Low-metallicity massive single stars with rotation.
II. Predicting
spectra and spectral classes of chemically-homogeneously evolving stars
Kubatova, B.; Szecsi, D.; Sander, A. A. C.; Kubat, J.; Tramper, F.; Krticka, J.; Kehrig, C.; Hamann, W.-R.; Hainich, R.; Shenar, T.
Context. Metal-poor massive stars are supposed to be progenitors of
certain supernovae, gamma-ray bursts and compact object mergers,
potentially contributing to the early epochs of the Universe with their
strong ionizing radiation. However, they remain mainly theoretical as
individual spectroscopic observations of such objects have rarely been
carried out below the metallicity of the SMC.
Aims. This work aims at exploring what our state-of-the-art theories of
stellar evolution combined with those of stellar atmospheres predict
about a certain type of metal-poor (0.02 Zsun) hot massive stars,
the chemically homogeneously evolving ones, called TWUIN stars.
Methods. Synthetic spectra corresponding to a broad range in masses
(20-130 Msun) and covering several evolutionary phases from the
zero-age main-sequence up to the core helium-burning stage were
computed.
Results. We find that TWUIN stars show almost no emission lines during
most of their {core hydrogen-burning} lifetimes. Most metal lines are
completely absent, including nitrogen. During their core helium-burning
stage, lines switch to emission and even some metal lines (oxygen and
carbon, but still almost no nitrogen) show up. Mass loss and clumping
play a significant role in line-formation in later evolutionary phases,
particularly during core helium-burning. Most of our spectra are
classified as an early O type giant or supergiant, and we find
Wolf-Rayet stars of type WO in the core helium-burning phase.
Conclusions. An extremely hot, early O type star observed in a
low-metallicity galaxy could be the outcome of chemically homogeneous
evolution - and therefore the progenitor of a long-duration
gamma-ray burst or a type Ic supernova. TWUIN stars may play an
important role in reionizing the Universe due to their being hot without
showing prominent emission lines during the majority of their lifetimes.