Context. The internal rotation of the Sun constitutes a fundamental constraint when modelling angular momentum transport in stellar interiors. In addition to the more external regions of the solar ...radiative zone probed by pressure modes, measurements of rotational splittings of gravity modes would offer an invaluable constraint on the rotation of the solar core. Aims. We study the constraints that a measurement of the core rotation rate of the Sun could bring on magnetic angular momentum transport in stellar radiative zones. Methods. Solar models accounting for angular momentum transport by hydrodynamic and magnetic instabilities were computed for different initial velocities and disc lifetimes on the pre-main sequence to reproduce the surface rotation velocities observed for solar-type stars in open clusters. The internal rotation of these solar models was then compared to helioseismic measurements. Results. We first show that models computed with angular momentum transport by magnetic instabilities and a recent prescription for the braking of the stellar surface by magnetized winds can reproduce the observations of surface velocities of stars in open clusters. These solar models predict both a flat rotation profile in the external part of the solar radiative zone probed by pressure modes and an increase in the rotation rate in the solar core, where the stabilizing effect of chemical gradients plays a key role. A rapid rotation of the core of the Sun, as suggested by reported detections of gravity modes, is thus found to be compatible with angular momentum transport by magnetic instabilities. Moreover, we show that the efficiency of magnetic angular momentum transport in regions of strong chemical gradients can be calibrated by the solar core rotation rate independently from the unknown rotational history of the Sun. In particular, we find that a recent revised prescription for the transport of angular momentum by the Tayler instability can be easily distinguished from the original Tayler–Spruit dynamo, with a faster rotating solar core supporting the original prescription. Conclusions. By calibrating the efficiency of magnetic angular momentum transport in regions of strong chemical gradients, a determination of the solar core rotation rate through gravity modes is of prime relevance not only for the Sun, but for stars in general, since radial differential rotation precisely develops in these regions during the more advanced stages of evolution.
With four years of nearly continuous photometry from Kepler, we are finally in a good position to apply asteroseismology to ... Doradus stars. In particular, several analyses have demonstrated the ...possibility to detect non-uniform period spacings, which have been predicted to be directly related to rotation. In this paper, we define a new seismic diagnostic for rotation in ... Doradus stars which are too rapidly rotating to present rotational splittings. Based on the non-uniformity of their period spacings, we define the observable S as the slope of the period spacing when plotted as a function of period. We provide a one-to-one relation between this observable S and the internal rotation, which applies widely in the instability strip of ... Doradus stars. We apply the diagnostic to a handful of stars observed by Kepler. Thanks to g modes in ... Doradus stars, we are now able to determine the internal rotation of stars on the lower main sequence, which is still not possible for Sun-like stars. (ProQuest: ... denotes formulae/symbols omitted.)
Since the CoRoT and Kepler missions, the availability of high-quality seismic spectra for red giants has made them the standard clocks and rulers for Galactic Archeology. With the expected excellent ...data from the TESS and PLATO missions, red giants will again play a key role in Galactic studies and stellar physics, thanks to the precise masses and radii determined by asteroseismology. The determination of these quantities is often based on so-called scaling laws, which have been used extensively for main-sequence stars. We show how the SOLA inversion technique can provide robust determinations of the mean density of red giants within 1 per cent of the real value, using only radial oscillations. Combined with radii determinations from Gaia of around 2 per cent precision, this approach provides robust, less model-dependent masses with an error lower than 10 per cent. It will improve age determinations, helping to accurately dissect the Galactic structure and history. We present results on artificial data of standard models, models including an extended atmosphere from averaged 3D simulations and non-adiabatic frequency calculations to test surface effects, and on eclipsing binaries. We show that the inversions provide very robust mean density estimates, using at best seismic information. However, we also show that a distinction between red-giant branch and red-clump stars is required to determine a reliable estimate of the mean density. The stability of the inversion enables an implementation in automated pipelines, making it suitable for large samples of stars.
Context.
Asteroseismic observations enable the characterisation of the internal rotation of evolved stars. These measurements reveal that an unknown efficient angular momentum (AM) transport ...mechanism is needed for subgiant and red giant stars in addition to hydrodynamic transport processes. A revised prescription for AM transport by the magnetic Tayler instability has been recently proposed as a possible candidate for such a missing mechanism.
Aims.
We compare the rotational properties predicted by this magnetic AM transport to asteroseismic constraints obtained for evolved stars with a particular focus on the subgiant phase.
Methods.
We computed models accounting for the recent prescription for AM transport by the Tayler instability with the Geneva stellar evolution code for subgiant and red giant stars, for which an asteroseismic determination of both core and surface rotation rates is available.
Results.
The revised prescription for the transport by the Tayler instability leads to low core rotation rates after the main sequence that are in better global agreement with asteroseismic measurements than those predicted by models with purely hydrodynamic processes or with the original Tayler-Spruit dynamo. A detailed comparison with asteroseismic data shows that the rotational properties of at most two of the six subgiants can be correctly reproduced by models accounting for this revised magnetic transport process. This result is obtained independently of the value adopted for the calibration parameter in this prescription. We also find that this transport by the Tayler instability faces difficulties in simultaneously reproducing asteroseismic measurements available for subgiant and red giant stars. The low values of the calibration parameter needed to correctly reproduce the rotational properties of two of the six subgiants lead to core rotation rates during the red giant phase that are too high. Inversely, the higher values of this parameter needed to reproduce the core rotation rates of red giants lead to a very low degree of radial differential rotation before the red giant phase, which is in contradiction with the internal rotation of subgiant stars.
Conclusions.
In its present form, the revised prescription for the transport by the Tayler instability does not provide a complete solution to the missing AM transport revealed by asteroseismology of evolved stars.
Context. The space-based Kepler mission provided four years of highly precise and almost uninterrupted photometry for hundreds of γ Doradus stars and tens of slowly pulsating B-type (SPB) stars, ...finally allowing us to apply asteroseismology to these gravity mode pulsators. Without rotation, gravity modes are equally spaced in period. This simple structure does not hold in rotating stars for which rotation needs to be taken into account to accurately interpret the oscillation spectrum. Aims. We aim to develop a stellar-model-independent method to analyse and interpret the oscillation spectrum of γ Dor and SPB stars. Methods. Within the traditional approximation of rotation, we highlight the possibility of recovering the equidistance of period spacings by stretching the pulsation periods. The stretching function depends on the degree and azimuthal order of gravity modes and the rotation rate of the star. In this new stretched space, the pulsation modes are regularly spaced by the stellar buoyancy radius. Results. On the basis of this property, we implemented a method to search for these new regularities and simultaneously infer the rotation frequency and buoyancy radius. Tests on synthetic spectra computed with a non-perturbative approach show that we can retrieve these two parameters with reasonable accuracy along with the mode identification. In uniformly rotating models of a typical γ Dor star, and for the most observed prograde dipole modes, we show that the accuracy on the derived parameters is better than 5% on both the internal rotation rate and the buoyancy radius. Finally, we apply the method to two stars of the Kepler field, a γ Dor and an SPB, and compare our results with those of other existing methods. Conclusions. We provide a stellar-model-independent method to obtain the near-core rotation rate, the buoyancy radius, and the mode identification from gravity-mode spectra of γ Dor and SPB stars.
Context.
The
α
Cen stellar system is the closest neighbour to our Sun. Its main component is a binary composed of two main-sequence stars, one more massive than the Sun and one less massive. The ...system’s bright magnitude led to a wealth of astronomical observations over a long period, making it an appealing testbed for stellar physics. In particular, detection of stellar pulsations in both
α
Cen A and B has revealed the potential of asteroseismology for determining its fundamental stellar parameters. Asteroseismic studies have also focused on the presence of a convective core in the A component, but as yet without definitive confirmation.
Aims.
Progress in the determination of solar surface abundances and stellar opacities have yielded new input for stellar theoretical models. We investigate their impact on a reference system such as
α
Cen AB. We seek to confirm the presence of a convective core in
α
Cen A by analysing the role of different stellar physics and the potential of asteroseismic inverse methods.
Methods.
First, we present a new series of asteroseismic calibrations of the binary carried out using forward approach modelling and including updated chemical mixture and opacities in the models. We took advantage of the most up-to-date orbital solution as non-seismic constraints. We then complement our analysis with help of recent asteroseismic diagnostic tools based on inverse methods developed for solar-like stars.
Results.
The inclusion of an updated chemical mixture -that is less metal-rich- appears to reduce the predicted asteroseismic masses of each component. Neither classical asteroseismic indicators such as the frequency ratios, nor asteroseismic inversions favour the presence of a convective core in
α
Cen A. The quality of the observational seismic dataset is the main limiting factor to settle the issue. Implementing new observing strategies to improve the precision on the pulsation frequencies would certainly refine the outcome of asteroseismology for this binary system.
The advent of space photometry with CoRoT and
Kepler
has allowed for the gathering of exquisite and extensive time series for a wealth of main-sequence stars, including
γ
Doradus stars, whose ...detailed seismology was not achievable from the ground.
γ
Doradus stars present an incredibly rich pulsation spectra, with gravito-inertial modes, in some cases supplemented with
δ
Scuti-like pressure modes – for the
hybrid
stars – and, in many cases, with Rossby modes. The present paper aims to show that in addition to these modes which have been established in the radiative envelope, pure inertial modes that are trapped in the convective core can be detected in
Kepler
observations of
γ
Doradus stars thanks to their resonance with the gravito-inertial modes. We started 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 Poincaré equation. We then computed coupling factors, which helped 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
γ
Doradus stars, we are able to show that the pure inertial and gravito-inertial resonances appear as “dips” in the gravito-inertial mode period spacing series at spin parameters that are close to those predicted by the simple model. We find the first evidence of such dips in the
Kepler
γ
Doradus star KIC 5608334. Finally, using complete calculations in isolated convective cores, we find that the spin parameters of the pure inertial and 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 that have been observed in some
Kepler
stars are, in fact, signatures of resonances with pure-inertial modes that are trapped in the convective core. This holds the promise that it would be possible to finally access the central conditions, namely, the rotation and density stratification, of intermediate-mass stars in the main sequence.
Recent asteroseismic studies have revealed that the convective core of
γ
Doradus stars rotates faster than their radiative interior. We study the development of differential rotation near the ...convective core to test angular momentum transport processes that are typically adopted in stellar evolution models. Models that only include the advection of angular momentum by meridional circulation and shear instabilities cannot reproduce current rotational constraints, irrespective of the initial conditions. The latest formulation of internal magnetic fields based on the Tayler instability is indeed able to reproduce the internal rotation rate of post-main sequence stars; however, it appears too efficient during the main sequence and has thus been disfavoured. A less efficient version of the same transport process can simultaneously reproduce the rotation rate of the convective core, the rotation rate in radiative regions as probed by gravity-modes, and the surface rotational velocities of
γ
Doradus stars. Our work suggests that there are additional physical processes apart from internal magnetic fields at work in the stellar interiors of post-main sequence stars.
Context. The 16Cyg binary system hosts the solar-like Kepler targets with the most stringent observational constraints. Indeed, we benefit from very high quality oscillation spectra, as well as ...spectroscopic and interferometric observations. Moreover, this system is particularly interesting since both stars are very similar in mass but the A component is orbited by a red dwarf, whereas the B component is orbited by a Jovian planet and thus could have formed a more complex planetary system. In our previous study, we showed that seismic inversions of integrated quantities could be used to constrain microscopic diffusion in the A component. In this study, we analyse the B component in the light of a more regularised inversion. Aims. We wish to analyse independently the B component of the 16Cyg binary system using the inversion of an indicator dedicated to analyse core conditions, denoted tu. Using this independent determination, we wish to analyse any differences between both stars due to the potential influence of planetary formation on stellar structure and/or their respective evolution. Methods. First, we recall the observational constraints for 16CygB and the method we used to generate reference stellar models of this star. We then describe how we improved the inversion and how this approach could be used for future targets with a sufficient number of observed frequencies. The inversion results were then used to analyse the differences between the A and B components. Results. The inversion of the tu indicator for 16CygB shows a disagreement with models including microscopic diffusion and sharing the chemical composition previously derived for 16CygA. We show that small changes in chemical composition are insufficient to solve the problem but that extra mixing can account for the differences seen between both stars. We use a parametric approach to analyse the impact of extra mixing in the form of turbulent diffusion on the behaviour of the tu values. We conclude on the necessity of further investigations using models with a physically motivated implementation of extra mixing processes including additional constraints to further improve the accuracy with which the fundamental parameters of this system are determined.
Although much work has been done on factors which influence the patterning of species and species trait assemblages in a variety of groups such as plants, vertebrates and invertebrates, few studies ...have been realized at a broad geographic scale. We analyzed patterns of relationships between species, species trait distribution/assembly, and environmental variables from the west of Europe to Slovakia, Poland and Sweden. We created a database by compiling traits and occurrence data of European collembolan species, using literature and personal field studies embracing a large range of environmental gradients (vertical stratification, habitat closure, humus form, soil acidity and moisture, temperature, rainfall, altitude) over which Collembola are supposed to be distributed. Occurrences of the 58 best-documented species, environmental variables and species traits allowed us to (i) show which environmental variables impact the distribution of the 58 species at broad scale and (ii) document to what extent environmental variables and species trait assemblages are related and which trends could be found in trait/environment relationships. The impact of vertical stratification, habitat closure, humus form, soil acidity, soil moisture, temperature, and to a lesser extent rainfall and altitude on species distribution, firstly revealed by indirect gradient analysis (correspondence analysis, CA), was further shown to be significant by direct gradient analysis (canonical correspondence analysis, CCA). RLQ analyses were performed to find linear combination of variables of table R (environmental variables) and linear combinations of the variables of table Q (species traits) of maximum covariance weighted by species occurrence data contained in table L. RLQ followed by permutation tests showed that all tested environmental variables apparently contributed significantly to the assemblages of the twelve species traits studied. A convergence was observed between traits related to vertical stratification and those related to habitat closure/aperture. Well-developed locomotory organs (furcula, legs), presence of sensorial organs sensitive to air movements and light (e.g. trichobothria and eye spots), spherical body, large body size, pigmentation (UV protection and signaling) and sexual reproduction largely occur in epigeic and open habitats, while most of woodland and edaphic habitats are characterized by short locomotory appendages, small body size, high number of defense organs (pseudocelli), presence of post-antennal organs and parthenogenesis. Climate and especially temperature exert an effect on the assemblage of traits that are mostly present above-ground and in open habitats. The contribution of combinations of some environmental variables to the occurrence of each species trait was tested by linear, logistic or multinomial regression (Generalized Linear Models). Vertical stratification, followed by temperature, played a dominant role in the variation of the twelve studied traits. Relationships between traits and environment tested here shows that it is possible to use some traits as proxies to identify potential ecological preferences or tolerances of invertebrate species. However, a significant part of species distribution remained unexplained, probably partly because some traits, like ecophysiological ones, or traits involved in biotic interactions (e.g. competition) were unavailable. The present work is thus a first step towards the creation of models predicting changes in collembolan communities. Further studies are required to inform ecophysiological traits, in order to complete such models. Moreover the niche width of species will have to be determined.
•A database compiling traits and occurrence data of European springtails has been created.•The distribution of best documented species (58) was impacted by environmental variables.•RLQ analyses showed that traits were impacted by vertical stratification, humus form, climate and habitat closure.•The contribution of each environmental variable was tested by GLM.
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