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.
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.
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
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
naturally arises. In my Master's thesis (german) and this
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