O(He) Stars

O(He) stars are extremely hot (Teff > 80 kK), hydrogen-deficient stars that show almost pure He II absorption-line spectra. From their effective temperatures and surface gravities, they are found in the post-AGB region in the Teff - log g diagram, just amongst the luminous PG 1159 stars, and hydrogen-rich central stars of planetary nebulae (CSPNe). Because of their fast evolutionary rate (typically 103 to a few 105 years), the number of know O(He) stars is rather low. So far only ten objects are known that belong to this exotic class. Three of them are CSPNe and around the other seven O(He) stars no planetary nebulae was detected.

Pa 5
H α images of the newly discovered
O(He)-type CSPN Pa 5. It is one of
the six CSPNe discovered in the
Kepler field and has Teff = 145 kK.

O(He) stars
H α (left) and [O III] images (right) of the O(He)-type
CSPNe LoTr4 (top) and K1-27 (bottom).

The existence of these stars is a challenge for stellar evolutionary theory. The key question is: How did these stars lose their hydrogen rich envelope?
Quantitative spectral analyses of hot, post-AGB stars revealed two distinct evolutionary sequences within the last decade. Besides the well known "usual" hydrogen-rich sequence, a hydrogen-deficient sequence was discovered. It is composed of Wolf-Rayet type stars that evolve into PG 1159 stars and finally might envolve into non-DA white dwarfs. The surface abundances of these carbon-dominated stars can be explained by AFTP, LTP or VLTP scenarios. However, such scenarios could never reproduce the helium-dominated atmospheres as they are found in O(He) stars, but also in helium-rich sdO, RCB, EHe or [WN] stars.
Therefore the question of a possible second hydrogen-deficient sequence naturally arises. In my Master's thesis (german) and this paper I discuss different evolutionary models in order to explain the origin of the O(He) stars and make possible evolutionary links to other helium-dominated stars. In this talk I summarize Recent Progress on our Understanding of He-Dominated Stellar Evolution.