Effects of rotation on the evolution and asteroseismic properties of red giants

Abstract

Context. The recent observations of solar-like oscillations in many red giant stars with the CoRoT satellite stimulated the theoretical study of the effects of various physical processes on the modelling of these stars. Aims. The influence of rotation on the properties of red giants is studied in the context of the asteroseismic modelling of these stars. Methods. The effects of rotation on the global and asteroseismic properties of red giant stars with a mass larger than the mass limit for degenerate He burning are investigated by comparing rotating models computed with a comprehensive treatment of shellular rotation to non-rotating ones. Results. While red giants exhibit low surface rotational velocities, we find that the rotational history of the star has a large impact on its properties during the red giant phase. In particular, for stars massive enough to ignite He burning in non-degenerate conditions, rotational mixing induces a significant increase of the stellar luminosity and shifts the location of the core helium burning phase to a higher luminosity in the HR diagram. This of course results in a change of the seismic properties of red giants at the same evolutionary state. As a consequence the inclusion of rotation significantly changes the fundamental parameters of a red giant star as determined by performing an asteroseismic calibration. In particular rotation decreases the derived stellar mass and increases the age. Depending on the rotation law assumed in the convective envelope and on the initial velocity of the star, non-negligible values of rotational splitting can be reached, which may complicate the observation and identification of non-radial oscillation modes for red giants exhibiting moderate surface rotational velocities. By comparing the effects of rotation and overshooting, we find that the main-sequence widening and the increase of the H-burning lifetime induced by rotation (V ini = 150 km s −1 ) are well reproduced by non-rotating models with an overshooting parameter of 0.1, while the increase of luminosity during the post-main sequence evolution is better reproduced by non-rotating models with overshooting parameters twice as large. This illustrates the fact that rotation not only increases the size of the convective core but also changes the chemical composition of the radiative zone.

Angular momentum transport in stellar interiors constrained by rotational splittings of mixed modes in red giants

“…1. As expected from previous computations of rotating models including shellular rotation, the rotation profile during the red giant phase is very steep as a result of the central contraction occurring at the end of the main sequence (Palacios et al 2006;Eggenberger et al 2010c).…”

Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes

“…By comparison with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior 1,5,6 .…”

“…By comparison with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior 1,5,6 .The asteroseismic approach to studying stellar interiors exploits information from oscillation modes of different radial order n and angular degree l, which propagate in cavities extending at different depths 7 . Stellar rotation lifts the degeneracy of non-radial modes, producing a multiplet of (2l 1 1) frequency peaks in the power spectrum for each mode.…”

Thermohaline instability and rotation-induced mixing

“…For red giants, rotating models are found to decrease the determined value of the stellar mass of a star located at a given luminosity in the HR diagram and to increase the value of its age. Consequently, the inclusion of rotation significantly changes the fundamental parameters determined for a star by performing an asteroseismic calibration (Eggenberger et al 2010b). …”