The close circumstellar environment of Betelgeuse - V. Rotation velocity and molecular envelope properties from ALMA

TL;DR

This study uses high-resolution ALMA observations to measure Betelgeuse's rotation velocity, revealing its co-rotating chromosphere and suggesting that rogue convection cells influence its anisotropic mass loss.

Contribution

First direct measurement of Betelgeuse's rotation velocity and axis orientation using ALMA, linking surface features to mass loss mechanisms.

Findings

Betelgeuse's projected equatorial rotation velocity is 5.47 km/s.

The star's rotation period is approximately 36 years.

Polar regions show focused mass loss linked to convection cells.

Abstract

We observed Betelgeuse using ALMA's extended configuration in band 7 (f~340 GHz, {\lambda}~0.88 mm), resulting in a very high angular resolution of 18 mas. Using a solid body rotation model of the 28SiO(v=2,J=8-7) line emission, we show that the supergiant is rotating with a projected equatorial velocity of v_eq sin i = 5.47 +/- 0.25 km/s at the equivalent continuum angular radius R_star = 29.50 +/- 0.14 mas. This corresponds to an angular rotation velocity of {\omega} sin i = (5.6 +/- 1.3) x 10^(-9) rad/s. The position angle of its north pole is PA = 48.0 +/- 3.5{\deg}. The rotation period of Betelgeuse is estimated to P/sin i = 36 +/- 8 years. The combination of our velocity measurement with previous observations in the ultraviolet shows that the chromosphere is co-rotating with the star up to a radius of ~10 au (45 mas or 1.5x the ALMA continuum radius). The coincidence of the position angle of the polar axis of Betelgeuse with that of the major ALMA continuum hot spot, a molecular plume, and a partial dust shell (from previous observations) suggests that focused mass loss is currently taking place in the polar region of the star. We propose that this hot spot corresponds to the location of a particularly strong "rogue" convection cell, which emits a focused molecular plume that subsequently condenses into dust at a few stellar radii. Rogue convection cells therefore appear to be an important factor shaping the anisotropic mass loss of red supergiants.