5 research outputs found
Turbulent transport by diffusive stratified shear flows: from local to global models. III. A closure model
Being able to account for the missing mixing in stellar radiative zones is a
key step toward a better understanding of stellar evolution. Zahn (1974) argued
that thermally diffusive shear-induced turbulence might be responsible for some
of this mixing. In Part I and Part II of this series of papers we showed that
Zahn's (1974, 1992) mixing model applies when the properties of the turbulence
are local. But we also discovered limitations of the model when this locality
condition fails, in particular near the edge of a turbulent region. In this
paper, we propose a second-order closure model for the transport of momentum
and chemical species by shear-induced turbulence in strongly stratified,
thermally diffusive environments (the so-called low P\'eclet number limit),
which builds upon the work of Garaud \& Ogilvie (2005). Comparison against
direct numerical simulations (DNSs) shows that the model is able to predict the
vertical profiles of the mean flow and of the stress tensor (including the
momentum transport) in diffusive shear flows, often with a reasonably good
precision, and at least within a factor of order unity in the worst case
scenario. The model is sufficiently simple to be implemented in stellar
evolution codes, and all the model constants have been calibrated against DNSs.
While significant limitations to its use remain (e.g. it can only be used in
the low P\'eclet number, slowly rotating limit), we argue that it is more
reliable than most of the astrophysical prescriptions that are used in stellar
evolution models today
Turbulent transport in a strongly stratified forced shear layer with thermal diffusion
This work presents numerical results on the transport of heat and chemical
species by shear-induced turbulence in strongly stratified but thermally
diffusive environments. The shear instabilities driven in this regime are
sometimes called "secular" shear instabilities, and can take place even when
the gradient Richardson number of the flow (the square of the ratio of the
buoyancy frequency to the shearing rate) is large, provided the P\'eclet number
(the ratio of the thermal diffusion timescale to the turnover timescale of the
turbulent eddies) is small. We have identified a set of simple criteria to
determine whether these instabilities can take place or not. Generally
speaking, we find that they may be relevant whenever the thermal diffusivity of
the fluid is very large (typically larger than cm/s), which is the
case in the outer layers of high-mass stars () for instance.
Using a simple model setup in which the shear is forced by a spatially
sinusoidal, constant-amplitude body-force, we have identified several regimes
ranging from effectively unstratified to very strongly stratified, each with
its own set of dynamical properties. Unless the system is in one of the two
extreme regimes (effectively unstratified or completely stable), however, we
find that (1) only about 10% of the input power is used towards heat transport,
while the remaining 90% is viscously dissipated; (2) that the effective
compositional mixing coefficient is well-approximated by the model of Zahn
(1992), with where is the thermal
diffusivity and is the gradient Richardson number. These results need to be
confirmed, however, with simulations in different model setups and at higher
effective Reynolds number.Comment: Submitted to Ap
Characterizing stellar parameters from high resolution spectra of cold/young stars for SPIRou legacy survey
National audienceWe propose to create a high resolution spectral library in the optical and infrared range with PHOENIX model atmospheres code that can compute molecular lines in order to estimate stellar parameters. The chosen grid of stellar parameters will be as follows: ÎŽT eff =25K, ÎŽlogg = 0.05, ÎŽ[M/H] = 0.05, ranging between: T eff = [2500K, 4000K], logg = [4.0, â5.5], [M/H] = [â1, â1] but first, we need to calibrate on F to K stars. We present here the preliminary tests and calibration on the Sun and 18sco. The method yield respectively T eff = 5770 ± 3K, logg = 4.458 ± 0.006 and T eff = 5763 ± 3.5K, logg = 4.451 ± 0.00
Characterizing stellar parameters from high-resolution spectra of main sequence cool stars. I. The G2V-K2V stars
International audienceThe goal of the present study is to construct, test, and validate a high-resolution synthetic spectral library using PHOENIX model atmospheres and develop a reliable tool to estimate stellar parameters from high-resolution optical and/or near-infrared spectra of M dwarfs. We report here the preliminary results of tests characterizing main sequence GâK stars from high-resolution spectra. We anchored the atomic line-list using the stellar standards Sun, Ο Boo A, and Ï” Eri to ensure the synthetic spectra computed with PHOENIX reproduce their observed counterparts. These stars were chosen because their parameters are very well characterized, and on which the absolute accuracy of our method depends on.We successfully estimated the stellar parameters with associated error bars for 17 stars. Using a pseudo Monte Carlo statistical analysis, we present overall improved uncertainties on the stellar parameters compared to those in the literature (on average 9 K, 0.014 dex, and 0.008 dex for the effective temperature, the surface gravity, and the metallicity, respectively). Our estimated stellar parameters are also in good agreement with values found in the literature