ABSTRACT: In cold Cepheids close to the red edge of the classical instability strip, a
strong coupling between the stellar pulsations and the surface convective
motions occurs. This coupling is by now poorly described by 1-D models of
convection, the so-called "time-dependent convection models" (TDC). The
intrinsic weakness of such models comes from the large number of unconstrained
free parameters entering in the description of turbulent convection. A way to
overcome these limits is to compute two-dimensional direct simulations (DNS),
in which all the nonlinearities are correctly solved. Two-dimensional DNS of
the convection-pulsation coupling are presented here. In an appropriate
parameter regime, convective motions can actually quench the radial pulsations
of the star, as suspected in Cepheids close to the red edge of the instability
strip. These nonlinear simulations can also be used to determine the limits and
the relevance of the TDC models.
ABSTRACT: Aims. We aim to investigate the long-term temporal evolution of the magnetic
field of the solar-type star \xi Bootis A, both from direct magnetic field
measurements and from the simultaneous estimate of indirect activity
indicators. Methods. We obtained seven epochs of high-resolution,
circularly-polarized spectra from the NARVAL spectropolarimeter between 2007
and 2011, for a total of 76 spectra. Using approximately 6,100 photospheric
spectral lines covering the visible domain, we employed a cross-correlation
procedure to compute a mean polarized line profile from each spectrum. The
large-scale photospheric magnetic field of the star was then modelled by means
of Zeeman-Doppler Imaging, allowing us to follow the year-to-year evolution of
the reconstructed magnetic topology. Simultaneously, we monitored the width of
several magnetically sensitive spectral lines, the radial velocity, the line
asymmetry of intensity line profiles, and the chromospheric emission in the
cores of the Ca II H and Halpha lines. Results. During the highest observed
activity states, in 2007 and 2011, the large-scale field of \xi Boo A is almost
completely axisymmetric and is dominated by its toroidal component. The
magnetic topologies reconstructed for these activity maxima are very similar,
suggesting a form of short cyclicity in the large-scale field distribution.
Correlated temporal evolution, due to both rotational modulation and seasonal
variability, is observed between the Ca II emission, the Halpha emission and
the width of magnetically sensitive lines. When measurable, the differential
rotation reveals a strong latitudinal shear in excess of 0.2 rad/d.
ABSTRACT: 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.
ABSTRACT: Phase-resolved observations of the solar-type star Ksi Bootis A were obtained using the NARVAL spectropolarimeter at the Telescope Bernard Lyot (Pic du Midi, France) during years 2007, 2008, 2009 and 2010. The data sets enable us to study both the rotational modulation and the long-term evolution of various magnetic and activity tracers. Here, we focus on the large-scale photospheric magnetic field (reconstructed by Zeeman-Doppler Imaging), the Zeeman broadening of the FeI 846.84 nm magnetic line, and the chromospheric CaII H and H alpha emission. Comment: 6 pages, 4 figures, Cool Stars 16 proceeding
ABSTRACT: 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 component of the 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. Comment: 6 pages, 2 figures, IAU Symposium 273 "Physics of Sun and Star Spots", 22-26 August 2010
ABSTRACT: 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.
ABSTRACT: 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)
ABSTRACT: We present a purely-radiative hydrodynamic model of the kappa-mechanism that sustains radial oscillations in Cepheid variables. We determine the physical conditions favourable for the kappa-mechanism to occur by the means of a configurable hollow in the radiative conductivity profile. By starting from these most favourable conditions, we complete nonlinear direct numerical simulations (DNS) and compare them with the results given by a linear-stability analysis of radial modes. We find that well-defined instability strips are generated by changing the location and shape of the conductivity hollow. For a given position in the layer, the hollow amplitude and width stand out as the key parameters governing the appearance of unstable modes driven by the kappa-mechanism. The DNS confirm both the growth rates and structures of the linearly-unstable modes. The nonlinear saturation that arises is produced by intricate couplings between the excited fundamental mode and higher damped overtones. These couplings are measured by projecting the DNS fields onto an acoustic subspace built from regular and adjoint eigenvectors and a 2:1 resonance is found to be responsible for the saturation of the kappa-mechanism instability.
ABSTRACT: We present here additional results of a spectropolarimetric survey of a small sample of stars ranging from spectral type M0 to M8 aimed at investigating observationally how dynamo processes operate in stars on both sides of the full convection threshold (spectral type M4). The present paper focuses on early M stars (M0--M3), i.e. above the full convection threshold. Applying tomographic imaging techniques to time series of rotationally modulated circularly polarised profiles collected with the NARVAL spectropolarimeter, we determine the rotation period and reconstruct the large-scale magnetic topologies of 6 early M dwarfs. We find that early-M stars preferentially host large-scale fields with dominantly toroidal and non-axisymmetric poloidal configurations, along with significant differential rotation (and long-term variability); only the lowest-mass star of our subsample is found to host an almost fully poloidal, mainly axisymmetric large-scale field ressembling those found in mid-M dwarfs. This abrupt change in the large-scale magnetic topologies of M dwarfs (occuring at spectral type M3) has no related signature on X-ray luminosities (measuring the total amount of magnetic flux); it thus suggests that underlying dynamo processes become more efficient at producing large-scale fields (despite producing the same flux) at spectral types later than M3. We suspect that this change relates to the rapid decrease in the radiative cores of low-mass stars and to the simultaneous sharp increase of the convective turnover times (with decreasing stellar mass) that models predict to occur at M3; it may also be (at least partly) responsible for the reduced magnetic braking reported for fully-convective stars. Comment: MNRAS, in press
ABSTRACT: We present in this paper the first results of a spectropolarimetric analysis of a small sample (~ 20) of active stars ranging from spectral type M0 to M8, which are either fully-convective or possess a very small radiative core. This study aims at providing new constraints on dynamo processes in fully-convective stars. The present paper focuses on 5 stars of spectral type ~M4, i.e. with masses close to the full convection threshold (~ 0.35 Msun), and with short rotational periods. Tomographic imaging techniques allow us to reconstruct the surface magnetic topologies from the rotationally modulated time-series of circularly polarised profiles. We fnd that all stars host mainly axisymmetric large-scale poloidal fields. Three stars were observed at two different epochs separated by ~1 yr; we find the magnetic topologies to be globally stable on this timescale. We also provide an accurate estimation of the rotational period of all stars, thus allowing us to start studying how rotation impacts the large-scale magnetic field. Comment: 17 pages, 14 figures, accepted for publication in MNRAS
ABSTRACT: Context: We present a purely-radiative hydrodynamical model of the
κ-mechanism that sustains radial oscillations in Cepheid
variables. Aims: We determine the physical conditions favourable
for the κ-mechanism to occur inside a layer, with a configurable
conductivity-hollow. We complete nonlinear direct numerical simulations
(DNS) that initiate from these most favourable conditions. Methods: We compare the results of a linear-stability analysis,
applied to radial modes using a spectral solver, and a DNS, which is
developed from a high-order finite difference code. Results: We
find that by changing the location and shape of the hollow, we can
generate well-defined instability strips. For a given position in the
layer, the amplitude and width of the hollow appear to be key parameters
to vary to attain unstable modes driven by the κ-mechanism. The
DNS, starting from the favourable conditions, confirm both the growth
rates and the structures of linearly-unstable modes. Nonlinear
saturation is produced by intricate couplings between excited
fundamental mode and higher damped overtones.
Astronomy and Astrophysics 05/2008; 484:29-42. · 4.59 Impact Factor
ABSTRACT: 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&A
ABSTRACT: 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&A
ABSTRACT: Context. We present a purely-radiative hydrodynamical model of the $\kappa$-mechanism that sustains radial oscillations in Cepheid variables.Aims. We determine the physical conditions favourable for the $\kappa$-mechanism to occur inside a layer, with a configurable conductivity-hollow. We complete nonlinear direct numerical simulations (DNS) that initiate from these most favourable conditions.Methods. We compare the results of a linear-stability analysis, applied to radial modes using a spectral solver, and a DNS, which is developed from a high-order finite difference code.Results. We find that by changing the location and shape of the hollow, we can generate well-defined instability strips. For a given position in the layer, the amplitude and width of the hollow appear to be key parameters to vary to attain unstable modes driven by the $\kappa$-mechanism. The DNS, starting from the favourable conditions, confirm both the growth rates and the structures of linearly-unstable modes. Nonlinear saturation is produced by intricate couplings between excited fundamental mode and higher damped overtones.
C. Charbonnel, F. Combes; Samadi, R.: SF2A-2008: Proc. Annual Meeting French Society of Astronomy and Astrophysics, Société Française d'Astronomie et d'Astrophysique (2008).
Strassmeier, K. G.; Kosovichev, A. G.; Beckmann, J. E.: Cosmic Magnetic Fields: From Planets to Stars and Galaxies, Proc. IAU Symposium 259, 2008, 441-442 (2009).