arXiv subjects

B. Dintrans

Publications and source records attributed to B. Dintrans.

33 records · Page 2Linked to original sources

arXiv2009

A polarity reversal in the large-scale magnetic field of the rapidly rotating Sun HD 190771

Aims. We investigate the long-term evolution of the large-scale photospheric magnetic field geometry of the solar-type star HD 190771. With fundamental parameters very close to those of the Sun except for a shorter rotation period of 8.8 d, HD 190771 provides us with a first insight into the specific impact of the rotation rate in the dynamo generation of magnetic fields in 1 $M_\odot$ stars. Methods. We use circularly polarized, high-resolution spectra obtained with the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and compute cross-correlation line profiles with high signal-to-noise ratio to detect polarized Zeeman signatures. From three phase-resolved data sets collected during the summers of 2007, 2008, and 2009, we model the large-scale photospheric magnetic field of the star by means of Zeeman-Doppler imaging and follow its temporal evolution. Results. The comparison of the magnetic maps shows that a polarity reversal of the axisymmetric component of the large-scale magnetic field occurred between 2007 and 2008, this evolution being observed in both the poloidal and toroidal magnetic components. Between 2008 and 2009, another type of global evolution occured, characterized by a sharp decrease of the fraction of magnetic energy stored in the toroidal component. These changes were not accompanied by significant evolution in the total photospheric magnetic energy. Using our spectra to perform radial velocity measurements, we also detect a very low-mass stellar companion to HD 190771.

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arXiv2008

EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars

We observe the slowly-rotating, active, single giant, EK Eri, to study and infer the nature of its magnetic field directly. We used the spectropolarimeter NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least Square Deconvolution method to create high signal-to-noise ratio Stokes V profiles. We fitted the Stokes V profiles with a model of the large-scale magnetic field. We studied the classical activity indicators, the CaII H and K lines, the CaII infrared triplet, and Hαline. We detected the Stokes V signal of EK Eri securely and measured the longitudinal magnetic field Bl for seven individual dates spanning 60% of the rotational period. The measured longitudinal magnetic field of EK Eri reached about 100 G and was as strong as fields observed in RSCVn or FK Com type stars: this was found to be extraordinary when compared with the weak fields observed at the surfaces of slowly-rotating MS stars or any single red giant previously observed with NARVAL. From our modeling, we infer that the mean surface magnetic field is about 270 G, and that the large scale magnetic field is dominated by a poloidal component. This is compatible with expectations for the descendant of a strongly magnetic Ap star.

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arXiv2008

DNS of the kappa-mechanism

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.

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arXiv2008

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.

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arXiv2008

Large-scale magnetic topologies of mid-M dwarfs

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.

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arXiv2008

Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold

From a set of stellar spectropolarimetric observations, we report the detection of surface magnetic fields in a sample of four solar-type stars, namely HD 73350, HD 76151, HD 146233 (18 Sco) and HD 190771. Assuming that the observed variability of polarimetric signal is controlled by stellar rotation, we establish the rotation periods of our targets, with values ranging from 8.8 d (for HD 190771) to 22.7 d (for HD 146233). Apart from rotation, fundamental parameters of the selected objects are very close to the Sun's, making this sample a practical basis to investigate the specific impact of rotation on magnetic properties of Sun-like stars. We reconstruct the large-scale magnetic geometry of the targets as a low-order (l<10) spherical harmonics expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (a) The magnetic energy of the large-scale field increases with rotation rate. The increase of chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (b) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ~12 days is necessary for the toroidal magnetic energy to dominate over the poloidal component.

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arXiv2008

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.

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arXiv2007

Why are some A stars magnetic, while most are not?

A small fraction of intermediate-mass main sequence (A and B type) stars have strong, organised magnetic fields. The large majority of such stars, however, show no evidence for magnetic fields, even when observed with very high precision. In this paper we describe a simple model, motivated by qualitatively new observational results, that provides a natural physical explanation for the small fraction of observed magnetic stars.

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arXiv2007

The stable magnetic field of the fullly-convective star V374 Peg

We report in this paper phase-resolved spectropolarimetric observations of the rapidly-rotating fully-convective M4 dwarf V374 Peg, on which a strong, mainly axisymmetric, large-scale poloidal magnetic field was recently detected. In addition to the original data set secured in 2005 August, we present here new data collected in 2005 September and 2006 August. From the rotational modulation of unpolarised line profiles, we conclude that starspots are present at the surface of the star, but their contrast and fractional coverage are much lower than those of non-fully convective active stars with similar rotation rate. Applying tomographic imaging on each set of circularly polarised profiles separately, we find that the large-scale magnetic topology is remarkably stable on a timescale of 1 yr; repeating the analysis on the complete data set suggests that the magnetic configuration is sheared by very weak differential rotation (about 1/10th of the solar surface shear) and only slightly distorted by intrinsic variability. This result is at odds with various theoretical predictions, suggesting that dynamo fields of fully-convective stars should be mostly non-axisymmetric unless they succeed at triggering significant differential rotation.

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arXiv2007

Weak magnetic fields in Ap/Bp stars: Evidence for a dipole field lower limit and a tentative interpretation of the magnetic dichotomy

We have investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields, with the aim of exploring the weak part of the magnetic field distribution of Ap/Bp stars. Using the MuSiCoS and NARVAL spectropolarimeters we have obtained 282 LSD Stokes V signatures of our 28 sample stars. All stars were detected, showing clearly that when observed with sufficient precision, all firmly classified Ap/Bp stars show detectable surface magnetic fields. To better characterise the surface magnetic field intensities and geometries of the sample, we have inferred the dipolar field intensity and the magnetic obliquity. The distribution of derived dipole strengths for these stars exhibits a plateau at about 1 kG, falling off to larger and smaller field strengths. Remarkably, in this sample of stars selected for their presumably weak magnetic fields, we find only 2 stars for which the derived dipole strength is weaker than 300 G. We interpret this "magnetic threshold" as a critical value necessary for the stability of large-scale magnetic fields, and develop a simple quantitative model that is able to approximately reproduce the observed threshold characteristics. This scenario leads to a natural explanation of the small fraction of intermediate-mass magnetic stars. It may also explain the near-absence of magnetic fields in more massive B and O-type stars.

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arXiv2007

Magnetic topologies of cool stars

Stellar magnetic fields can be investigated using several, very complementary approaches. While conventional spectroscopy is capable of estimating the average magnetic strength of potentially complex field configurations thanks to its low sensitivity to the vector properties of the field, spectropolarimetry can be used to map the medium- and large-scale structure of magnetic topologies. In particular, the latter approach allows one to retrieve information about the poloidal and toroidal components of the large-scale dynamo fields in low-mass stars, and thus to investigate the physical processes that produce them. Similarly, this technique can be used to explore how magnetic fields couple young stars to their massive accretion disc and thus to estimate how much mass and angular momentum are transfered to the newly-born low-mass star. We present here the latest results in this field obtained with spectropolarimetry, with special emphasis on the surprising discoveries obtained on very-low mass fully-convective stars and classical T Tauri stars thanks to the ESPaDOnS spectropolarimeter recently installed on the 3.6m Canada-France-Hawaii Telescope.

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arXiv2005

The magnetism of the solar interior for a complete MHD solar vision

The solar magnetism is no more considered as a purely superficial phenomenon. The SoHO community has shown that the length of the solar cycle depends on the transition region between radiation and convection. Nevertheless, the internal solar (stellar) magnetism stays poorly known. Starting in 2008, the American instrument HMI/SDO and the European microsatellite PICARD will enrich our view of the Sun-Earth relationship. Thus obtaining a complete MHD solar picture is a clear objective for the next decades and it requires complementary observations of the dynamics of the radiative zone. For that ambitious goal, space prototypes are being developed to improve gravity mode detection. The Sun is unique to progress on the topology of deep internal magnetic fields and to understand the complex mechanisms which provoke photospheric and coronal magnetic changes and possible longer cycles important for human life. We propose the following roadmap in Europe to contribute to this "impressive" revolution in Astronomy and in our Sun-Earth relationship: SoHO (1995-2007), PICARD (2008-2010), DynaMICS (2009-2017) in parallel to SDO (2008-2017) then a world-class mission located at the L1 orbit or above the solar poles.

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arXiv2005

Spectrum and amplitudes of internal gravity waves excited by penetrative convection in solar-type stars

The excitation of internal gravity waves by penetrative convective plumes is investigated using 2-D direct simulations of compressible convection. The wave generation is quantitatively studied from the linear response of the radiative zone to the plumes penetration, using projections onto the g-modes solutions of the associated linear eigenvalue problem for the perturbations. This allows an accurate determination of both the spectrum and amplitudes of the stochastically excited modes. Using time-frequency diagrams of the mode amplitudes, we then show that the lifetime of a mode is around twice its period and that during times of significant excitation up to 40% of the total kinetic energy may be contained into g-modes.

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arXiv2004

Shearing and embedding box simulations of the magnetorotational instability

Two different computational approaches to the magnetorotational instability (MRI) are pursued: the shearing box approach which is suited for local simulations and the embedding box approach whereby a Taylor Couette flow is embedded in a box so that numerical problems with the coordinate singularity are avoided. New shearing box simulations are presented and differences between regular and hyperviscosity are discussed. Preliminary simulations of spherical nonlinear Taylor Couette flow in an embedding box are presented and the effects of an axial field on the background flow are studied.

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arXiv2002

More concerning the anelastic and subseismic approximations for low-frequency modes in stars

Two approximations, namely the subseismic approximation and the anelastic approximation, are presently used to filter out the acoustic modes when computing low frequency modes of a star (gravity modes or inertial modes). In a precedent paper (Dintrans & Rieutord 2001), we observed that the anelastic approximation gave eigenfrequencies much closer to the exact ones than the subseismic approximation. Here, we try to clarify this behaviour and show that it is due to the different physical approach taken by each approximation: On the one hand, the subseismic approximation considers the low frequency part of the spectrum of (say) gravity modes and turns out to be valid only in the central region of a star; on the other hand, the anelastic approximation considers the Brunt-Vaisala frequency as asymptotically small and makes no assumption on the order of the modes. Both approximations fail to describe the modes in the surface layers but eigenmodes issued from the anelastic approximation are closer to those including acoustic effects than their subseismic equivalent. We conclude that, as far as stellar eigenvalue problems are concerned, the anelastic approximation is better suited for simplifying the eigenvalue problem when low-frequency modes of a star are considered, while the subseismic approximation is a useful concept when analytic solutions of high order low-frequency modes are needed in the central region of a star.

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