Internal rotation of 13 low-mass low-luminosity red giants in theKeplerfield

Abstract

Context. The Kepler space telescope has provided time series of red giants of such unprecedented quality that a detailed asteroseismic analysis becomes possible. For a limited set of about a dozen red giants, the observed oscillation frequencies obtained by peak-bagging together with the most recent pulsation codes allowed us to reliably determine the core/envelope rotation ratio. The results so far show that the current models are unable to reproduce the rotation ratios, predicting higher values than what is observed and thus indicating that an efficient angular momentum transport mechanism should be at work. Here we provide an asteroseismic analysis of a sample of 13 low-luminosity low-mass red giant stars observed by Kepler during its first nominal mission. These targets form a subsample of the 19 red giants studied previously Corsaro et al. (2015), which not only have a large number of extracted oscillation frequencies, but also unambiguous mode identifications. Aims. We aim to extend the sample of red giants for which internal rotation ratios obtained by theoretical modeling of peak-bagged frequencies are available. We also derive the rotation ratios using different methods, and compare the results of these methods with each other. Methods. We built seismic models using a grid search combined with a Nelder-Mead simplex algorithm and obtained rotation averages employing Bayesian inference and inversion methods. We compared these averages with those obtained using a previously developed model-independent method. Results. We find that the cores of the red giants in this sample are rotating 5 to 10 times faster than their envelopes, which is consistent with earlier results. The rotation rates computed from the different methods show good agreement for some targets, while some discrepancies exist for others.

Period spacings in red giants

“…The case of KIC 3955033 is illustrative, with a negative surface rotation when using ζ; the use of ζ m provides a null value (5±20 nHz). This case also confirms the general situation shown by previous works (Goupil et al 2013;Di Mauro et al 2016;Triana et al 2017): deriving surface rotation can be achieved for the low RGB only.…”

“…As shown by subsequent works (e.g., Deheuvels et al 2014;Di Mauro et al 2016;Triana et al 2017), it is difficult to derive from the observed rotational splittings more than these two mean quantities. Again, we have to solve the ambiguity of the meaning of ζ in Eq.…”

Seismic probing of the first dredge-up event through the eccentric red-giant and red-giant spectroscopic binary KIC 9163796

“…For KIC 9163796 we have, however, an estimate for the surface rotation rate from the dominant period of the light curve modulation ( f 1 = 87±8 nHz). The core-surface rotation gradient is typical for RGB stars (Ω core /Ω envelope ≃ 6 to 30 times, Beck et al 2012Beck et al , 2014Deheuvels et al 2012Deheuvels et al , 2014Di Mauro et al 2016;Goupil et al 2013;Triana et al 2017) but appears a the lower end of the values found so far. Yet we note that this is small number statistics.…”

Asteroseismic sensitivity to internal rotation along the red-giant branch

“…Internal rotation rates as a function of depth have been estimated for a number of red giants using a forward modelling approach (Beck et al 2014(Beck et al , 2018 or by means of rotational inversions (Deheuvels et al 2012(Deheuvels et al , 2014Di Mauro et al 2016Triana et al 2017). However, the exact shape of the internal rotation profile remains unknown, as only core and surface rotation rates can currently be measured.…”