Seismic constraints on the radial dependence of the internal rotation profiles of sixKeplersubgiants and young red giants

Abstract

Context. We still do not understand which physical mechanisms are responsible for the transport of angular momentum inside stars. The recent detection of mixed modes that contain the clear signature of rotation in the spectra of Kepler subgiants and red giants gives us the opportunity to make progress on this question. Aims. Our aim is to probe the radial dependence of the rotation profiles for a sample of Kepler targets. For this purpose, subgiants and early red giants are particularly interesting targets because their rotational splittings are more sensitive to the rotation outside the deeper core than is the case for their more evolved counterparts. Methods. We first extracted the rotational splittings and frequencies of the modes for six young Kepler red giants. We then performed a seismic modeling of these stars using the evolutionary codes Cesam2k and astec. By using the observed splittings and the rotational kernels of the optimal models, we inverted the internal rotation profiles of the six stars. Results. We obtain estimates of the core rotation rates for these stars, and upper limits to the rotation in their convective envelope. We show that the rotation contrast between the core and the envelope increases during the subgiant branch. Our results also suggest that the core of subgiants spins up with time, while their envelope spins down. For two of the stars, we show that a discontinuous rotation profile with a deep discontinuity reproduces the observed splittings significantly better than a smooth rotation profile. Interestingly, the depths that are found to be most probable for the discontinuities roughly coincide with the location of the H-burning shell, which separates the layers that contract from those that expand. Conclusions. We characterized the differential rotation pattern of six young giants with a range of metallicities, and with both radiative and convective cores on the main sequence. This will bring observational constraints to the scenarios of angular momentum transport in stars. Moreover, if the existence of sharp gradients in the rotation profiles of young red giants is confirmed, it is expected to help in distinguishing between the physical processes that could transport angular momentum in the subgiant and red giant branches.

Asteroseismic Signatures of Evolving Internal Stellar Magnetic Fields

“…Several asteroseismic studies (Beck et al 2012;Mosser et al 2012;Deheuvels et al 2014Deheuvels et al , 2015Di Mauro et al 2016) have measured the core rotation rates of RGB/clump stars. The relatively slow core rotation rates indicate that strong angular momentum transport mechanisms are at work (Cantiello et al 2014), coupling the radiative cores with the convective envelope.…”

“…This technique utilizes observations of mixed modes, oscillations that behave as pressure waves (p-modes) near the stellar surface and gravity waves (g-modes) in the stellar core (Scuflaire 1974;Osaki 1975;Aizenman et al 1977;Dziembowski et al 2001;Christensen-Dalsgaard 2004;Dupret et al 2009). Mixed modes have been used successfully to determine the evolutionary status of red giant stars Stello et al 2013;Mosser et al 2014), and to measure their internal rotation rate (Beck et al 2012;Mosser et al 2012;Deheuvels et al 2014Deheuvels et al , 2015. Turbulent convection in the red giant envelope excites these modes, with part of the wave energy leaking through the evanescent region into the gmode cavity.…”

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

“…The measurement of rotational splittings of mixed modes (Beck et al 2012;Deheuvels et al 2012;Mosser et al 2012) has given direct insight into the internal rotation of red giants and their variations with evolution. In the subgiant phase, Deheuvels et al (2014) showed that the core spins up and the envelope spins A&A 605, A75 (2017) down. This was qualitatively expected, considering that the deepest layers below the H-burning shell contract, while the layers above expand.…”

Asteroseismology of 19 low-luminosity red giant stars fromKepler

“…The splitting of mixed modes show that red-giant cores rotate faster than their convective envelope (Beck et al 2012;Deheuvels et al 2012) and opens up the possibility of probing their internal rotation rates (Deheuvels et al 2014(Deheuvels et al , 2015Di Mauro et al 2016). In fact, for our target stars Corsaro et al (2015a) determined the frequencies of many mixed dipolar A&A 591, A99 (2016) KIC ∆ν ν max T eff log g Z/X 3744043 A 9.90 ± 0.05 112.5 ± 0.2 4906 ± 91 3.05 ± 0.11 −0.37 ± 0.04 6117517 B 10.16 ± 0.05 120.3 ± 0.2 4734 ± 91 3.01 ± 0.11 0.38 ± 0.03 6144777 C 11.01 ± 0.06 129.7 ± 0.2 4788 ± 91 3.07 ± 0.11 0.24 ± 0.03 7060732 D 10.94 ± 0.05 132.3 ± 0.2 4892 ± 200 − − 7619745 E 13.13 ± 0.07 170.8 ± 0.2 4932 ± 91 3.13 ± 0.11 −0.04 ± 0.03 8366239 F 13.70 ± 0.07 185.6 ± 0.4 4948 ± 91 3.10 ± 0.11 −0.00 ± 0.03 8475025 G 9.66 ± 0.05 112.9 ± 0.3 4854 ± 91 3.01 ± 0.11 −0.04 ± 0.03 8718745 H 11.40 ± 0.06 129.3 ± 0.2 4769 ± 91 2.94 ± 0.11 −0.32 ± 0.04 9145955 I 11.00 ± 0.06 131.7 ± 0.2 4925 ± 91 3.04 ± 0.11 −0.32 ± 0.03 9267654 J 10.34 ± 0.05 118.6 ± 0.2 4824 ± 91 3.22 ± 0.11 −0.04 ± 0.03 9475697 K 9.88 ± 0.05 115.1 ± 0.2 4791 ± 91 2.90 ± 0.11 0.19 ± 0.03 9882316 L 13.78 ± 0.07 182.0 ± 0.5 5093 ± 91 3.20 ± 0.11 −0.41 ± 0.04 10123207 M 13.67 ± 0.07 160.9 ± 0.2 4840 ± 91 2.98 ± 0.11 −0.45 ± 0.04 10200377 N 12.47 ± 0.06 142.5 ± 0.2 4828 ± 91 3.00 ± 0.11 −0.63 ± 0.04 10257278 O 12.20 ± 0.06 149.5 ± 0.3 4887 ± 91 2.99 ± 0.11 0.06 ± 0.03 11353313 P 10.76 ± 0.05 126.5 ± 0.2 4955 ± 91 3.01 ± 0.11 −0.42 ± 0.04 11913545 Q 10.18 ± 0.05 117.1 ± 0.2 4960 ± 200 − − 11968334 R 11.41 ± 0.06 141.4 ± 0.3 4826 ± 91 3.10 ± 0.11 0.35 ± 0.03 12008916 S 12.90 ± 0.06 161.9 ± 0.3 5107 ± 200 − −…”