Rotation of the asymptotic giant branch star R Doradus

TL;DR

This study reports the first direct measurement of stellar rotation in an AGB star, R Doradus, revealing faster-than-expected rotation likely due to a close companion, which may influence its mass loss behavior.

Contribution

It provides the first direct observational evidence of stellar rotation in an AGB star using ALMA, suggesting the presence of a close companion affecting the star's angular momentum.

Findings

Measured stellar rotation velocity of R Doradus is about 1 km/s.

Rotation is two orders of magnitude faster than expected for a solitary AGB star.

Rotation likely influenced by a close-in, low-mass companion.

Abstract

High resolution observations of the extended atmospheres of asymptotic giant branch (AGB) stars can now directly confront the theories that describe stellar mass loss. Using Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution ($30\times42$~mas) observations we have, for the first time, resolved stellar rotation of an AGB star, R~Dor. We measure an angular rotation velocity of $\omega_R\sin{i}=(3.5\pm0.3)\times10^{-9}$~rad~s$^{-1}$ which indicates a rotational velocity of $|\upsilon_{\rm rot}\sin{i}|=1.0\pm0.1$~km~s$^{-1}$ at the stellar surface ($R_*=31.2$~mas at $214$~GHz). The rotation axis projected on the plane of the sky has a position angle $\Phi=7\pm6^\circ$. We find that the rotation of R Dor is two orders of magnitude faster than expected for a solitary AGB star that will have lost most of its angular momentum. Its rotational velocity is consistent with angular momentum transfer from a close companion. As a companion has not been directly detected we thus suggest R~Dor has a low-mass, close-in, companion. The rotational velocity approaches the critical velocity, set by the local sound speed in the extended envelope, and is thus expected to affect the mass loss characteristics of R~Dor.