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Despite their importance in stellar evolution, little is known about magnetic fields in the interior of stars. The recent seismic detection of magnetic fields in the core of several red giant stars has given measurements of their strength and information on their topology. We revisit the puzzling case of hydrogen-shell burning giants that show deviations from the expected regular period spacing of gravity modes. These stars also tend to have a too low measured period spacing compared to their counterparts. We here show that these two features are well accounted for by strong magnetic fields in the cores of these stars. For 11 Kepler red giants showing these anomalies, we place lower limits on the core field strengths ranging from 40 to 610 kG. For one star, the measured field exceeds the critical field above which gravity waves no longer propagate in the core. We find that this star shows mixed mode suppression at low frequency, which further suggests that this phenomenon might be related to strong core magnetic fields., Comment: 10 pages, 7 figures, accepted as an Letter in A&A

Aims. The very weak magnetic field detected at the surface of Vega hints at a widespread population of weakly magnetic stars of A and B spectral types. We contribute here to gather more clues about the origin of this magnetism by investigating the long-term stability of the field geometry of this prototypical star. Methods. We use spectropolarimetric data collected as part of a long-term campaign, with more than 2,000 observations spread between 2008 and 2018. Using various sub-sets extracted from the whole time series, we reconstruct several maps of the large-scale surface magnetic field. Results. We confirm that the polarimetric signal is modulated according to a $\sim 0.68$ d period, which we interpret as the stellar rotation period. The surface magnetic field is organized in a complex geometry. We confirm the existence of a very localized, polar magnetic spot previously reported for Vega, with a radial field strength of about -5 G. We show that the surface of the star is also covered by a dipole, with a polar strength close to 9 G and a dipole obliquity close to $90^\circ$. Both magnetic structures are remarkably stable over one decade. The available data suggest that smaller-scale magnetic spots may not be limited to the polar region, although the poor reliability of their reconstruction does not allow us to firmly conclude about their temporal evolution., Comment: A&A, in press

Some contracting or expanding stars are thought to host a large-scale magnetic field in their radiative interior. By interacting with the contraction-induced flows, such fields may significantly alter the rotational history of the star. They thus constitute a promising way to address the problem of angular momentum transport during the rapid phases of stellar evolution. In this work, we aim at studying the interplay between flows and magnetic fields in a contracting radiative zone. We propose a scenario that may account for the post-main sequence evolution of solar-like stars, in which a quasi-solid rotation can be maintained by a large-scale magnetic field during a contraction timescale. Then, an axisymmetric instability would destroy this large-scale structure and enables the differential rotation to set in. Such a contraction driven instability could also be at the origin of the observed dichotomy between strongly and weakly magnetic intermediate-mass stars., Comment: 31 pages, 29 figures, 3 tables, accepted for publication in Astronomy & Astrophysics

Context. Stars experience rapid contraction or expansion at different phases of their evolution. Modelling the angular momentum and chemical elements transport occurring during these phases remains an unsolved problem. Aims. We study a stellar radiative zone undergoing radial contraction and investigate the induced differential rotation and meridional circulation. Methods. We consider a rotating spherical layer crossed by an imposed radial velocity field that mimics the contraction and solve numerically the axisymmetric hydrodynamical equations in both the Boussinesq and anelastic approximations. An extensive parametric study is conducted to cover regimes of contraction, rotation, stable stratification and density stratification that are relevant for stars. Results. The differential rotation and the meridional circulation result from a competition between the contraction-driven inward transport of angular momentum and an outward transport dominated by either viscosity or an Eddington-Sweet type circulation, depending on the value of the $P_r \left( N_0 / \Omega_0 \right)^2 $ parameter, where $P_r$ is the Prandtl number, $N_0$ the Brunt-V\"ais\"ail\"a frequency and $\Omega_0$ the rotation rate. Taking the density stratification into account is important to study more realistic radial contraction fields but also because the resulting flow is less affected by unwanted effects of the boundary conditions. In these different regimes and for a weak differential rotation, we derive scaling laws that relate the amplitude of the differential rotation to the contraction timescale., Comment: 24 pages, 16 figures, 8 tables

Gamma Doradus stars present an incredibly rich pulsation spectra, with gravito-inertial modes, in some cases supplemented with delta Scuti-like pressure modes and in numerous cases with Rossby modes. The present paper aims at showing that, in addition to these modes established in the radiative envelope, pure inertial modes, trapped in the convective core, can be detected in Kepler observations of gamma Doradus stars, thanks to their resonance with the gravito-inertial modes. We start by using a simplified model of perturbations in a full sphere of uniform density. Under these conditions, the spectrum of pure inertial modes is known from analytical solutions of the so-called Poincare equation. We then compute coupling factors which help select the pure inertial modes which interact best with the surrounding dipolar gravito-inertial modes. Using complete calculations of gravito-inertial modes in realistic models of gamma Doradus stars, we are able to show that the pure inertial/gravito-inertial resonances appear as dips in the gravito-inertial mode period spacing series at spin parameters close to those predicted by the simple model. We find the first evidence of such dips in the Kepler gamma Doradus star KIC5608334. Finally, using complete calculations in isolated convective cores, we find that the spin parameters of the pure inertial/gravito-inertial resonances are also sensitive to the density stratification of the convective core. In conclusion, we have discovered that certain dips in gravito-inertial mode period spacings observed in some Kepler stars are in fact the signatures of resonances with pure-inertial modes that are trapped in the convective core. This holds the promise to finally access the central conditions , i.e. rotation and density stratification, of intermediate-mass stars on the main sequence., Comment: 11 pages, 8 figures, accepted for publication in A&A

The present study aims at studying the flow and field produced by a stellar radiative zone which is initially made to rotate differentially in the presence of a large-scale poloidal magnetic field threading the whole domain. We focus both on the axisymmetric configurations produced by the initial winding-up of the magnetic field lines and on the possible instabilities of those configurations. We aim in particular at assessing the role of the stratification at stabilising the system. We perform 2D and 3D global Boussinesq numerical simulations started from an initial radial or cylindrical differential rotation and a large-scale poloidal magnetic field. Under the conditions of a large rotation frequency compared to the Alfv\'en frequency, a magnetic configuration strongly dominated by its toroidal component is built. The parameters of the simulations are chosen to respect the ordering of time scales of a typical stellar radiative zone. In this framework, the axisymmetric evolution is studied by varying the relative effects of the thermal diffusion, the Br\"unt-V\"ais\"al\"a frequency, the rotation and the initial poloidal field strength. We find that the axisymmetric state only depends on $t_{es}/t_{Ap}$, the ratio between the Eddington-Sweet circulation time scale and the Alfv\'en time scale. A scale analysis of the Boussinesq equations allows us to recover this result. In the cylindrical case, a magneto-rotational instability develops when the thermal diffusivity is sufficiently high to enable the favored wavenumbers to be insensitive to the effects of the stable stratification. In the radial case, the magneto-rotational instability is driven by the latitudinal shear created by the back-reaction of the Lorentz force on the flow. Increasing the level of stratification then leaves the growth rate of the instability mainly unaffected while its horizontal length scale grows., Comment: 20 pages, 13 figures, accepted for publication in A&A

The topic of turbulent transport in a stellar radiative zone is vast and poorly understood. Many physical processes can potentially drive turbulence in stellar radiative zone but the limited observational constraints and the uncertainties in modelling turbulent transport make it difficult to identify the most relevant one. Here, we focus on the effect of stable stratification on the radial turbulent transport of chemicals and more particularly on the case where the turbulence is driven by a radial shear. Results of numerical simulations designed to test phenomenological models of turbulent transport will be presented. While this may appears little ambitious, stable stratification will influence the radial turbulent transport whatever the mechanism that drives the turbulent motions and the present considerations should be useful for these other mechanism too., Comment: 30 pages, 12 figures, Proceedings of the Evry Schatzman School 2018. Edited by M. Rieutord, I. Baraffe, Y. Lebreton. EDP Sciences Proceedings, to appear

Stably stratified layers are present in stellar interiors (radiative zones) as well as planetary interiors - recent observations and theoretical studies of the Earth's magnetic field seem to indicate the presence of a thin, stably stratified layer at the top of the liquid outer core. We present direct numerical simulations of this region, which is modelled as an axisymmetric spherical Couette flow for a stably stratified fluid embedded in a dipolar magnetic field. For strong magnetic fields, a super-rotating shear layer, rotating nearly 30% faster than the imposed rotation rate difference between the inner convective dynamo region and the outer boundary, is generated in the stably stratified region. In the Earth context, and contrary to what was previously believed, we show that this super-rotation may extend towards the Earth magnetostrophic regime if the density stratification is sufficiently large. The corresponding differential rotation triggers magnetohydrodynamic instabilities and waves in the stratified region, which feature growth rates comparable to the observed timescale for geomagnetic secular variations and jerks. In the stellar context, we perform a linear analysis which shows that similar instabilities are likely to arise, and we argue that it may play a role in explaining the observed magnetic dichotomy among intermediate-mass stars., Comment: 18 pages, 6 figures. Accepted for publication in GAFD

Spectral signatures of surface spots were recently discovered from high cadence observations of the A star Vega. We aim at constraining the surface distribution of these photospheric inhomogeneities, and investigating a possible short term evolution of the spot pattern. Using data collected over five consecutive nights, we employ the Doppler Imaging method to reconstruct three different maps of the stellar surface, from three consecutive subsets of the whole time-series. The surface maps display a complex distribution of dark and bright spots, covering most of the visible fraction of the stellar surface. A number of surface features are consistently recovered in all three maps, but other features seem to evolve over the time span of observations, suggesting that fast changes can affect the surface of Vega within a few days at most. The short-term evolution is observed as emergence or disappearance of individual spots, and may also show up as zonal flows, with low-latitude and high latitude belts rotating faster than intermediate latitudes. It is tempting to relate the surface brightness activity to the complex magnetic field topology previously reconstructed for Vega, although strictly simultaneous brightness and magnetic maps will be necessary to assess this potential link., Comment: MNRAS Letters, in press

Context: Mode identification has remained a major obstacle in the interpretation of pulsation spectra in rapidly rotating stars. Aims: We would like to test mode identification methods and seismic diagnostics in rapidly rotating stars, using oscillation spectra based on new theoretical predictions. Methods: We investigate the auto-correlation function and Fourier transform of theoretically calculated frequency spectra, in which modes are selected according to their visibilities. Given the difficulties in predicting intrinsic mode amplitudes, we experimented with various ad-hoc prescriptions for setting these, including using random values. Furthermore, we analyse the ratios between mode amplitudes observed in different photometric bands. Results: When non-random intrinsic mode amplitudes are used, our results show that it is possible to extract the large frequency separation or half its value, and sometimes twice the rotation rate, from the auto-correlation function. The Fourier transforms are mostly sensitive to the large frequency separation or half its value. When the intrinsic mode amplitudes include random factors, the results are far less favourable. We also find that amplitude ratios provide a good way of grouping together modes with similar characteristics. By analysing the frequencies of these groups, it is possible to constrain mode identification as well as determine the large frequency separation and the rotation rate., Comment: 15 pages, 12 figures, accepted in A&A

An extremely weak circularly polarized signature was recently discovered in spectral lines of the chemically peculiar Am star Sirius A. A weak surface magnetic field was proposed to account for the observed polarized signal, but the shape of the phase-averaged signature, dominated by a prominent positive lobe, is not expected in the standard theory of the Zeeman effect. We aim at verifying the presence of weak circularly polarized signatures in two other bright Am stars, beta UMa and theta Leo, and investigating the physical origin of Sirius-like polarized signals further. We present here a set of deep spectropolarimetric observations of beta UMa and theta Leo, observed with the NARVAL spectropolarimeter. We analyzed all spectra with the Least Squares Deconvolution multiline procedure. To improve the signal-to-noise ratio and detect extremely weak signatures in Stokes V profiles, we co-added all available spectra of each star (around 150 observations each time). Finally, we ran several tests to evaluate whether the detected signatures are consistent with the behavior expected from the Zeeman effect. The line profiles of the two stars display circularly polarized signatures similar in shape and amplitude to the observations previously gathered for Sirius A. Our series of tests brings further evidence of a magnetic origin of the recorded signal. These new detections suggest that very weak magnetic fields may well be present in the photospheres of a significant fraction of intermediate-mass stars. The strongly asymmetric Zeeman signatures measured so far in Am stars (featuring a dominant single-sign lobe) are not expected in the standard theory of the Zeeman effect and may be linked to sharp vertical gradients in photospheric velocities and magnetic field strengths.

The theoretically studied impact of rapid rotation on stellar evolution needs to be confronted with the results of high resolution spectroscopy-velocimetry observations. A weak surface magnetic field had recently been detected in the A0 prototype star Vega, potentially leading to a (yet undetected) structured surface. The goal of this article is to present a thorough analysis of the line profile variations and associated estimators in the early-type standard star Vega (A0) in order reveal potential activity tracers, exoplanet companions and stellar oscillations. Vega was monitored in high-resolution spectroscopy with the velocimeter Sophie/OHP. A total of 2588 high S/N spectra was obtained during 5 nights (August 2012) at R = 75000 and covering the visible domain. For each reduced spectrum, Least Square Deconvolved (LSD) equivalent photospheric profiles were calculated with a Teff = 9500 and logg = 4.0 spectral line mask. Several methods were applied to study the dynamic behavior of the profile variations (evolution of radial velocity, bisectors, vspan, 2D profiles, amongst others). We present the discovery of a starspotted stellar surface in an A-type standard star with faint spot amplitudes Delta F/Fc ~5 10^{-4}. A rotational modulation of spectral lines with a period of rotation P = 0.68 d has clearly been exhibited, confirming the results of previous spectropolarimetric studies. Either a very thin convective layer can be responsible for magnetic field generation at small amplitudes, or a new mechanism has to be invoked in order to explain the existence of activity tracing starspots. This first strong evidence that standard A-type stars can show surface structures opens a new field of research and asks the question about a potential link with the recently discovered weak magnetic field discoveries in this category of stars., Comment: accepted for publication by Astronomy & Astrophysics (23rd of March 2015)

We investigate the magnetic field at the surface of 48 red giants selected as promising for detection of Stokes V Zeeman signatures in their spectral lines. We use the spectropolarimeters Narval and ESPaDOnS to detect circular polarization within the photospheric absorption lines of our targets and use the least-squares deconvolution (LSD) method. We also measure the classical S-index activity indicator, and the stellar radial velocity. To infer the evolutionary status of our giants and to interpret our results, we use state-of-the-art stellar evolutionary models with predictions of convective turnover times. We unambiguously detect magnetic fields via Zeeman signatures in 29 of the 48 red giants in our sample. Zeeman signatures are found in all but one of the 24 red giants exhibiting signs of activity, as well as 6 out of 17 bright giant stars.The majority of the magnetically detected giants are either in the first dredge up phase or at the beginning of core He burning, i.e. phases when the convective turnover time is at a maximum: this corresponds to a 'magnetic strip' for red giants in the Hertzsprung-Russell diagram. A close study of the 16 giants with known rotational periods shows that the measured magnetic field strength is tightly correlated with the rotational properties, namely to the rotational period and to the Rossby number Ro. Our results show that the magnetic fields of these giants are produced by a dynamo. Four stars for which the magnetic field is measured to be outstandingly strong with respect to that expected from the rotational period/magnetic field relation or their evolutionary status are interpreted as being probable descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4 bright giants (Aldebaran, Alphard, Arcturus, eta Psc) are detected with magnetic field strength at the sub-gauss level., Comment: 34 pages, 22 Figures, accepted for publication in Astronomy & Astrophysics

We obtained 128 high signal-to-noise ratio Stokes $V$ spectra of the B3V star $\iota$~Her on 5 consecutive nights in 2012 with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope, with the aim of searching for the presence of weak and/or complex magnetic fields. Least-Squares Deconvolution mean profiles were computed from individual spectra, averaged over individual nights and over the entire run. No Zeeman signatures are detected in any of the profiles. The longitudinal magnetic field in the grand average profile was measured to be $-0.24 \pm 0.32 $ G, as compared to $-0.22 \pm 0.32$ G in the null profile. Our observations therefore provide no evidence for the presence of Zeeman signatures analogous to those observed in the A0V star Vega by Lignieres et al. (2009). We interpret these observations in three ways. First, we compare the LSD profiles with synthetic Stokes $V$ profiles corresponding to organised (dipolar) magnetic fields, for which we find an upper limit of about 8~G on the polar strength of any surface dipole present. Secondly, we compare the grand average profile with calculations corresponding to the random magnetic spot topologies of Kochukhov & Sudnik (2013), inferring that spots, if present, of 2 degrees radius with strengths of 2-4 G and a filling factor of 50% should have been detected in our data. Finally, we compare the observations with synthetic $V$ profiles corresponding to the surface magnetic maps of Vega (Petit et al. 2010) computed for the spectral characteristics of $\iota$ Her. We conclude that while it is unlikely we would have detected a magnetic field identical to Vega's, we would have likely detected one with a peak strength of about 30 G, i.e. approximately four times as strong as that of Vega., Comment: 12 pages. MNRAS, accepted and in press

Until recently, the detection of magnetic fields at the surface of intermediate-mass main-sequence stars has been limited to Ap/Bp stars, a class of chemically peculiar stars. This class represents no more than 5-10% of the stars in this mass range. This small fraction is not explained by the fossil field paradigm that describes the Ap/Bp type magnetism as a remnant of an early phase of the star-life. Also, the limitation of the field measurements to a small and special group of stars is obviously a problem to study the effect of the magnetic fields on the stellar evolution of a typical intermediate-mass star. Thanks to the improved sensitivity of a new generation of spectropolarimeters, a lower bound to the magnetic fields of Ap/Bp stars, a two orders of magnitude desert in the longitudinal magnetic field and a new type of sub-gauss magnetism first discovered on Vega have been identified. These advances provide new clues to understand the origin of intermediate-mass magnetism as well as its influence on stellar evolution. In particular, a scenario has been proposed whereby the magnetic dichotomy between Ap/Bp and Vega-like magnetism originate from the bifurcation between stable and unstable large scale magnetic configurations in differentially rotating stars. In this paper, we review these recent observational findings and discuss this scenario., Comment: 10 pages, 5 figures, invited review, to appear in the proceedings of IAUS 302 "Magnetic Fields Throughout Stellar Evolution"

The continuous high-precision photometric observations provided by the CoRoT and Kepler space missions have allowed us to better understand the structure and dynamics of red giants using asteroseismic techniques. A small fraction of these stars shows dipole modes with unexpectedly low amplitudes. The reduction in amplitude is more pronounced for stars with higher frequency of maximum power. In this work we want to characterize KIC 8561221 in order to confirm that it is currently the least evolved star among this peculiar subset and to discuss several hypotheses that could help explain the reduction of the dipole mode amplitudes. We used Kepler short- and long-cadence data combined with spectroscopic observations to infer the stellar structure and dynamics of KIC 8561221. We then discussed different scenarios that could contribute to the reduction of the dipole amplitudes such as a fast rotating interior or the effect of a magnetic field on the properties of the modes. We also performed a detailed study of the inertia and damping of the modes. We have been able to characterize 37 oscillations modes, in particular, a few dipole modes above nu_max that exhibit nearly normal amplitudes. We have inferred a surface rotation period of around 91 days and uncovered the existence of a variation in the surface magnetic activity during the last 4 years. As expected, the internal regions of the star probed by the l = 2 and 3 modes spin 4 to 8 times faster than the surface. With our grid of standard models we are able to properly fit the observed frequencies. Our model calculation of mode inertia and damping give no explanation for the depressed dipole modes. A fast rotating core is also ruled out as a possible explanation. Finally, we do not have any observational evidence of the presence of a strong deep magnetic field inside the star., Comment: Accepted in A&A. 17 pages, 16 figures

Stellar magnetic field measurements obtained from spectropolarimetry offer key data for activity and dynamo studies, and we present the results of a major high-resolution spectropolarimetric Bcool project magnetic snapshot survey of 170 solar-type stars from observations with the Telescope Bernard Lyot and the Canada-France-Hawaii Telescope. For each target star a high signal-to-noise circularly polarised Stokes V profile has been obtained using Least-Squares Deconvolution, and used to detect surface magnetic fields and measure the corresponding mean surface longitudinal magnetic field ($B_{l}$). Chromospheric activity indicators were also measured. Surface magnetic fields were detected for 67 stars, with 21 of these stars classified as mature solar-type stars, a result that increases by a factor of four the number of mature solar-type stars on which magnetic fields have been observed. In addition, a magnetic field was detected for 3 out of 18 of the subgiant stars surveyed. For the population of K-dwarfs the mean value of $B_{l}$ ($|B_{l}|_{mean}$) was also found to be higher (5.7 G) than $|B_{l}|_{mean}$ measured for the G-dwarfs (3.2 G) and the F-dwarfs (3.3 G). For the sample as a whole $|B_{l}|_{mean}$ increases with rotation rate and decreases with age, and the upper envelope for $|B_{l}|$ correlates well with the observed chromospheric emission. Stars with a chromospheric S-index greater than about 0.2 show a high magnetic field detection rate and so offer optimal targets for future studies. This survey constitutes the most extensive spectropolarimetric survey of cool stars undertaken to date, and suggests that it is feasible to pursue magnetic mapping of a wide range of moderately active solar-type stars to improve understanding of their surface fields and dynamos., Comment: 36 pages, 26 figures, 7 tables, submitted to MNRAS

We have studied the magnetic field of about 50 active and non-active single G-K-type red giants by means of spectropolarimetry with Narval and ESPaDOnS. 30 giants have been significantly Zeeman-detected. A close study of the 17 giants with known rotational periods shows that the measured magnetic field strength is correlated to the rotation, in particular to the Rossby number. 4 giants for which the magnetic field is measured to be outstandingly strong with respect to the rotational period or the evolutionary state are identified as probable Ap-star descendants. We detail their magnetic properties and propose criteria to identify Ap-star descendants., Comment: 8 pages, to appear in the proceedings of the conference "Putting A Stars into Context: Evolution, Environment, and Related Stars"

14 Ceti is a subgiant star of F spectral class that displays variations in the S-index of its CaII H & K lines and an X-ray emission that is stronger than the mean observed for its spectral class, which may be due to some magnetic activity. We attempt to Zeeman-detect and study the magnetic field of 14 Ceti and to infer its origin. We used the spectropolarimeter Narval at the Telescope Bernard Lyot, Pic du Midi Observatory, and the least squares deconvolution method to create high signal-to-noise ratio Stokes V profiles. We derived the surface-averaged longitudinal magnetic field Bl. We also measured the S-index, and the radial velocity for each observation. 14 Ceti is Zeeman-detected for the 30 observed dates spanning from August 2007 to January 2012. The average longitudinal magnetic field does not reverse its sign, reaches about -35 G, and shows some month-long-timescale variations in our 2008 and 2011-2012 observations. The S-index follows the same long-term trend as Bl. 14 Ceti is confirmed as a single star without H-K emission cores. The strength of the observed surface magnetic field of 14 Ceti is one order of magnitude greater than the observed one for late F main-sequence stars, and is comparable to the values measured in the active late F pre-main-sequence star HR 1817. On the other hand, taking into account the post-main-sequence evolution of an Ap star, an oblique rotator model can explain the strength of the magnetic field of 14 Ceti. The variations with a timescale of months observed for both the Bl and S-index could be due to the rotation. The most probable scenario to explain our observations appears to be that 14 Ceti is the descendant of a cool Ap star ., Comment: Accepted for publication in Astronomy & Astrophysics, 7 pages, 5 figures

We present the result of a highly sensitive spectropolarimetric study dedicated to intermediate mass stars. We report the detection of sub-gauss surface magnetic fields on the normal, rapidly-rotating A-type star Vega and on the moderately-rotating Am star Sirius A. These magnetic detections constitute the first evidence that tepid stars that do not belong to the class of Ap/Bp stars can also host magnetized photospheres, suggesting that a significant fraction of stars in this mass regime are magnetic. We present here the observational clues gathered so far to progress towards understanding the physical processes at the origin of this newly identified Vega-like magnetism., Comment: Submitted to Astronomical Notes (proceedings of the 7th Potsdam Thinkshop on "Magnetic Fields in Stars and Exoplanets")

We present complete 2D computations of g modes in distorted polytropic models of stars performed with the Two-dimensional Oscillation Program (TOP). We computed low-degree modes (l=1 modes with radial order n=-1...-14, and l=2,3 modes with n=-1...-5 and -16...-20) of a nonrotating model and followed them by slowly increasing the rotation rate up to 70 % of the Keplerian break-up velocity. We use these computations to determine the domain of validity of perturbative methods up to the 3rd order. We study the evolution of the regularities of the spectrum and show quantitative agreement with the traditional approximation for not too large values of the ratio of the rotation rate to the pulsation frequency. We also show the appearance of new types of modes, called "rosette" modes due to their spatial structure. Thanks to the ray theory for gravito-inertial waves that we developed, we can associate these modes with stable periodic rays., Comment: 9 pages, 4 figures. To appear in the ASP proceedings of "The 61st Fujihara seminar: Progress in solar/stellar physics with helio- and asteroseismology", 13th-17th March 2011, Hakone, Japan

EK Eri is one of the most slowly rotating active giants known, and has been proposed to be the descendant of a strongly magnetic Ap star. We have performed a spectropolarimetric study of EK Eri over 4 photometric periods with the aim of inferring the topology of its magnetic field. We used the NARVAL spectropolarimeter at the Bernard Lyot telescope at the Pic du Midi Observatory, along with the least-squares deconvolution method, to extract high signal-to-noise ratio Stokes V profiles from a timeseries of 28 polarisation spectra. We have derived the surface-averaged longitudinal magnetic field Bl. We fit the Stokes V profiles with a model of the large-scale magnetic field and obtained Zeeman Doppler images of the surface magnetic strength and geometry. Bl variations of up to about 80 G are observed without any reversal of its sign, and which are in phase with photometric ephemeris. The activity indicators are shown to vary smoothly on a timescale compatible with the rotational period inferred from photometry (308.8 d.), however large deviations can occur from one rotation to another. The surface magnetic field variations of EK Eri appear to be dominated by a strong magnetic spot (of negative polarity) which is phased with the dark (cool) photometric spot. Our modeling shows that the large-scale magnetic field of EK Eri is strongly poloidal. For a rotational axis inclination of i = 60{\deg}, we obtain a model that is almost purely dipolar. In the dipolar model, the strong magnetic/photometric spot corresponds to the negative pole of the dipole, which could be the remnant of that of an Ap star progenitor of EK Eri. Our observations and modeling conceptually support this hypothesis, suggesting an explanation of the outstanding magnetic properties of EK Eri as the result of interaction between deep convection and the remnant of an Ap star magnetic dipole., Comment: 8 pages, 6 figures, accepted for publication in Astronomy & Astrophysics

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., Comment: 16 pages, 8 figures, accepted for publication in A&A, abstract shortened to respect the 1920 characters required

We present new spectropolarimetric observations of the normal A-type star Vega, obtained during the summer of 2010 with NARVAL at T\'elescope Bernard Lyot (Pic du Midi Observatory). This new time-series is constituted of 615 spectra collected over 6 different nights. We use the Least-Square-Deconvolution technique to compute, from each spectrum, a mean line profile with a signal-to-noise ratio close to 20,000. After averaging all 615 polarized observations, we detect a circularly polarized Zeeman signature consistent in shape and amplitude with the signatures previously reported from our observations of 2008 and 2009. The surface magnetic geometry of the star, reconstructed using the technique of Zeeman-Doppler Imaging, agrees with the maps obtained in 2008 and 2009, showing that most recognizable features of the photospheric field of Vega are only weakly distorted by large-scale surface flows (differential rotation or meridional circulation)., Comment: Proceedings of the conference "Stellar polarimetry: from birth to death", 2011 Jun 27-30, Madison

We aim at a highly sensitive search for weak magnetic fields in main sequence stars of intermediate mass, by scanning classes of stars with no previously reported magnetic members. After detecting a weak magnetic field on the normal, rapidly rotating A-type star Vega, we concentrate here on the bright star Sirius A, taken as a prototypical, chemically peculiar, moderately rotating Am star. We employed the NARVAL and ESPaDOnS high-resolution spectropolarimeters to collect 442 circularly polarized spectra, complemented by 60 linearly polarized spectra. Using a list of about 1,100 photospheric spectral lines, we computed a cross correlation line profile from every spectrum, leading to a signal-to-noise ratio of up to 30,000 in the polarized profile. We report the repeated detection of circularly polarized, highly asymmetric signatures in the line profiles, interpreted as Zeeman signatures of a large-scale photospheric magnetic field, with a line-of-sight component equal to $0.2 \pm 0.1$ G. This is the first polarimetric detection of a surface magnetic field on an Am star. Using rough estimates of the physical properties of the upper layers of Sirius A, we suggest that a dynamo operating in the shallow convective envelope cannot account for the field strength reported here. Together with the magnetic field of Vega, this result confirms that a new class of magnetic objects exists among non Ap/Bp intermediate-mass stars, and it may indicate that a significant fraction of tepid stars are magnetic., Comment: Accepted by Astronomy and Astrophysics

Despite more and more observational data, stellar acoustic oscillation modes are not well understood as soon as rotation cannot be treated perturbatively. In a way similar to semiclassical theory in quantum physics, we use acoustic ray dynamics to build an asymptotic theory for the subset of regular modes which are the easiest to observe and identify. Comparisons with 2D numerical simulations of oscillations in polytropic stars show that both the frequency and amplitude distributions of these modes can accurately be described by an asymptotic theory for almost all rotation rates. The spectra are mainly characterized by two quantum numbers; their extraction from observed spectra should enable one to obtain information about stellar interiors., Comment: 5 pages, 4 figures, discussion added

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

According to a recent ray-based asymptotic theory, the high-frequency p-mode spectrum of rapidly rotating stars is a superposition of frequency subsets associated with dynamically independent regions of the ray-dynamics phase space. At high rotation rates corresponding to typical $\delta$ Scuti stars, two frequency subsets are expected to be visible : a regular frequency subset described by a Tassoul like formula and an irregular frequency subset with specific statistical properties. In this paper, we investigate whether the regular patterns can be detected in the resulting spectrum. We compute the autocorrelation function of synthetic spectra where the frequencies follow the asymptotic theory, the relative amplitudes are simply given by the modes' disk-averaging factors, and the frequency resolution is that of a CoRoT long run. Our first results are that (i) the detection of regular patterns strongly depends on the ratio of regular over irregular modes, (ii) low inclination angle configurations are more favorable than near equator-on configurations, (iii) in the absence of differential rotation, the $2 \Omega$ rotational splitting between $m=1$ and $m=-1$ modes is an easy feature to detect., Comment: 4 pages, 4 figures, proceedings of the HELAS-IV International Conference, accepted for publication in Astronomische Nachrichten

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

This work aims at determining the impact of slow to moderate rotation on the regular patterns often present in solar-like oscillation spectra. We focus on the well-known asteroseismic diagnostic echelle diagrams, examining how rotation may modify the estimates of the large and small spacings, as well as the identification of modes. We illustrate the work with a real case: the solar-like star $\eta$ Bootis. The modeling takes into account rotation effects on the equilibrium models through an effective gravity and on the oscillation frequencies through both perturbative and non-perturbative calculations. We compare the results of both type of calculations in the context of the regular spacings (like the small spacings and the scaled small spacings) and echelle diagrams. We show that for echelle diagrams the perturbative approach remains valid for rotational velocities up to 40-50 km/s. We show that for the rotational velocities measured in solar-like stars, theoretical oscillation frequencies must be corrected up to the second-order in terms of rotation rate, including near degeneracy effects. For rotational velocities of about 16 km/S and higher, diagnostics on large spacings and on modal identification through echelle diagrams can be significantly altered by the presence of the $m\neq0$ components of the rotationally split modes. We found these effects to be detectable in the observed frequency range. Analysis of the effects of rotation on small spacings and scaled small spacings reveals that these can be of the order of, or even larger than surface effects, typically turbulence, microscopic diffusion, etc. Furthermore, we show that scaled spacings are significantly affected by stellar distortion even for small stellar rotational velocities (from 10-15 km/s) and therefore some care must be taken when using them as indicators for probing deep stellar interiors., Comment: 10 pages,5 figures, accepted for publication in ApJ; http://iopscience.iop.org/0004-637X/721/1/537/

We investigate the magnetic dichotomy between Ap/Bp and other A-type stars by carrying out a deep spectropolarimetric study of Am and HgMn stars. Using the NARVAL spectropolarimeter at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France), we obtained high-resolution circular polarisation spectroscopy of 12 Am stars and 3 HgMn stars. Using Least Squares Deconvolution (LSD), no magnetic field is detected in any of the 15 observed stars. Uncertaintiies as low as 0.3 G (respectively 1 G) have been reached for surface-averaged longitudinal magnetic field measurements for Am (respectively HgMn) stars. Associated with the results obtained previously for Ap/Bp stars, our study confirms the existence of a magnetic dichotomy among A-type stars. Our data demonstrate that there is at least one order of magnitude difference in field strength between Zeeman detected stars (Ap/Bp stars) and non Zeeman detected stars (Am and HgMn stars). This result confirms that the spectroscopically-defined Ap/Bp stars are the only A-type stars harbouring detectable large-scale surface magnetic fields., Comment: 6 pages, 3 figures, accepted for publication in A&A

The recent discovery of a weak surface magnetic field on the normal intermediate-mass star Vega raises the question of the origin of this magnetism in a class of stars that was not known to host magnetic fields. We aim to confirm the field detection and provide additional observational constraints about the field characteristics, by modelling the magnetic geometry of the star and by investigating the seasonal variability of the reconstructed field. We analyse a total of 799 circularly-polarized spectra collected with the NARVAL and ESPaDOnS spectropolarimeters during 2008 and 2009. We employ a cross-correlation procedure to compute, from each spectrum, a mean polarized line profile with a signal-to-noise ratio of about 20,000. The technique of Zeeman-Doppler Imaging is then used to determine the rotation period of the star and reconstruct the large-scale magnetic geometry of Vega at two different epochs. We confirm the detection of circularly polarized signatures in the mean line profiles. The amplitude of the signatures is larger when spectral lines of higher magnetic sensitivity are selected for the analysis, as expected for a signal of magnetic origin. The short-term evolution of polarized signatures is consistent with a rotational period of 0.732 \pm 0.008 d. The reconstructed magnetic topology unveils a magnetic region of radial field orientation, closely concentrated around the rotation pole. This polar feature is accompanied by a small number of magnetic patches at lower latitudes. No significant variability in the field structure is observed over a time span of one year. The repeated observation of a weak photospheric magnetic field on Vega suggests that a previously unknown type of magnetic stars exists in the intermediate-mass domain. Vega may well be the first confirmed member of a much larger, as yet unexplored, class of weakly-magnetic stars., Comment: Accepted by Astronomy & Astrophysics. Abstract shortened to respect the arXiv limit of 1920 characters

CoRoT and Kepler missions are now providing high-quality asteroseismic data for a large number of stars. Among intermediate-mass and massive stars, fast rotators are common objects. Taking the rotation effects into account is needed to correctly understand, identify, and interpret the observed oscillation frequencies of these stars. A classical approach is to consider the rotation as a perturbation. In this paper, we focus on gravity modes, such as those occurring in gamma Doradus, slowly pulsating B (SPB), or Be stars. We aim to define the suitability of perturbative methods. With the two-dimensional oscillation program (TOP), we performed complete computations of gravity modes -including the Coriolis force, the centrifugal distortion, and compressible effects- in 2-D distorted polytropic models of stars. We started with the modes l=1, n=1-14, and l=2-3, n=1-5,16-20 of a nonrotating star, and followed these modes by increasing the rotation rate up to 70% of the break-up rotation rate. We then derived perturbative coefficients and determined the domains of validity of the perturbative methods. Second-order perturbative methods are suited to computing low-order, low-degree mode frequencies up to rotation speeds ~100 km/s for typical gamma Dor stars or ~150 km/s for B stars. The domains of validity can be extended by a few tens of km/s thanks to the third-order terms. For higher order modes, the domains of validity are noticeably reduced. Moreover, perturbative methods are inefficient for modes with frequencies lower than the Coriolis frequency 2Omega. We interpret this failure as a consequence of a modification in the shape of the resonant cavity that is not taken into account in the perturbative approach., Comment: 8 pages, 6 figures, Astronomy & Astrophysics (in press)

Using the Two-dimensional Oscillation Program (TOP), we have explored the effects of rapid rotation on gravity modes in polytropic stars. Coriolis force, centrifugal distortion as well as compressible effects have been taken into account. Thanks to our complete calculation, we have first studied the validity domain of perturbative methods and started to explore properties of these modes. We focus on l=1 in this analysis., Comment: Proc. HELAS Workshop on `New insights into the Sun', Ponte de Lima, Portugal, 2009, M. S. Cunha & M. J. Thompson (eds)

The asteroseismology of rapidly rotating pulsating stars is hindered by our poor knowledge of the effect of the rotation on the oscillation properties. Here we present an asymptotic analysis of high-frequency acoustic modes in rapidly rotating stars. We study the Hamiltonian dynamics of acoustic rays in uniformly rotating polytropic stars and show that the phase space structure has a mixed character, regions of chaotic trajectories coexisting with stable structures like island chains or invariant tori. In order to interpret the ray dynamics in terms of acoustic mode properties, we then use tools and concepts developed in the context of quantum physics. Accordingly, the high-frequency acoustic spectrum is a superposition of frequency subsets associated with dynamically independent phase space regions. The sub-spectra associated with stable structures are regular and can be modelled through EBK quantization methods while those associated with chaotic regions are irregular but with generic statistical properties. The results of this asymptotic analysis are successfully confronted with the properties of numerically computed high-frequency acoustic modes. The implications for the asteroseismology of rapidly rotating stars are discussed., Comment: 21 pages, 11 figures, accepted for publication in Astronomy and Astrophysics

We report the detection of a magnetic field on Vega through spectropolarimetric observations. We acquired 257 Stokes V, high signal-to-noise and high-resolution echelle spectra during four consecutive nights with the NARVAL spectropolarimeter at the 2-m Telescope Bernard Lyot of Observatoire du Pic du Midi (France). A circularly polarized signal in line profiles is unambiguously detected after combining the contribution of about 1200 spectral lines for each spectrum and summing the signal over the 257 spectra. Due to the low amplitude of the polarized signal, various tests have been performed to discard the possibility of a spurious polarized signal. They all point towards a stellar origin of the polarized signal. Interpreting this polarization as a Zeeman signature leads to a value of $-0.6 \pm 0.3$ G for the disk-averaged line-of-sight component of the surface magnetic field. This is the first strong evidence of a magnetic field in an A-type star which is not an Ap chemically peculiar star. Moreover, this longitudinal magnetic field is smaller by about two orders of magnitude than the longitudinal magnetic field (taken at its maximum phase) of the most weakly magnetic Ap stars. Magnetic fields similar to the Vega magnetic field could be present but still undetected in many other A-type stars., Comment: 4 pages, accepted for publication as a Letter in Astronomy & Astrophysics

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 H\alpha line. 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., Comment: 8 pages, 6 figures. Accepted for publication in A&A

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

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

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., Comment: MNRAS (in press)

Effects of rapid stellar rotation on acoustic oscillation modes are poorly understood. We study the dynamics of acoustic rays in rotating polytropic stars and show using quantum chaos concepts that the eigenfrequency spectrum is a superposition of regular frequency patterns and an irregular frequency subset respectively associated with near-integrable and chaotic phase space regions. This opens new perspectives for rapidly rotating star seismology and also provides a new and potentially observable manifestation of wave chaos in a large scale natural system., Comment: 5 pages, 3 figures; accepted for publication in Phys. Rev. E

Context: Rapid rotation modifies the structure of the frequency spectrum of pulsating stars, thus making mode identification difficult. Aims: We look for new forms of organisation for the frequency spectrum that can provide a basis for mode identification at high rotation rates. Methods: Acoustic modes in uniformly rotating polytropic models of stars are computed using a numerical code that fully takes the effects of rotation (centrifugal distortion and Coriolis acceleration) into account. All low-degree modes, l=0 to 3, with radial orders n=1-10 and 21-25 for N=3 polytropic models and n=1-10 for N=1.5 polytropic models are followed from a zero rotation rate up to 59 % of the break-up velocity. Results: We find an empirical formula that gives a good description of the high-frequency range of the computed acoustic spectrum for high rotation rates. Differences between this formula and complete eigenmode calculations are shown to be substantially smaller than those obtained with a third order perturbative method valid at low rotation rates., Comment: 4 pages, 3 figures, 2 tables, accepted for publication in A&A

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., Comment: 6 pages, 2 figures. Proceedings of Solar Polarisation Workshop #5

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., Comment: Accepted by Astronomy & Astrophysics, 17 Sept. 07. 23 pages, 32 figures

We present the results of a very large aspect ratio (42.6) numerical simulation of fully compressible turbulent convection in a polytropic atmosphere, and focus on the properties of large-scale flows. Mesoscale patterns dominate the turbulent energy spectrum. We show that these structures, which had already been observed in Boussinesq simulations by Cattaneo et al. (2001), have a genuine convective origin and do not result directly from collective interactions of the smaller scales of the flow, even though their growth is strongly affected by nonlinear transfers. If this result is relevant to the solar photosphere, it suggests that the dominant convective mode below the Sun's surface may be at mesoscales., Comment: 5 pages, 7 figures (reduced quality) -- published in A&A in 2005 but not uploaded to the arxiv

Context: With the launch of space missions devoted to asteroseismology (like COROT), the scientific community will soon have accurate measurements of pulsation frequencies in many rapidly rotating stars. Aims: The present work focuses on the effects of rotation on pulsations of rapidly rotating stars when both the Coriolis and centrifugal accelerations require a non-perturbative treatment. Method: We develop a 2-dimensional spectral numerical approach which allows us to compute acoustic modes in centrifugally distorted polytropes including the full influence of the Coriolis force. This method is validated through comparisons with previous studies, and the results are shown to be highly accurate. Results: In the frequency range considered and with COROT's accuracy, we establish a domain of validity for perturbative methods, thus showing the need for complete calculations beyond v.sin i = 50 km/s for a R = 2.3 R_\odot, M = 1.9 M_\odot polytropic star. Furthermore, it is shown that the main differences between complete and perturbative calculations come essentially from the centrifugal distortion., Comment: published in A&A, corrected minor mistakes and updated some references

A new non-perturbative method to compute accurate oscillation modes in rapidly rotating stars is presented. In this paper, the effect of the centrifugal force is fully taken into account while the Coriolis force is neglected. This assumption is valid when the time scale of the oscillation is much shorter than the inverse of the rotation rate and is expected to be suitable for high radial order p-modes of $\delta$ Scuti stars. Axisymmetric p-modes have been computed in uniformly rotating polytropic models of stars. In the frequency and rotation range considered, we found that as rotation increases (i) the asymptotic structure of the non-rotating frequency spectrum is first destroyed then replaced by a new form of organization (ii) the mode amplitude tends to concentrate near the equator (iii) differences with perturbative methods become significant as soon as the rotation rate exceeds about fifteen percent of the Keplerian limit. The implications for the seismology of rapidly rotating stars are then discussed., Comment: accepted for publication in A&A

With this work, we have conducted an extensive search for magnetic fields in the photosphere of $\alpha$~And. We have acquired new circular polarisation spectra with the MuSiCoS and ESPaDOnS sp ectropolarimeters. We have also obtained FORS1 circular polarisation spectra fro m the ESO Archive, and considered all previously published magnetic data. This e xtensive dataset has been used to systematically test for the presence of magnet ic fields in the photosphere of $\alpha$~And. We have also examined the high-res olution spectra for line profile variability. The polarimetric and magnetic data provide no convincing evidence for photospheric magnetic fields. The highest-S/N phase- and velocity-resolved Stokes $V$ profiles, obtained with ESPaDOnS, allow us to place a $3\sigma$ upper limit of abou t 100~G on the possible presence of any undetected pure dipolar, quadrupolar or octupolar surface magnet ic fields (and just 50~G for fields with significant obliquity). We also conside r and dismiss the possible existence of more complex fossil and dynamo-generated fields, and discuss the implications of these results for explaining the non-un iform surface distribution of Hg. The very high-quality ESPaDOnS spectra have al lowed us to confidently detect variability of Hg~{\sc ii} $\lambda 6149$, $\lamb da 5425$ and $\lambda 5677$. The profile variability of the Hg~{\sc ii} lines is strong, and similar to that of the Hg~{\sc ii} $\lambda 3984$ line. On the ot her hand, variability of other lines (e.g. Mn, Fe) is much weaker, and appears to be attributable to orbital modulation, continuum normalisation differences and weak, variable fringing.

We investigate the magnetic geometry of the active G8 dwarf xi Bootis A, from spectropolarimetric observations obtained in 2003 with the MuSiCoS echelle spectropolarimeter at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We repeatedly detect a photospheric magnetic field, with periodic variations consistent with rotational modulation. Circularly polarized (Stokes V) line profiles present a systematic asymmetry, showing up as an excess in amplitude and area of the blue lobe of the profiles. A direct modeling of Stokes V profiles suggests that the global magnetic field is composed of two main components, with an inclined dipole and a large-scale toroidal field. We derive a dipole intensity of about 40 G, with an inclination of 35 degrees of the dipole with respect to the rotation axis. The toroidal field strength is of order of 120 G. A noticeable evolution of the field geometry is observed over the 40 nights of our observing window and results in an increase of the field strength and of the dipole inclination. This study is the first step of a long-term monitoring of xi Bootis A and other active solar-type stars, with the aim to investigate secular fluctuations of stellar magnetic geometries induced by activity cycles., Comment: accepted by MNRAS