
amount of internal mixing or enhanced mass-loss. The results suggest that a
single-star evolution model is sufficient to describe the physics inside this
binary system. Qualitatively, the high turbulent diffusion required to
reproduce the observations could be partly attributed to stellar rotation.
Higher-order terms in the apsidal motion are negligible. Only a very severe
misalignment of the rotation axes could significantly impact the rate of
apsidal motion, but such a high misalignment is highly unlikely in such a
binary system. We infer an age estimate of Myr for the binary and
initial masses of M for both stars.Comment: 16 pages, A&A accepte
Statistical properties of energy levels of chaotic systems: Wigner or non-Wigner
For systems whose classical dynamics is chaotic, it is generally believed
that the local statistical properties of the quantum energy levels are well
described by Random Matrix Theory. We present here two counterexamples - the
hydrogen atom in a magnetic field and the quartic oscillator - which display
nearest neighbor statistics strongly different from the usual Wigner
distribution. We interpret the results with a simple model using a set of
regular states coupled to a set of chaotic states modeled by a random matrix.Comment: 10 pages, Revtex 3.0 + 4 .ps figures tar-compressed using uufiles
package, use csh to unpack (on Unix machine), to be published in Phys. Rev.
Let
- …