The Betelgeuse Project III: Merger Characteristics

Abstract

We previously proposed that Betelgeuse might have been spun up by accreting a companion of about 1 M . Here we explore in more detail the possible systematics of such a merger and a larger range of accreted masses. We use the stellar evolutionary code mesa to add angular momentum to a primary star in core helium burning, core carbon burning, or shell carbon burning. Our models provide a reasonable "natural" explanation for why Betelgeuse has a large, but sub-Keplerian equatorial velocity. They eject sufficient mass and angular momentum in rotationally-induced mass loss to reproduce the observed ratio of the equatorial velocity to escape velocity of Betelgeuse, ≈ 0.23, within a factor of three nearly independent of the primary mass, the secondary mass, and the epoch at which merger occurs. Our models suggest that merger of a primary of somewhat less than 15 M with secondaries of from 1 to 10 M during core helium burning or core carbon burning could yield the equatorial rotational velocity of ∼ 15 km s −1 attributed to Betelgeuse. For accreting models, a wave of angular momentum is halted at the composition boundary at the edge of the helium core. The inner core is thus not affected by the accretion of the companion in these simulations. Accretion has relatively little effect on the production of magnetic fields in the inner core. Our results do not prove, but do not negate that Betelgeuse might have ingested a companion of several M .1. INTRODUCTION Betelguese (α Orionis) is a nearby, massive red supergiant (RSG) that provides clues to a broad range of issues of the evolution and explosion of massive stars. It has been difficult to obtain tight constraints on the evolutionary state of Betelgeuse and hence when it might explode and information about the internal rotational state and associated mixing. It is thus important to understand Betelgeuse in greater depth. The recent extreme dimming episode has only added impetus to this quest (Guinan et al. 2020;Levesque & Massey 2020;Harper et al. 2020;Dharmawardena et al. 2020).The distance to Betelgeuse has been known to only 20% (D ≈ 197 ± 45 pc;Harper et al. 2008Harper et al. , 2017 a situation that is not improved by Gaia that saturates on such a bright star. Key properties such as radius and luminosity are thus somewhat uncertain. Within this uncertainty, models of Betelgeuse might be brought into agreement with observations of L, R, and T ef f at either the minimum-luminosity base of the giant branch or at the tip of the red supergiant branch (RSB). By