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

Abstract

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 in the light of the corresponding viscosity determined for the more massive red giant KIC 8366239. 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 ν add = 1 × 10 3 -1.3 × 10 4 cm 2 s −1 . 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 modeling 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 ν add is reduced to 1-4 × 10 3 cm 2 s −1 . In addition to being sensitive to the evolutionary stage of the star, the efficiency of the unknown process for internal transport of angular momentum increases with the stellar mass.

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

“…This is an improvement compared to the results of Deheuvels et al (2012) who could only provide a lower limit of five for the core-envelope rotation ratio. This measurement could be used to improve our knowledge of the timescale over which angular momentum is transported in red giants (Eggenberger et al 2017).…”

Progress in Global Helioseismology: A New Light on the Solar Modeling Problem and Its Implications for Solar-Like Stars

“…Hence, it also serves the purpose of reminding the model-dependence of asteroseismic investigations, but also how the high-quality asteroseismic data can help us better understand the physical processes acting inside stars by providing other experimental conditions to those at hand in helioseismology. A good illustration of the limitations of stellar models, and thus a central point for which a revision of their ingredients could have a significant impact is found for example in the current discussions related to the transport of angular momentum on both the main-sequence and the red giant branch (Eggenberger et al, 2017;Benomar et al, 2018;Ouazzani et al, 2018;Eggenberger et al, 2019). In those cases, the sensitivity of the proposed mechanisms to the chemical composition gradients, and thus their validity, could be influenced by revisions of some of the physical ingredients at play when studying the solar modelling problem.…”

Massive stars, successes and challenges

“…In case the angular momentum would be conserved, present day stellar models predict in general too fast rotating neutron stars when compared to the rotation rate of young observed pulsars (Heger et al 2005). If true it would indicate that massive star models with rotation still miss some angular momentum transport mechanism, a process that appears well confirmed for small mass stars (see Beck et al 2012, Eggenberger et al 2017 and references therein). The physics of rotation is complex and involves turbulence, a feature that, as recalled above cannot be described easily in numerical simulation.…”