Near-degeneracy effects on the frequencies of rotationally-split mixed modes in red giants

Deheuvels, S.; Ouazzani, R. M.; Basu, Sarbani

doi:10.1051/0004-6361/201730786

Cited by 38 publications

(47 citation statements)

References 47 publications

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“…Recent developments in asteroseismology have led to a wealth of information on the internal properties of mainly low-and intermediate-mass stars (recently reviewed in Aerts et al 2019). In particular, the analysis of mixed modes have been used to infer core rotation rates in post-main sequence stars (Beck et al 2012;Deheuvels et al 2012Deheuvels et al , 2014Deheuvels et al , 2015Deheuvels et al , 2017Mosser et al 2012;Di Mauro et al 2016Gehan et al 2018;Tayar et al 2019). A large discrepancy exists between the predicted values of stellar evolution models and observed values (Eggenberger et al 2012;Ceillier et al 2013;Marques et al 2013;Cantiello et al 2014), which confirms the findings of earlier works (Pinsonneault et al 1989;Chaboyer et al 1995;Eggenberger et al 2005;Suijs et al 2008;Denissenkov et al 2010).…”

Section: Introductionsupporting

confidence: 69%

Asteroseismology of evolved stars to constrain the internal transport of angular momentum

Hartogh

Eggenberger

Deheuvels

2020

A&A

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Context. The internal characteristics of stars, such as their core rotation rates, are obtained via asteroseismic observations. A comparison of core rotation rates found in this way with core rotation rates as predicted by stellar evolution models demonstrate a large discrepancy. This means that there must be a process of angular momentum transport missing in the current theory of stellar evolution. A new formalism was recently proposed to fill in for this missing process, which has the Tayler instability as its starting point (by Fuller et al. 2019, MNRAS, 485, 3661, hereafter referred to as “Fuller-formalism”). Aims. We investigate the effect of the Fuller-formalism on the internal rotation of stellar models with an initial mass of 2.5 M⊙. Methods. Stellar evolution models, including the Fuller-formalism, of intermediate-mass stars were calculated to make a comparison between asteroseismically obtained core rotation rates in the core He burning phase and in the white dwarf phase. Results. Our main results show that models including the Fuller-formalism can match the core rotation rates obtained for the core He burning phases. However, these models are unable to match the rotation rates obtained for white dwarfs. When we exclude the Fuller-formalism at the end of the core He burning phase, the white dwarf rotation rates of the models match the observed rates. Conclusions. We conclude that in the present form, the Fuller-formalism cannot be the sole solution for the missing process of angular momentum transport in intermediate-mass stars.

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Section: Introductionsupporting

confidence: 69%

Asteroseismology of evolved stars to constrain the internal transport of angular momentum

Hartogh

Eggenberger

Deheuvels

2020

A&A

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“…So, even relatively weak fossil fields (Br 10 −2 G) may enforce nearly rigid rotation of the radiative core. While the internal field strengths of red giants are not well known 3 , a rigidly rotating core enforced by fossil fields would necessitate large differential rotation in the 2 We are concerned that measurement bias limits the number of core rotation measurements for stars with more rapidly rotating cores higher up the RGB, where the rotational frequency splitting becomes comparable to the mixed mode period spacing, and the asteroseismic power spectrum becomes difficult to interpret (Deheuvels et al 2017). 3 While some red giants with suppressed dipole oscillation modes may have very strong (Br 10 5 G) magnetic fields (Fuller et al 2015;Stello et al 2016), those whose internal rotation has been measured must have weaker fields in order for gravity waves to propagate in their cores such that the core rotation rate can be measured.…”

Section: Discussionmentioning

confidence: 99%

Slowing the spins of stellar cores

Fuller

Piro

Jermyn

2019

Monthly Notices of the Royal Astronomical Society

303

383

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The angular momentum (AM) evolution of stellar interiors, along with the resulting rotation rates of stellar remnants, remains poorly understood. Asteroseismic measurements of red giant stars reveal that their cores rotate much faster than their surfaces, but much slower than theoretically predicted, indicating an unidentified source of AM transport operates in their radiative cores. Motivated by this, we investigate the magnetic Tayler instability and argue that it saturates when turbulent dissipation of the perturbed magnetic field energy is equal to magnetic energy generation via winding. This leads to larger magnetic field amplitudes, more efficient AM transport, and smaller shears than predicted by the classic Tayler-Spruit dynamo. We provide prescriptions for the effective AM diffusivity and incorporate them into numerical stellar models, finding they largely reproduce (1) the nearly rigid rotation of the Sun and main sequence stars, (2) the core rotation rates of low-mass red giants during hydrogen shell and helium burning, and (3) the rotation rates of white dwarfs. We discuss implications for stellar rotational evolution, internal rotation profiles, rotational mixing, and the spins of compact objects.

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“…Finally, using our best-fit models for the two stars, we could estimate the minimum separation between consecutive mixed modes, in order to test whether asymmetries in rotational multiplets due to near-degeneracy effects are expected (Deheuvels et al 2017). For KIC8524425, we found minimum separations of 30.7 µHz for l = 1 modes and 10.7 µHz for l = 2 modes.…”

Section: Figmentioning

confidence: 94%

Seismic evidence for near solid-body rotation in two Kepler subgiants and implications for angular momentum transport

Deheuvels

Ballot

Eggenberger

et al. 2020

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Context. Asteroseismic measurements of the internal rotation of subgiants and red giants all show the need for invoking a more efficient transport of angular momentum than theoretically predicted. Constraints on the core rotation rate are available starting from the base of the red giant branch (RGB) and we are still lacking information on the internal rotation of less evolved subgiants. Aims. We identify two young Kepler subgiants, KIC8524425 and KIC5955122, whose mixed modes are clearly split by rotation. We aim to probe their internal rotation profile and assess the efficiency of the angular momentum transport during this phase of the evolution. Methods. Using the full Kepler data set, we extracted the mode frequencies and rotational splittings for the two stars using a Bayesian approach. We then performed a detailed seismic modeling of both targets and used the rotational kernels to invert their internal rotation profiles using the MOLA inversion method. We thus obtained estimates of the average rotation rates in the g-mode cavity (Ω g) and in the p-mode cavity (Ω p). Results. We found that both stars are rotating nearly as solid bodies, with core-envelope contrasts of Ω g/ Ω p = 0.68 ± 0.47 for KIC8524425 and Ω g/ Ω p = 0.72 ± 0.37 for KIC5955122. This result shows that the internal transport of angular momentum has to occur faster than the timescale at which differential rotation is forced in these stars (between 300 Myr and 600 Myr). By modeling the additional transport of angular momentum as a diffusive process with a constant viscosity ν add , we found that values of ν add > 5 × 10 4 cm 2 .s −1 are required to account for the internal rotation of KIC8524425, and ν add > 1.5 × 10 5 cm 2 .s −1 for KIC5955122. These values are lower than or comparable to the efficiency of the core-envelope coupling during the main sequence, as given by the surface rotation of stars in open clusters. On the other hand, they are higher than the viscosity needed to reproduce the rotation of subgiants near the base of the RGB. Conclusions. Our results yield further evidence that the efficiency of the internal redistribution of angular momentum decreases during the subgiant phase. We thus bring new constraints that will need to be accounted for by mechanisms that are proposed as candidates for angular momentum transport in subgiants and red giants.

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Near-degeneracy effects on the frequencies of rotationally-split mixed modes in red giants

Cited by 38 publications

References 47 publications

Asteroseismology of evolved stars to constrain the internal transport of angular momentum

Asteroseismology of evolved stars to constrain the internal transport of angular momentum

Slowing the spins of stellar cores

Seismic evidence for near solid-body rotation in two Kepler subgiants and implications for angular momentum transport

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