Asteroseismic rotation rates of hot subdwarf B stars hint at transient accretion from leftover common envelope matter

TL;DR

This study uses asteroseismology to measure internal rotation rates of hot subdwarf B stars in binary systems, revealing that accretion of matter from leftover common envelope material can significantly spin up these stars.

Contribution

It demonstrates that accretion of circumstellar matter can explain the higher observed rotation rates in sdB stars, challenging previous models of angular momentum content.

Findings

Predicted core rotation rates are 2-10 times lower than observed.

Envelope rotation rates are 2-5 orders of magnitude lower than observed.

Accretion of small amounts of matter can spin up stars to match observations.

Abstract

Asteroseismology enabled measuring the rotation rate in the deep stellar interiors of stars across several evolutionary phases, advancing the theory of angular momentum transport in single stars from the main sequence to the white dwarf phase. However, binary stellar evolution products have not yet been studied in the context of angular momentum transport constrained by asteroseismology. Hot subdwarf B (sdB) stars can pulsate in non-radial modes, enabling probing of their internal rotation. Those in binary systems form through mass transfer, thus they can be used to probe theories of internal rotation in post-mass transfer stars. Here, we interpret observed asteroseismic core and envelope rotation rates of sdB stars in unsynchronised binary systems that formed through the common-envelope channel, using stellar evolution models of rotating sdB stars with internal magnetic fields. We find that when sdB stars form with the angular momentum content of red giant cores prior to common-envelope ejection, their predicted core rotation rates are two to ten times lower than measured asteroseismic rotation rates, and their envelope rotation rates are lower by two to five orders of magnitude. This suggests that the angular momentum content of sdB stars increases during their formation. Since sdB stars in close binary systems may host circumstellar matter from a past common-envelope ejection, we show that if they accrete a small amount of matter, the combination of internal magnetic fields with angular momentum transfer through accretion spins up both the core and envelope to match their measured asteroseismic rotation rates.