69 research outputs found
The coupling between internal waves and shear-induced turbulence in stellar radiation zones: the critical layer
Internal gravity waves (hereafter IGWs) are known as one of the candidates
for explaining the angular velocity profile in the Sun and in solar-type
main-sequence and evolved stars, due to their role in the transport of angular
momentum. Our bringing concerns critical layers, a process poorly explored in
stellar physics, defined as the location where the local relative frequency of
a given wave to the rotational frequency of the fluid tends to zero (i.e that
corresponds to co-rotation resonances). IGW propagate through stably-stratified
radiative regions, where they extract or deposit angular momentum through two
processes: radiative and viscous dampings and critical layers. Our goal is to
obtain a complete picture of the effects of this latters. First, we expose a
mathematical resolution of the equation of propagation for IGWs in adiabatic
and non-adiabatic cases near critical layers. Then, the use of a dynamical
stellar evolution code, which treats the secular transport of angular momentum,
allows us to apply these results to the case of a solar-like star.The analysis
reveals two cases depending on the value of the Richardson number at critical
layers: a stable one, where IGWs are attenuated as they pass through a critical
level, and an unstable turbulent case where they can be reflected/transmitted
by the critical level with a coefficient larger than one. Such
over-reflection/transmission can have strong implications on our vision of
angular momentum transport in stellar interiors. This paper highlights the
existence of two regimes defining the interaction between an IGW and a critical
layer. An application exposes the effect of the first regime, showing a
strengthening of the damping of the wave. Moreover, this work opens new ways
concerning the coupling between IGWs and shear instabilities in stellar
interiors.Comment: 17 pages, 8 figure
Tidal dynamics of extended bodies in planetary systems and multiple stars
With the discovery during the past decade of a large number of extrasolar
planets orbiting their parent stars at a distance lower than 0.1 astronomical
unit (and the launch and the preparation of dedicated space missions such as
CoRoT and KEPLER), with the position of inner natural satellites around giant
planets in our Solar System and with the existence of very closed but separated
binary stars, tidal interaction has to be carefully studied. In particular, a
question arises about the validity of usual approximations used in the
modelling of this interaction. The purpose of this paper is to examine the step
beyond the ponctual approximation for the tidal perturber. To achieve this aim,
the gravitational interaction between two extended bodies and more precisely
the interaction between mass multipole moments of their gravitational fields
and the associated tidal phenomena are studied. Use of Cartesian Symmetric
Trace Free (STF) tensors, of their relation with spherical harmonics and of the
Kaula's transform enables to derive analytically the tidal and mutual
interaction potentials as well as the associated disturbing functions in
extended bodies systems. The tidal and mutual interaction potentials of two
extended bodies are derived. Next, the external gravitational potential of such
tidally disturbed extended body is obtained, using the Love's number theory, as
well as the associated disturbing function. Finally, the dynamical evolution
equations for such systems are given in their more general form without any
linearization. The dynamical equations for the gravitational and tidal
interactions between extended bodies and associated dynamics are derived in a
form where they could be directly implemented to perform coherent numerical
simulations of planetary systems or multiple stars tidal evolution.Comment: accepted for publication in Astronomy and Astrophysic
Angular Momentum Transport in Stellar Interiors
Stars lose a significant amount of angular momentum between birth and death,
implying that efficient processes transporting it from the core to the surface
are active. Space asteroseismology delivered the interior rotation rates of
more than a thousand low- and intermediate-mass stars, revealing that: 1)
single stars rotate nearly uniformly during the core hydrogen and core helium
burning phases; 2) stellar cores spin up to a factor 10 faster than the
envelope during the red giant phase; 3) the angular momentum of the
helium-burning core of stars is in agreement with the angular momentum of white
dwarfs. Observations reveal a strong decrease of core angular momentum when
stars have a convective core. Current theory of angular momentum transport
fails to explain this. We propose improving the theory with a data-driven
approach, whereby angular momentum prescriptions derived from multi-dimensional
(magneto)hydrodynamical simulations and theoretical considerations are
continously tested against modern observations. The TESS and PLATO space
missions have the potential to derive the interior rotation of large samples of
stars, including high-mass and metal-poor stars in binaries and clusters. This
will provide the powerful observational constraints needed to improve theory
and simulations.Comment: Manuscript submitted to Annual Reviews of Astronomy and Astrophysics
for Volume 57. This is the authors' submitted version. Revisions and the
final version will only become available from
https://www.annualreviews.org/journal/astr
Sub-Inertial Gravity Modes in the B8V Star KIC 7760680 Reveal Moderate Core Overshooting and Low Vertical Diffusive Mixing
KIC 7760680 is so far the richest slowly pulsating B star, by exhibiting 36
consecutive dipole () gravity (g-) modes. The monotonically decreasing
period spacing of the series, in addition to the local dips in the pattern
confirm that KIC 7760680 is a moderate rotator, with clear mode trapping in
chemically inhomogeneous layers. We employ the traditional approximation of
rotation to incorporate rotational effects on g-mode frequencies. Our detailed
forward asteroseismic modelling of this g-mode series reveals that KIC 7760680
is a moderately rotating B star with mass M. By
simultaneously matching the slope of the period spacing, and the number of
modes in the observed frequency range, we deduce that the equatorial rotation
frequency of KIC 7760680 is 0.4805 day, which is 26\% of its Roche break
up frequency. The relative deviation of the model frequencies and those
observed is less than one percent. We succeed to tightly constrain the
exponentially-decaying convective core overshooting parameter to . This means that convective core overshooting can
coexist with moderate rotation. Moreover, models with exponentially-decaying
overshoot from the core outperform those with the classical step-function
overshoot. The best value for extra diffusive mixing in the radiatively stable
envelope is confined to (with in cm sec), which is notably smaller than theoretical
predictions.Comment: 12 Figures, 2 Tables, all data publicly available for download;
accepted for publication in Astrophysical Journa
Extracting Star Formation Histories from Medium-resolution Galaxy Spectra
We adapt an existing data compression algorithm, MOPED, to the extraction of
median-likelihood star formation (SF) histories from medium-resolution galaxy
spectra. By focusing on the high-pass components of galaxy spectra, we minimize
potential uncertainties arising from the spectro-photometric calibration and
intrinsic attenuation by dust. We validate our approach using model high-pass
spectra of galaxies with different SF histories covering the wavelength range
3650-8500 A at a resolving power of about 2000. We show that the method can
recover the full SF histories of these models, without prior knowledge of the
metallicity, to within an accuracy that depends sensitively on signal-to-noise
ratio. The investigation of the sensitivity of the flux at each wavelength to
the mass fraction of stars of different ages allows us to identify new
age-sensitive features in galaxy spectra. We also highlight a fundamental
limitation in the recovery of the SF histories of galaxies for which the
optical signatures of intermediate-age stars are masked by those of younger and
older stars. We apply this method to derive average SF histories from the
highest-quality spectra of morphologically identified early- and late-type
galaxies in the SDSS EDR [...]. We also investigate the constraints set by the
high-pass signal in the stacked spectra of a magnitude-limited sample of
SDSS-EDR galaxies on the global SF history of the Universe. We confirm that the
stellar populations in the most massive galaxies today appear to have formed on
average earlier than those in the least massive ones. Our results do not
support the recent suggestion of a statistically significant peak in the SF
activity of the Universe at redshifts below unity, although such a peak is not
ruled out [abridged].Comment: 18 pages, 14 figures, to appear in MNRAS; version with full
resolution figures available at http://www.mpa-garching.mpg.de/~charlot/SFH
Spin alignment of stars in old open clusters
Stellar clusters form by gravitational collapse of turbulent molecular
clouds, with up to several thousand stars per cluster. They are thought to be
the birthplace of most stars and therefore play an important role in our
understanding of star formation, a fundamental problem in astrophysics. The
initial conditions of the molecular cloud establish its dynamical history until
the stellar cluster is born. However, the evolution of the cloud's angular
momentum during cluster formation is not well understood. Current observations
have suggested that turbulence scrambles the angular momentum of the
cluster-forming cloud, preventing spin alignment amongst stars within a
cluster. Here we use asteroseismology to measure the inclination angles of spin
axes in 48 stars from the two old open clusters NGC~6791 and NGC~6819. The
stars within each cluster show strong alignment. Three-dimensional
hydrodynamical simulations of proto-cluster formation show that at least 50 %
of the initial proto-cluster kinetic energy has to be rotational in order to
obtain strong stellar-spin alignment within a cluster. Our result indicates
that the global angular momentum of the cluster-forming clouds was efficiently
transferred to each star and that its imprint has survived after several
gigayears since the clusters formed.Comment: 14 pages, 3 figures, 1 table. Published in Nature Astronom
La(, ) cross sections constrained with statistical decay properties of La nuclei
The nuclear level densities and -ray strength functions of
La were measured using the La(He, ),
La(He, He) and La(d, p) reactions. The
particle- coincidences were recorded with the silicon particle
telescope (SiRi) and NaI(Tl) (CACTUS) arrays. In the context of these
experimental results, the low-energy enhancement in the A140 region is
discussed. The La( cross sections were calculated
at - and -process temperatures using the experimentally measured nuclear
level densities and -ray strength functions. Good agreement is found
between La( calculated cross sections and previous
measurements
The star formation histories of galaxies in the Sloan Digital Sky Survey
We present the results of a MOPED analysis of ~3 x 10^5 galaxy spectra from
the Sloan Digital Sky Survey Data Release Three (SDSS DR3), with a number of
improvements in data, modelling and analysis compared with our previous
analysis of DR1. The improvements include: modelling the galaxies with
theoretical models at a higher spectral resolution of 3\AA; better calibrated
data; an extended list of excluded emission lines, and a wider range of dust
models. We present new estimates of the cosmic star formation rate, the
evolution of stellar mass density and the stellar mass function from the fossil
record. In contrast to our earlier work the results show no conclusive peak in
the star formation rate out to a redshift around 2 but continue to show
conclusive evidence for `downsizing' in the SDSS fossil record. The star
formation history is now in good agreement with more traditional instantaneous
measures. The galaxy stellar mass function is determined over five decades of
mass, and an updated estimate of the current stellar mass density is presented.
We also investigate the systematic effects of changes in the stellar population
modelling, the spectral resolution, dust modelling, sky lines, spectral
resolution and the change of data set. We find that the main changes in the
results are due to the improvements in the calibration of the SDSS data,
changes in the initial mass function and the theoretical models used.Comment: replaced to match accepted version in MNRA
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