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Liège, Belgium

Eggenberger P.,Observatoire de Geneva | Montalban J.,Luniversite Of Liege | Miglio A.,University of Birmingham
Astronomy and Astrophysics

Context. Recent asteroseismic observations have led to the determination of rotational frequency splittings for ℓ = 1 mixed modes in red giants. Aims. We investigate how these observed splittings can constrain the modelling of the physical processes transporting angular momentum in stellar interiors. Methods. We first compare models including a comprehensive treatment of shellular rotation only, with the rotational splittings observed for the red giant KIC 8366239. We then study how these asteroseismic constraints can give us information about the efficiency of an additional mechanism for the internal transport of angular momentum. This is done by computing rotating models of KIC 8366239 that include a constant viscosity corresponding to this physical process, in addition to the treatment of shellular rotation. Results. We find that models of red giant stars including shellular rotation only predict steep rotation profiles, which are incompatible with the measurements of rotational splittings in the red giant KIC 8366239. Meridional circulation and shear mixing alone are found to produce an insufficient internal coupling so that an additional mechanism for the internal transport of angular momentum is needed during the post-main sequence evolution. We show that the viscosity ν add corresponding to this mechanism is strongly constrained to be ν add = 3 × 10 4 cm 2 s -1 thanks to the observed ratio of the splittings for modes in the wings to those at the centre of the dipole forests. Such a value of viscosity may suggest that the same unknown physical process is at work during the main sequence and the post-main sequence evolution. © 2012 ESO. Source

Samadi R.,University Paris Diderot | Belkacem K.,University Paris Diderot | Dupret M.-A.,Luniversite Of Liege | Ludwig H.-G.,University of Heidelberg | And 6 more authors.
Astronomy and Astrophysics

Context. A growing number of solar-like oscillations has been detected in red giant stars thanks to the CoRoT and Kepler space-crafts. In the same way as for main-sequence stars, mode driving is attributed to turbulent convection in the uppermost convective layers of those stars. Aims. The seismic data gathered by CoRoT on red giant stars allow us to test the mode driving theory in physical conditions different from main-sequence stars. Methods. Using a set of 3D hydrodynamical models representative of the upper layers of sub-and red giant stars, we computed the acoustic mode energy supply rate (p mex). Assuming adiabatic pulsations and using global stellar models that assume that the surface stratification comes from the 3D hydrodynamical models, we computed the mode amplitude in terms of surface velocity. This was converted into intensity fluctuations using either a simplified adiabatic scaling relation or a non-adiabatic one. Results. From L and M (the luminosity and mass), the energy supply rate {p max} is found to scale as (L/M) 2.6 for both main-sequence and red giant stars, extending previous results. The theoretical amplitudes in velocity under-estimate the Doppler velocity measurements obtained so far from the ground for red giant stars by about 30%. In terms of intensity, the theoretical scaling law based on the adiabatic intensity-velocity scaling relation results in an under-estimation by a factor of about 2.5 with respect to the CoRoT seismic measurements. On the other hand, using the non-adiabatic intensity-velocity relation significantly reduces the discrepancy with the CoRoT data. The theoretical amplitudes remain 40% below, however, the CoRoT measurements. Conclusions. Our results show that scaling relations of mode amplitudes cannot be simply extended from main-sequence to red giant stars in terms of intensity on the basis of adiabatic relations because non-adiabatic effects for red giant stars are important and cannot be neglected. We discuss possible reasons for the remaining differences. © 2012 ESO. Source

Mosser B.,University Pierre and Marie Curie | Benomar O.,University of Tokyo | Benomar O.,University of Sydney | Belkacem K.,University Pierre and Marie Curie | And 24 more authors.
Astronomy and Astrophysics

Context. The detection of oscillations with a mixed character in subgiants and red giants allows us to probe the physical conditions in their cores. Aims. With these mixed modes, we aim at determining seismic markers of stellar evolution. Methods. Kepler asteroseismic data were selected to map various evolutionary stages and stellar masses. Seismic evolutionary tracks were then drawn with the combination of the frequency and period spacings. Results. We measured the asymptotic period spacing for 1178 stars at various evolutionary stages. This allows us to monitor stellar evolution from the main sequence to the asymptotic giant branch and draw seismic evolutionary tracks. We present clear quantified asteroseismic definitions that characterize the change in the evolutionary stages, in particular the transition from the subgiant stage to the early red giant branch, and the end of the horizontal branch. Conclusions. The seismic information is so precise that clear conclusions can be drawn independently of evolution models. The quantitative seismic information can now be used for stellar modeling, especially for studying the energy transport in the helium-burning core or for specifying the inner properties of stars entering the red or asymptotic giant branches. Modeling will also allow us to study stars that are identified to be in the helium-subflash stage, high-mass stars either arriving or quitting the secondary clump, or stars that could be in the blue-loop stage. © ESO, 2014. Source

Buldgen G.,Luniversite Of Liege | Reese D.R.,University of Birmingham | Reese D.R.,University Paris Diderot | Dupret M.A.,Luniversite Of Liege
Astronomy and Astrophysics

Context. Constraining additional mixing processes and chemical composition is a central problem in stellar physics as their impact on determining stellar age leads to biases in our studies of stellar evolution, galactic history and exoplanetary systems. In two previous papers, we have shown how seismic inversion techniques could be used to offer strong constraints on such processes by pointing out weaknesses in current theoretical models. The theoretical approach having been tested, we now wish to apply our technique to observations. In that sense, the solar analogues 16CygA and 16CygB, being amongst the best targets in the Kepler field, are probably currently the most well suited stars to test the diagnostic potential of seismic inversions. Aims. We wish to use seismic indicators obtained through inversion techniques to constrain additional mixing processes in the components of the binary system 16Cyg. The combination of various seismic indicators will help to point out the weaknesses of stellar models and thus obtain more constrained and accurate fundamendal parameters for these stars. Methods. First, we used the latest seismic, spectroscopic and interferometric observational constraints in the literature for this system to independently determine suitable reference models for both stars. We then carried out seismic inversions of the acoustic radius, the mean density and a core conditions indicator. These additional constraints will be used to improve the reference models for both stars. Results. The combination of seismic, interferometric and spectroscopic constraints allows us to obtain accurate reference models for both stars. However, we note that it is possible to achieve similar accuracy for a range of model parameters. Namely, changing the diffusion coefficient or the chemical composition within the observational values could lead to a 5% uncertainty in mass, a 3% uncertainty in radius and up to an 8% uncertainty in age. We used acoustic radius and mean density inversions to further improve our reference models and then carried out inversions for a core conditions indicator, denoted tu. Thanks to the sensitivity of this indicator to microscopic diffusion and chemical composition mismatches, we were able to reduce the mass uncertainties to 2%, namely between [0.96 Mo, 1.0 Mo], the radius uncertainties to 1%, namely between [1.188 Ro, 1.200 Ro] and the age uncertainties to 3%, namely between [7.0 Gy, 7.4 Gy], for 16CygA. For 16CygB, tu offered a consistency check for the models but could not be used to independently reduce the initial scatter observed for the fundamental parameters. Nonetheless, assuming consistency with the age of 16CygA can help to further constrain its mass and radius. We thus find that the mass of 16CygB should be between 0.93 Mo and 0.96 Mo and its radius between 1.08 Ro and 1.10 Ro © ESO, 2016. Source

Fontaine G.,University of Montreal | Brassard P.,University of Montreal | Charpinet S.,Toulouse 1 University Capitole | Charpinet S.,French National Center for Scientific Research | And 3 more authors.
Astronomy and Astrophysics

We present the results of about a decade of efforts toward building an empirical mass distribution for hot B subdwarf stars on the basis of asteroseismology. So far, our group has published detailed analyses pertaining to 16 pulsating B subdwarfs, including estimates of the masses of these pulsators. Given that measurements of the masses of B subdwarfs through more classical methods (such as full orbital solutions in binary stars) have remained far and few, asteroseismology has proven a tool of choice in this endeavor. On the basis of a first sample of 15 pulsators, we find a relatively sharp mass distribution with a mean mass of 0.470 M ⊙, a median value of 0.470 M ⊙, and a narrow range 0.441-0.499 M ⊙ containing some 68.3% of the stars. We augmented our sample with the addition of seven stars (components of eclipsing binaries) with masses reliably established through light curve modeling and spectroscopy. The new distribution is very similar to the former one with a mean mass of 0.470 M ⊙, a median value of 0.471 M ⊙, and a slightly wider range 0.439-0.501 M ⊙ containing some 68.3% of the stars. Although still based on small-number statistics, our derived empirical mass distribution compares qualitatively very well with the expectations of stellar evolution theory. © 2012 ESO. Source

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