Asteroseismology and evolution of EHB stars

The properties of the Extreme Horizontal Branch stars are quite well understood, but much uncertainty surrounds the many paths that bring a star to this peculiar configuration. Asteroseismology of pulsating EHB stars has been performed on a number of objects, bringing us to the stage where comparisons of the inferred properties with evolutionary models becomes feasible. In this review I outline our current understanding of the formation and evolution of these stars, with emphasis on recent progress. The aim is to show how the physical parameters derived by asteroseismology can enable the discrimination between different evolutionary models.

💡 Research Summary

The paper provides a comprehensive review of Extreme Horizontal Branch (EHB) stars, focusing on how asteroseismology can be used to discriminate among their various formation and evolutionary pathways. EHB stars are core‑helium‑burning objects with very thin hydrogen envelopes (≤1 % of the total mass) and masses clustered around the canonical helium‑flash value of ~0.47 M⊙. Their lifetimes on the zero‑age to terminal‑age EHB are 100–150 Myr, after which they evolve directly through the sdO phase to the white‑dwarf cooling track without a second giant phase.

Observationally, large surveys such as PG, HS, SDSS, and especially the Bok‑Green (BG) survey have identified roughly 2,500 hot subdwarfs, of which the majority occupy a narrow region in the T_eff–log g plane consistent with a tight mass distribution (≈0.45–0.5 M⊙). Spectroscopic classifications (sdB, sdO, sdOB, He‑sdB, He‑sdO) describe atmospheric composition but do not uniquely map to evolutionary stage. A distinct population of He‑rich objects lies off the canonical tracks, suggesting additional processes such as late hot‑flasher mixing.

Binary interaction is identified as the dominant channel for producing EHB stars. Two primary binary pathways are discussed:

1. Common‑Envelope (CE) Ejection – When the mass donor (a red‑giant) is more massive than its companion, the orbit shrinks catastrophically, leading to a CE phase. Friction deposits orbital energy into the envelope, which is expelled, leaving a close sdB+WD or sdB+M‑dwarf binary with periods ≤30 days. Radial‑velocity surveys (e.g., Maxted et al. 2001) find a short‑period binary fraction of ~60 %. These systems tend to cluster at lower gravities on the EHB.

2. Stable Roche‑Lobe Overflow (RLOF) – If the companion is relatively massive, mass transfer proceeds stably, producing wide binaries with periods of hundreds to thousands of days. Detecting these long‑period systems is observationally challenging, leading to lower reported fractions.

The formation of apparently single EHB stars remains contentious. Proposed mechanisms include:

• Enhanced RGB mass loss parameterized by a broad distribution of Reimers η_R values, allowing the envelope to be stripped without binary interaction.
• He‑WD mergers, which can produce He‑rich subdwarfs but predict rapid rotation not commonly observed.
• Planet‑induced CE ejection, exemplified by the pulsating sdB V391 Peg, where a ~3 M_Jup planet on a 1,170‑day orbit may have contributed to envelope removal.
• Supernova disruption of a binary, leaving the former donor as a single EHB star with a peculiar Galactic orbit.

The paper also details the “hot‑flasher” scenario. If the helium flash occurs after the star has left the RGB, the timing determines surface composition. Early hot flashers retain a hydrogen shell that prevents deep mixing, while late hot flashers experience convective mixing that brings helium and CNO‑processed material to the surface, producing the observed He‑rich subdwarfs.

Asteroseismology emerges as the key diagnostic tool. Pulsating sdB stars (sdBV) are divided into p‑mode (V361 Hya) and g‑mode (V1093 Her) classes. By matching observed frequencies to theoretical models, internal parameters such as total mass, envelope mass, metallicity, and rotation can be inferred with high precision. Existing asteroseismic analyses have yielded masses between 0.42–0.48 M⊙ and envelope masses of 10⁻⁴–10⁻³ M⊙, values that can be directly compared with predictions from CE, RLOF, merger, or enhanced‑wind scenarios. Moreover, long‑term monitoring of pulsation timing can reveal low‑mass companions (planets or brown dwarfs) with sensitivities surpassing traditional spectroscopy.

In summary, the review underscores that EHB stars arise from multiple, competing channels—binary CE ejection, stable RLOF, mergers, and possibly extreme wind loss or planetary interactions. Their observable properties (binary fraction, helium enrichment, pulsation spectra) provide constraints that asteroseismology can exploit to test and refine evolutionary models. Future advances, including Gaia parallaxes, high‑precision space‑based photometry (TESS, Kepler), and extended radial‑velocity campaigns, promise to resolve remaining ambiguities and deliver a unified picture of EHB star formation and evolution.