Asteroseismic Rotation Rates of Hot Subdwarf B Stars Hint at Transient Accretion from Leftover Common Envelope Matter
26 Apr 2025, Yanjiang
Every day, somewhere in the Milky Way, a binary star system reaches a crisis point. Two stars that have orbited each other for billions of years suddenly find themselves sharing the same outer atmosphere — one star plunging deep inside the envelope of its companion, spiraling inward through a sea of hot plasma. This dramatic phase, known as common envelope evolution, lasts only a few thousand years. But it reshapes the stars forever.
What happens next has been a puzzle. The stripped cores that emerge from common envelope ejection become hot subdwarf B (sdB) stars — dense, compact objects that burn helium in their cores. Astronomers have measured their rotation rates using asteroseismology, reading the stars’ internal spins from their pulsation patterns. And the numbers don’t match expectations.
A team led by Zhengwei Liu at the Yunnan Observatories, Chinese Academy of Sciences, working with collaborators across China, Europe, and the United States, has now proposed an explanation. Their work, appearing in a preprint (arXiv:2604.22233), suggests that these stars may be spinning faster than expected because they briefly accrete matter from the debris left behind after common envelope ejection.
The Rotation Problem
Hot subdwarf B stars are fascinating objects. They are the exposed cores of red giants, stripped of their hydrogen envelopes during binary interactions. Some of them pulsate, and those pulsations — non-radial modes that travel through the star’s interior — carry information about the star’s internal rotation. By analyzing these pulsations, astronomers can measure how fast the core and envelope are spinning separately.
The measurements have produced a surprise. For sdB stars in unsynchronized binary systems — those where the star’s spin is not locked to its orbital period — the observed rotation rates are significantly higher than what theory predicts.
Liu and his colleagues calculated what the rotation rates should be if sdB stars inherited only the angular momentum of the red giant cores that preceded them. The mismatch was stark: the predicted core rotation rates were two to ten times lower than observed, and the predicted envelope rotation rates were lower by two to five orders of magnitude.
Something must be adding angular momentum to these stars during their formation.
A Common Envelope Leftover
The key clue lies in the binary environment. SdB stars in close binary systems formed through common envelope evolution. During this process, the original red giant’s envelope is ejected, but not all of it escapes into space. Some material remains in the vicinity — a circumstellar disk or cloud of leftover matter, still orbiting the newly formed binary.
The team’s idea is elegant: if the sdB star briefly accretes a small amount of this leftover material, the infalling matter would transfer angular momentum to the star. Combined with internal magnetic fields that couple the core and envelope, this accretion could spin up both regions simultaneously.
To test this, the researchers built stellar evolution models of rotating sdB stars with internal magnetic fields. They simulated the formation process, tracking how angular momentum is distributed and redistributed as the star evolves. The models that included transient accretion — just a tiny fraction of the star’s mass — produced rotation rates that matched the asteroseismic observations.
The amount of accreted matter is small. The team estimates that accreting between 0.001 and 0.01 solar masses of leftover common envelope material is sufficient to explain the observed spins. This is a remarkably modest amount, consistent with what might remain in the circumstellar environment after envelope ejection.
Why This Matters
This work connects two previously separate threads in stellar astrophysics. Asteroseismology has revolutionized our understanding of angular momentum transport in single stars, from the main sequence through the red giant phase to white dwarfs. But binary evolution products — stars that have been fundamentally altered by mass transfer — have remained largely unexplored in this context.
The result also has implications for how we understand the common envelope phase itself. This process is one of the least understood but most important stages in binary evolution, responsible for creating everything from cataclysmic variables to gravitational wave sources. If sdB stars are accreting leftover envelope material, it means the common envelope ejection is not as clean as simple models assume. Some of that ejected matter lingers, and it matters.
For the broader picture, this work suggests that binary interactions leave a lasting imprint on the internal rotation of their products. The angular momentum content of a star is not simply determined by its progenitor; it carries the signature of its violent formation history.
What Comes Next
The team’s models reproduce the observed rotation rates, but the work raises new questions. How exactly does the leftover common envelope matter get accreted? What determines how much material is available? And can we detect this circumstellar material directly?
Liu and colleagues are already thinking about these questions. Future observations with high-resolution spectroscopy might detect the signature of circumstellar material around sdB binaries. And more detailed models of the accretion process could predict specific patterns in the rotation rates that would confirm the scenario.
For now, the work offers a satisfying resolution to a long-standing puzzle. The stars spin too fast — and now we have a reason why. In the debris of their own violent birth, they found the extra push.
Yanjiang is an online editor of Loom Science
References
- Facundo D. Moyano et al., Asteroseismic rotation rates of hot subdwarf B stars hint at transient accretion from leftover common envelope matter, arXiv:2604.22233