99 research outputs found
A test of time-dependent theories of stellar convection
Context: In Cepheids close to the red edge of the classical instability
strip, a coupling occurs between the acoustic oscillations and the convective
motions close to the surface.The best topical models that account for this
coupling rely on 1-D time-dependent convection (TDC) formulations. However,
their intrinsic weakness comes from the large number of unconstrained free
parameters entering in the description of turbulent convection. Aims: We
compare two widely used TDC models with the first two-dimensional nonlinear
direct numerical simulations (DNS) of the convection-pulsation coupling in
which the acoustic oscillations are self-sustained by the kappa-mechanism.
Methods: The free parameters appearing in the Stellingwerf and Kuhfuss TDC
recipes are constrained using a chi2-test with the time-dependent convective
flux that evolves in nonlinear simulations of highly-compressible convection
with kappa-mechanism. Results: This work emphasises some inherent limits of TDC
models, that is, the temporal variability and non-universality of their free
parameters. More importantly, within these limits, Stellingwerf's formalism is
found to give better spatial and temporal agreements with the nonlinear
simulation than Kuhfuss's one. It may therefore be preferred in 1-D TDC
hydrocodes or stellar evolution codes.Comment: 7 pages, 5 figures, 2 tables, accepted for publication in A&
Numerical simulations of the kappa-mechanism with convection
A strong coupling between convection and pulsations is known to play a major
role in the disappearance of unstable modes close to the red edge of the
classical Cepheid instability strip. As mean-field models of time-dependent
convection rely on weakly-constrained parameters, we tackle this problem by the
means of 2-D Direct Numerical Simulations (DNS) of kappa-mechanism with
convection.
Using a linear stability analysis, we first determine the physical conditions
favourable to the kappa-mechanism to occur inside a purely-radiative layer.
Both the instability strips and the nonlinear saturation of unstable modes are
then confirmed by the corresponding DNS. We next present the new simulations
with convection, where a convective zone and the driving region overlap. The
coupling between the convective motions and acoustic modes is then addressed by
using projections onto an acoustic subspace.Comment: 5 pages, 6 figures, accepted for publication in Astrophysics and
Space Science, HELAS workshop (Rome june 2009
Convective quenching of stellar pulsations
Context: we study the convection-pulsation coupling that occurs in cold
Cepheids close to the red edge of the classical instability strip. In these
stars, the surface convective zone is supposed to stabilise the radial
oscillations excited by the kappa-mechanism.
Aims: we study the influence of the convective motions onto the amplitude and
the nonlinear saturation of acoustic modes excited by kappa-mechanism. We are
interested in determining the physical conditions needed to lead to a quenching
of oscillations by convection.
Methods: we compute two-dimensional nonlinear simulations (DNS) of the
convection-pulsation coupling, in which the oscillations are sustained by a
continuous physical process: the kappa-mechanism. Thanks to both a frequential
analysis and a projection of the physical fields onto an acoustic subspace, we
study how the convective motions affect the unstable radial oscillations.
Results: depending on the initial physical conditions, two main behaviours
are obtained: (i) either the unstable fundamental acoustic mode has a large
amplitude, carries the bulk of the kinetic energy and shows a nonlinear
saturation similar to the purely radiative case; (ii) or the convective motions
affect significantly the mode amplitude that remains very weak. In this second
case, convection is quenching the acoustic oscillations. We interpret these
discrepancies in terms of the difference in density contrast: larger
stratification leads to smaller convective plumes that do not affect much the
purely radial modes, while large-scale vortices may quench the oscillations.Comment: 15 pages, 17 figures, 3 tables, accepted for publication in A&
Pharmacokinetics of Micafungin in Critically Ill Patients
We investigated covariates of pharmacokinetics of micafungin in critically ill patients. After application of micafungin, plasma samples were collected. Non-linear mixed effects modelling (NONMEM 7.3) was used to develop the pharmacokinetic model. Using this model, the adequacy of a fixed 100 mg dosing regimen was evaluated in the study cohort. A two-compartment model with linear elimination was found to describe the obtained data. SOFA score was identified as a significant covariate on both clearance and central volume of distribution, respectively. Patients in highly critical condition, represented by a SOFA above 10 showed a 30.8% lower central volume of distribution than the less critically ill patients. For patients with bilirubin levels above 4 mg/dl, clearance was decreased by 21.1%. Renal replacement therapy (RRT) did not influence micafungin clearance or the volumes of distribution. In a posthoc evaluation of the modeled population, 100 mg micafungin was suitable when assessing the PKPD targets (AUC/MIC) for C. albicans and C. glabrata, with insufficient target attainment for C. parapsilosis. Micafungin pharmacokinetics appear not to be influenced by the status of RRT. A dose of 100 mg micafungin is suitable for infections with C. albicans and C. glabrata in critically ill patients
Direct numerical simulations of the kappa-mechanism II. The nonlinear saturation and Hertzsprung's progression
Context: We study the kappa-mechanism that excites radial oscillations in
Cepheid variables.
Aims: We address the mode couplings that manages the nonlinear saturation of
the instability in direct numerical simulations (DNS).
Methods: We project the DNS fields onto an acoustic subspace built from the
regular and adjoint eigenvectors that are solutions to the linear-oscillations
equations.
Results: We determine the time evolution of both the amplitude and kinetic
energy of each mode that propagates in the DNS. More than 98% of the total
kinetic energy is contained in two modes that correspond to the
linearly-unstable fundamental mode and the linearly-stable second overtone.
Because the eigenperiods ratio is close to 1/2, we discover that the nonlinear
saturation is due to a 2:1 resonance between these two modes. An interesting
application of this result concerns the reproduction of Hertzsprung's
progression observed in Bump Cepheids.Comment: 11 pages, 9 figures, 1 table, accepted for publication in A&
DNS of the kappa-mechanism. I. Radial modes in the purely radiative case
Context: Hydrodynamical model of the kappa-mechanism in a purely radiative
case. Aims: First, to determine the physical conditions propitious to
kappa-mechanism in a layer with a configurable conductivity hollow and second,
to perform the (nonlinear) direct numerical simulations (DNS) from the most
favourable setups. Methods: A linear stability analysis applied to radial modes
using a spectral solver and DNS thanks to a high-order finite difference code
are compared. Results: Changing the hollow properties (location and shape) lead
to well-defined instability strips. For a given position in the layer, the
amplitude and width of the hollow appear to be the key parameters to get
unstable modes driven by kappa-mechanism. The DNS achieved from these more
auspicious configurations confirm the growth rates as well as structures of
linearly unstable modes. The nonlinear saturation follows through intricate
couplings between the excited fundamental mode and higher damped overtones.Comment: 15 pages, 15 figures, 1 table, accepted for publication in A&
Exploring the magnetic topologies of cool stars
Magnetic fields of cool stars can be directly investigated through the study of the Zeeman effect on photospheric spectral lines using several approaches. With spectroscopic measurement in unpolarised light, the total magnetic flux averaged over the stellar disc can be derived but very little information on the field geometry is available. Spectropolarimetry provides a complementary information on the large-scale magnetic topology. With Zeeman-Doppler Imaging (ZDI), this information can be retrieved to produce a map of the vector magnetic field at the surface of the star, and in particular to assess the relative importance of the poloidal and toroidal components as well as the degree of axisymmetry of the field distribution. The development of high-performance spectropolarimeters associated with multi-lines techniques and ZDI allows us to explore magnetic topologies throughout the Hertzsprung-Russel diagram, on stars spanning a wide range of mass, age and rotation period. These observations bring novel constraints on magnetic field generation by dynamo effect in cool stars. In particular, the study of solar twins brings new insight on the impact of rotation on the solar dynamo, whereas the detection of strong and stable dipolar magnetic fields on fully convective stars questions the precise role of the tachocline in this process.Postprin
- …