The effects of the Reimers $\eta$ on the solar rotational period when our Sun evolves to the RGB tip

Abstract

Our Sun will expand enormously and lose substantial mass via a stellar wind during the red giant branch (RGB) phase; the rotational period will be prolonged by several orders of magnitude. It is difficult to predict how much mass the Sun will lose before it reaches the RGB tip. Therefore, the solar rotational period at the RGB tip is also quite indeterminate. In this work, the Sun is considered as a two-component system comprised of a core and a convective envelope, each being allowed to rotate freely. The angular momentum transfer from the inner planets to the solar envelope is taken into consideration. Using Eggleton’s stellar evolution code, we study how the solar rotational period at the RGB tip depends on the value of Reimers $\eta$ chosen. The solar envelope’s rotational period at the RGB tip varies from 1 792 to 736 934 years, as the Reimers $\eta$ is changed from 0.00 to 0.75. Recent observations show that the average Reimers $\eta$ of Sun-like stars is 0.477. Adopting this average value of the Reimers $\eta$, the solar envelope’s rotational period at the RGB tip will be 24 868 years. We also show how the envelope’s rotational evolves with age and luminosity. Other Sun-like stars, with different planetary configurations, may prematurely eject mass and lead to planetary nebulae, if they engulf a brown-dwarf companion at the RGB tip. Swallowing a planet with 13 Jupiter masses and a 3-day orbit, a Sun-like star can become a rapidly rotating giant star.