O(He) Stars

TL;DR

This paper discusses the spectral characteristics and potential evolutionary origins of O(He) stars, a rare class of hot, helium-rich post-AGB stars, proposing two possible evolutionary scenarios involving either a merger process or early-AGB departure.

Contribution

It introduces the concept of O(He) stars and explores their possible evolutionary pathways, linking them to other stellar classes and processes.

Findings

O(He) stars are a distinct class of hot, helium-rich post-AGB stars.

Two evolutionary scenarios are proposed: merger origin or early-AGB departure.

Potential connection to RCrB, sdO, and PG1159 stars.

Abstract

Spectral analyses of H-deficient post-AGB stars have shown that a small group of four extremely hot objects exists which have almost pure He absorption-line spectra in the optical. These are classified as O(He) stars. For their evolution there are two scenarios: They could be the long-sought hot successors of RCrB stars, which have not been identified up to now. If this turns out to be true, then a third post-AGB evolutionary sequence is revealed, which is probably the result of a double-degenerate merging process. An alternative explanation might be that O(He) stars are post early-AGB stars. These depart from the AGB just before they experience their first thermal pulse (TP) which will then occur as a late thermal pulse (LTP). This would be a link to the low-mass He-enriched sdO stars and low-mass, particularly He-rich PG1159 stars.