arXiv subjects

P. Eggenberger

Publications and source records attributed to P. Eggenberger.

At least 55 records · Page 3Linked to original sources

arXiv2019

Rotation rate of the solar core as a key constraint to magnetic angular momentum transport in stellar interiors

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. 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.

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arXiv2019

The s process in rotating low-mass AGB stars. Nucleosynthesis calculations in models matching asteroseismic constraints

Aims. We investigate the s-process during the AGB phase of stellar models whose cores are enforced to rotate at rates consistent with asteroseismology observations of their progenitors and successors. Methods. We calculated new 2M$_{\odot}$, Z=0.01 models, rotating at 0, 125, and 250 km/s at the start of main sequence. An artificial, additional viscosity was added to enhance the transport of angular momentum in order to reduce the core rotation rates to be in agreement with asteroseismology observations. We compared rotation rates of our models with observed rotation rates during the MS up to the end of core He burning, and the white dwarf phase. Results. We present nucleosynthesis calculations for these rotating AGB models that were enforced to match the asteroseismic constraints on rotation rates of MS, RGB, He-burning, and WD stars. In particular, we calculated one model that matches the upper limit of observed rotation rates of core He-burning stars and we also included a model that rotates one order of magnitude faster than the upper limit of the observations. The s-process production in both of these models is comparable to that of non-rotating models. Conclusions. Slowing down the core rotation rate in stars to match the above mentioned asteroseismic constraints reduces the rotationally induced mixing processes to the point that they have no effect on the s-process nucleosynthesis. This result is independent of the initial rotation rate of the stellar evolution model. However, there are uncertainties remaining in the treatment of rotation in stellar evolution, which need to be reduced in order to confirm our conclusions, including the physical nature of our approach to reduce the core rotation rates of our models, and magnetic processes.

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arXiv2019

Revisiting Kepler-444 Part I. Seismic modelling and inversions of stellar structure

Context. The CoRoT and Kepler missions have paved the way for synergies between exoplanetology and asteroseismology. The use of seismic data helps providing stringent constraints on the stellar properties which directly impact the results of planetary studies. Amongst the most interesting planetary systems discovered by Kepler, Kepler-444 is unique by the quality of its seismic and classical stellar constraints. Its magnitude, age and the presence of 5 small-sized planets orbiting this target makes it an exceptional testbed for exoplanetology. Aims. We aim at providing a detailed characterization of Kepler-444, focusing on the dependency of the results on variations of key ingredients of the theoretical stellar models. This thorough study will serve as a basis for future investigations of the planetary evolution of the system orbiting Kepler-444. Methods. We use local and global minimization techniques to study the internal structure of the exoplanet-host star Kepler-444. We combine seismic observations from the Kepler mission, Gaia DR2 data and revised spectroscopic parameters to precisely constrain its internal structure and evolution. Results. We provide updated robust and precise determinations of the fundamental parameters of Kepler-444 and demonstrate that this low-mass star bore a convective core during a significant portion of its life on the main-sequence. Using seismic data, we are able to estimate the lifetime of the convective core to approximately 8 Gyr out of the 11 Gyr of the evolution of Kepler-444. The revised stellar parameters found by our thorough study are M = 0.754 +- 0.03M_Sun , R = 0.753 +- 0.01R_Sun , Age = 11 +- 1 Gy.

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arXiv2019

Transiting planet candidate from K2 with the longest period

Context: We present the transit and follow-up of a single transit event from Campaign 14 of K2, EPIC248847494b, which has a duration of 54 hours and a 0.18% depth. Aims: Using photometric tools and conducting radial velocity follow-up, we vet and characterise this very strong candidate. Methods: Owing to the long, unknown period, standard follow-up methods needed to be adapted. The transit was fitted using Namaste, and the radial velocity slope was measured and compared to a grid of planet-like orbits with varying masses and periods. These used stellar parameters measured from spectra and the distance as measured by Gaia. Results: Orbiting around a sub-giant star with a radius of 2.70$\pm$0.12R$_{\rm Sol}$, the planet has a radius of 1.11$_{-0.07}^{+0.07}$R$_{\rm Jup}$ and a period of 3650$_{-1130}^{+1280}$ days. The radial velocity measurements constrain the mass to be lower than 13M$_{\rm Jup}$, which implies a planet-like object. Conclusions: We have found a planet at 4.5 AU from a single-transit event. After a full radial velocity follow-up campaign, if confirmed, it will be the longest-period transiting planet discovered.

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arXiv2019

Stellar models and isochrones from low-mass to massive stars including pre-main sequence phase with accretion

Grids of stellar models are useful tools to derive the properties of stellar clusters, in particular young clusters hosting massive stars, and to provide information on the star formation process in various mass ranges. Because of their short evolutionary timescale, massive stars end their life while their low-mass siblings are still on the pre-main sequence (pre-MS) phase. Thus the study of young clusters requires consistent consideration of all the phases of stellar evolution. But despite the large number of grids that are available in the literature, a grid accounting for the evolution from the pre-MS accretion phase to the post-MS phase in the whole stellar mass range is still lacking. We build a grid of stellar models at solar metallicity with masses from 0.8 $M_\odot$ to 120 $M_\odot$, including pre-MS phase with accretion. We use the {\sc genec} code to run stellar models on this mass range. The accretion law is chosen to match the observations of pre-MS objects on the Hertzsprung-Russell diagram. We describe the evolutionary tracks and isochrones of our models. The grid is connected to previous MS and post-MS grids computed with the same numerical method and physical assumptions, which provides the widest grid in mass and age to date. Numerical tables of our models and corresponding isochrones are available online.

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arXiv2019

Combining multiple structural inversions to constrain the Solar modelling problem

The Sun is the most studied of stars and a laboratory of fundamental physics. However, the understanding of our star is stained by the solar modelling problem which can stem from various causes. We combine inversions of sound speed, an entropy proxy and the Ledoux discriminant with the position of the base of the convective zone and the photospheric helium abundance to test combinations of ingredients such as equation of state, abundance and opacity tables. We study the potential of the inversions to constrain ad-hoc opacity modifications and additional mixing in the Sun. We show that they provide constraints on these modifications to the ingredients and that the solar problem likely occurs from various sources and using phase shifts with our approach is the next step to take.

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arXiv2019

Constraining transport of angular momentum in stars: Combining asteroseismic observations of core helium burning stars and white dwarfs

Transport of angular momentum has been a challenging topic within the stellar evolution community, even more since the recent asteroseismic surveys. All published studies on rotation using asteroseismic observations show a discrepancy between the observed and calculated rotation rates, indicating there is an undetermined process of angular momentum transport active in these stars. We aim to constrain the efficiency of this process by investigating rotation rates of 2.5 M$_{\odot}$ stars. First, we investigated whether the Tayler-Spruit dynamo could be responsible for the extra transport of angular momentum for stars with an initial mass of 2.5 M$_{\odot}$. Then, by computing rotating models including a constant additional artificial viscosity, we determined the efficiency of the missing process of angular momentum transport by comparing the models to the asteroseismic observations of core helium burning stars. Parameter studies were performed to investigate the effect of the stellar evolution code used, initial mass, and evolutionary stage. We evolved our models into the white dwarf phase, and provide a comparison to white dwarf rotation rates. The Tayler-Spruit dynamo is unable to provide enough transport of angular momentum to reach the observed values of the core helium burning stars investigated in this paper. We find that a value for the additional artificial viscosity $ν_{\rm{add}}$ around 10$^7$ cm$^2$ s$^{-1}$ provides enough transport of angular momentum. However, the rotational period of these models is too high in the white dwarf phase to match the white dwarf observations. From this comparison we infer that the efficiency of the missing process must decrease during the core helium burning phase. When excluding the $ν_{\rm{add}}$ during core helium burning phase, we can match the rotational periods of both the core helium burning stars and white dwarfs.

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arXiv2018

Disappearance of the extended main sequence turn-off in intermediate age clusters as a consequence of magnetic braking

Extended MS turn-offs are features commonly found in the colour-magnitude diagrams of young and intermediate age (less than about 2 Gyr) massive star clusters, where the MS turn-off is broader than can be explained by photometric uncertainties, crowding, or binarity. Rotation is suspected to be the cause of this feature, by accumulating fast rotating stars, strongly affected by gravity darkening and rotation-induced mixing, near the MS turn-off. This scenario successfully reproduces the tight relation between the age and the actual extent in luminosity of the extended MS turn-off of observed clusters. Below a given mass (dependent on the metallicity), stars are efficiently braked early on the MS due to the interaction of stellar winds and the surface magnetic field, making their tracks converge towards those of non-rotating tracks in the HRD. When these stars are located at the turn-off of a cluster, their slow rotation causes the extended MS turn-off feature to disappear. We investigate the maximal mass for which this braking occurs at different metallicities, and determine the age above which no extended MS turn-off is expected in clusters. Our models predict that the extended MS turn-off phenomenon disappears at ages older than about 2 Gyr. There is a trend with the metallicity, the age at which the disappearance occurs becoming older at higher metallicity. These results are robust between the two codes used in this work, despite some differences in the input physics and in particular in the detailed description of rotation-induced internal processes and of angular momentum extraction by stellar winds. Comparing our results with clusters in the LMC and Galaxy shows a very good fit to the observations. This strengthens the rotation scenario to explain the cause of the extended MS turn-off phenomenon.

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arXiv2018

Asteroseismology of evolved stars to constrain the internal transport of angular momentum. I. Efficiency of transport during the subgiant phase

Context: The observations of solar-like oscillations in evolved stars have brought important constraints on their internal rotation rates. To correctly reproduce these data, an efficient transport mechanism is needed in addition to meridional circulation and shear instability. Aims: We study the efficiency of the transport of angular momentum during the subgiant phase. Results: The precise asteroseismic measurements of both core and surface rotation rates available for the six Kepler targets enable a precise determination of the efficiency of the transport of angular momentum needed for each of these subgiants. These results are found to be insensitive to all the uncertainties related to the modelling of rotational effects before the post-main sequence phase. An interesting exception in this context is the case of young subgiants (typical values of log(g) close to 4), because their rotational properties are sensitive to the degree of radial differential rotation on the main sequence. These young subgiants constitute therefore perfect targets to constrain the transport of angular momentum on the main sequence from asteroseismic observations of evolved stars. As for red giants, we find that the efficiency of the additional transport process increases with the mass of the star during the subgiant phase. However, the efficiency of this undetermined mechanism decreases with evolution during the subgiant phase, contrary to what is found for red giants. Consequently, a transport process with an efficiency that increases with the degree of radial differential rotation cannot account for the core rotation rates of subgiants, while it correctly reproduces the rotation rates of red giant stars. This suggests that the physical nature of the additional mechanism needed for the internal transport of angular momentum may be different in subgiant and red giant stars.

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arXiv2018

Combining multiple structural inversions to constrain the Solar modelling problem

The Sun is the most studied of all stars. It is a reference for all other observed stars and a laboratory of fundamental physics helping us understand processes occuring in conditions irreproducible on Earth. However, our understanding of the Sun is currently stained by the solar modelling problem which can stem from various causes, such as the opacities, the equation of state and the mixing of chemical elements. We combine inversions of sound speed, an entropy proxy and the Ledoux discriminant with constraints such as the position of the base of the convective zone and the photospheric helium abundance. We test various combinations of standard ingredients for solar modelling such as abundance tables, equation of state, formalism for convection and diffusion and opacity tables and study the diagnostic potential of the inversions to constrain ad-hoc modifications of the opacity profile and additional mixing below the convective envelope. Combining inversions provides stringent constraints on the modifications on the models, far beyond what is achievable only from sound speed inversions. We constrain the form and amplitude of the opacity increase required and show that a 15% increase at log T=6.35 provides a significant improvement but that a more global increase of the opacity, within the uncertainties of current tables, coupled with an additional mixing at the bottom of the convective zone gives the best agreement for low metallicity models. We show that high metallicity models do not satisfy all the inversion results. We conclude that the solar problem likely occurs from various small sources, as ingredients such as the equation of state or the formalism of convection can induce small but significant changes and that using phase shift analyses combined with our approach is the next step for a better understanding of the inaccuracies of solar models just below the convective envelope.

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arXiv2018

Mean density inversions for red giants and red clump stars

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.

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arXiv2017

Close binary evolution. III. Impact of tides, wind magnetic braking, and internal angular momentum transport

Massive stars with solar metallicity lose important amounts of rotational angular momentum through their winds. When a magnetic field is present at the surface of a star, efficient angular momentum losses can still be achieved even when the mass-loss rate is very modest, at lower metallicities, or for lower-initial-mass stars. In a close binary system, the effect of wind magnetic braking also interacts with the influence of tides, resulting in a complex evolution of rotation. We study the interactions between the process of wind magnetic braking and tides in close binary systems. We discuss the evolution of a 10 M$_\odot$ star in a close binary system with a 7 M$_\odot$ companion using the Geneva stellar evolution code. The initial orbital period is 1.2 days. The 10 M$_\odot$ star has a surface magnetic field of 1 kG. Various initial rotations are considered. We use two different approaches for the internal angular momentum transport. In one of them, angular momentum is transported by shear and meridional currents. In the other, a strong internal magnetic field imposes nearly perfect solid-body rotation. The evolution of the primary is computed until the first mass-transfer episode occurs. The cases of different values for the magnetic fields and for various orbital periods and mass ratios are briefly discussed. We show that, independently of the initial rotation rate of the primary and the efficiency of the internal angular momentum transport, the surface rotation of the primary will converge, in a time that is short with respect to the main-sequence lifetime, towards a slowly evolving velocity that is different from the synchronization velocity. (abridged).

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arXiv2017

Inversions of the Ledoux discriminant: a closer look at the tachocline

Modelling the base of the solar convective envelope is a tedious problem. Since the first rotation inversions, solar modellers are confronted with the fact that a region of very limited extent has an enormous physical impact on the Sun. Indeed, it is the transition region from differential to solid body rotation, the tachocline, which furthermore is influenced by turbulence and is also supposed to be the seat of the solar magnetic dynamo. Moreover, solar models show significant disagreement with the sound speed profile in this region. In this paper, we show how helioseismology can provide further constraints on this region by carrying out an inversion of the Ledoux discriminant. We compare these inversions for Standard Solar Models built using various opacity tables and chemical abundances and discuss the origins of the discrepancies between Solar Models and the Sun.

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arXiv2017

Seismic inversion of the solar entropy: A case for improving the Standard Solar Model

The Sun is the most constrained and well-studied of all stars. As a consequence, the physical ingredients entering solar models are used as a reference to study all other stars observed in the Universe. However, our understanding of the solar structure is still imperfect, as illustrated by the current debate on the heavy element abundances in the Sun. We wish to provide additional information on the solar structure by carrying out structural inversions of a new physical quantity, a proxy of the entropy of the solar plasma which properties are very sensitive to the temperature gradient below the convective zone. We use new structural kernels to carry out direct inversions of an entropy proxy of the solar plasma and compare the solar structure to various standard solar models built using various opacity tables and chemical abundances. We also link our results to classical tests commonly found in the literature. Our analysis allows us to probe more efficiently the uncertain regions of the solar models, just below the convective zone, paving the way for new in-depth analyses of the Sun taking into account additional physical uncertainties of solar models beyond the specific question of chemical abundances.

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arXiv2017

PLATO as it is: a legacy mission for Galactic archaeology

Deciphering the assembly history of the Milky Way is a formidable task, which becomes possible only if one can produce high-resolution chrono-chemo-kinematical maps of the Galaxy. Data from large-scale astrometric and spectroscopic surveys will soon provide us with a well-defined view of the current chemo-kinematical structure of the Milky Way, but will only enable a blurred view on the temporal sequence that led to the present-day Galaxy. As demonstrated by the (ongoing) exploitation of data from the pioneering photometric missions CoRoT, Kepler, and K2, asteroseismology provides the way forward: solar-like oscillating giants are excellent evolutionary clocks thanks to the availability of seismic constraints on their mass and to the tight age-initial-mass relation they adhere to. In this paper we identify five key outstanding questions relating to the formation and evolution of the Milky Way that will need precise and accurate ages for large samples of stars to be addressed, and we identify the requirements in terms of number of targets and the precision on the stellar properties that are needed to tackle such questions. By quantifying the asteroseismic yields expected from PLATO for red-giant stars, we demonstrate that these requirements are within the capabilities of the current instrument design, provided that observations are sufficiently long to identify the evolutionary state and allow robust and precise determination of acoustic-mode frequencies. This will allow us to harvest data of sufficient quality to reach a 10% precision in age. This is a fundamental pre-requisite to then reach the more ambitious goal of a similar level of accuracy, which will only be possible if we have to hand a careful appraisal of systematic uncertainties on age deriving from our limited understanding of stellar physics, a goal which conveniently falls within the main aims of PLATO's core science.

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arXiv2017

Period-luminosity relations of fast-rotating B-type stars in the young open cluster NGC3766

We study the pulsational properties of rapidly rotating main-sequence B-type stars using linear non-adiabatic analysis of non-radial low-frequency modes taking into account the effect of rotation. We compare the properties of prograde sectoral $g$ and retrograde $r$ modes excited by the $κ$ mechanism at the Fe opacity peak with the newly discovered period-luminosity relation that is obeyed by a group of fast-rotating B-type stars in the young open cluster NGC 3766. The observed relation consists of two sequences in the period versus magnitude diagram, at periods shorter than 0.5 days. We find that this property is consistent with similar period-luminosity relations predicted for excited sectoral prograde $g$-modes of azimuthal orders $m=-1$ and $m=-2$ in fast-rotating stars along an isochrone. We further show that some of the rapidly rotating stars that have photometric variability with periods longer than a day may be caused by $r$-mode pulsation predicted to be excited in these stars. One fast-rotating star, in particular, shows both short and long periods that can be explained by the simultaneous excitation of $g$- and $r$-mode pulsations in models of fast-rotating stars.

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arXiv2016

Constraining the efficiency of angular momentum transport with asteroseismology of red giants: the effect of stellar mass

Context: Constraints on the internal rotation of red giants are now available thanks to asteroseismic observations. Preliminary comparisons with rotating stellar models indicate that an undetermined additional process for the internal transport of angular momentum is required in addition to purely hydrodynamic processes. Aims: We investigate how asteroseismic measurements of red giants can help us characterize the additional transport mechanism. Methods: We first determine the efficiency of the missing transport mechanism for the low-mass red giant KIC 7341231 by computing rotating models that include an additional viscosity corresponding to this process. We then discuss the change in the efficiency of this transport of angular momentum with the mass, metallicity and evolutionary stage. Results: In the case of the low-mass red giant KIC 7341231, we find that the viscosity corresponding to the additional mechanism is constrained to the range 1 x 10^3 - 1.3 x 10^4 cm^2/s. This constraint on the efficiency of the unknown additional transport mechanism during the post-main sequence is obtained independently of any specific assumption about the modelling of rotational effects during the pre-main sequence and the main sequence (in particular, the braking of the surface by magnetized winds and the efficiency of the internal transport of angular momentum before the post-main-sequence phase). When we assume that the additional transport mechanism is at work during the whole evolution of the star together with a solar-calibrated braking of the surface by magnetized winds, the range of nu_add is reduced to 1 - 4 x 10^3 cm^2/s. In addition to being sensitive to the evolutionary stage of the star, we show that the efficiency of the unknown process for internal transport of angular momentum increases with the stellar mass.

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arXiv2016

Stellar variability in open clusters. II. Discovery of a new period-luminosity relation in a class of fast-rotating pulsating stars in NGC 3766

$Context.$ Pulsating stars are windows to the physics of stars enabling us to see glimpses of their interior. Not all stars pulsate, however. On the main sequence, pulsating stars form an almost continuous sequence in brightness, except for a magnitude range between $δ$ Scuti and slowly pulsating B stars. Against all expectations, 36 periodic variables were discovered in 2013 in this luminosity range in the open cluster NGC 3766, the origins of which was a mystery. $Aims.$ We investigate the properties of those new variability class candidates in relation to their stellar rotation rates and stellar multiplicity. $Methods.$ We took multi-epoch spectra over three consecutive nights using ESO's Very Large Telescope. $Results.$ We find that the majority of the new variability class candidates are fast-rotating pulsators that obey a new period-luminosity relation. We argue that the new relation discovered here has a different physical origin to the period-luminosity relations observed for Cepheids. $Conclusions.$ We anticipate that our discovery will boost the relatively new field of stellar pulsation in fast-rotating stars, will open new doors for asteroseismology, and will potentially offer a new tool to estimate stellar ages or cosmic distances.

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