The close circumstellar environment of Betelgeuse - V. Rotation velocity and molecular envelope properties from ALMA
Résumé
We observed Betelgeuse using ALMA’s extended configuration in band 7 (f ≈ 340 GHz, λ ≈ 0.88 mm), resulting in a very high
angular resolution of 18 mas. Using a solid body rotation model of the 28SiO(3=2, J=8-7) line emission, we show that the supergiant
is rotating with a projected equatorial velocity of 3eq sin i = 5.47 ± 0.25 km s−1
at the equivalent continuum angular radius Rstar =
29.50 ± 0.14 mas. This corresponds to an angular rotation velocity of ω sin i = (5.6 ± 1.3) × 10−9
rad s−1
. The position angle of its
north pole is PA = 48.0 ± 3.5
◦
. 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 1.5× 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.
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