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  γ Cas stars: Normal Be stars with discs impacted by the wind of a helium-star companion?

Langer, N., Baade, D., Bodensteiner, J., Greiner, J., Rivinius, T., Martayan, C., et al. (2020). γ Cas stars: Normal Be stars with discs impacted by the wind of a helium-star companion? Astronomy and Astrophysics, 633: A40. doi:10.1051/0004-6361/201936736.

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Item Permalink: http://hdl.handle.net/21.11116/0000-0005-DA05-0 Version Permalink: http://hdl.handle.net/21.11116/0000-0005-DA06-F
Genre: Journal Article

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Langer, N., Author
Baade, D., Author
Bodensteiner, J., Author
Greiner, J.1, Author              
Rivinius, Th., Author
Martayan, Ch., Author
Borre, C. C., Author
Affiliations:
1High Energy Astrophysics, MPI for Extraterrestrial Physics, Max Planck Society, ou_159890              

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 Abstract: γ Cas stars are a ∼1% minority among classical Be stars with hard (≥5−10 keV), but only moderately strong continuous thermal X-ray flux, and mostly very early-B spectral type. The X-ray flux has been suggested to originate from matter accelerated via magnetic disc-star interaction, by a rapidly rotating neutron star (NS) companion via the propeller effect, or by accretion onto a white dwarf (WD) companion. In view of the growing number of identified γ Cas stars and the only imperfect matches between these suggestions and the observations, alternative models should be pursued. Two of the three best-observed γ Cas stars, γ Cas itself and π Aqr, have a low-mass companion with low optical flux, whereas interferometry of BZ Cru is inconclusive. Binary-evolution models are examined for their ability to produce such systems. The OB+He-star stage of post-mass transfer binaries, which is otherwise observationally unaccounted, can potentially reproduce many observed properties of γ Cas stars. The interaction of the fast wind of helium stars with the circumstellar disc and/or with the wind of Be stars may give rise to the production of hard X-rays. While not modelling this process, it is shown that the energy budget is favourable, and that the wind velocities may lead to hard X-rays, as observed in γ Cas stars. Furthermore, the observed number of these objects appears to be consistent with the evolutionary models. Within the Be+He-star binary model, the Be stars in γ-Cas stars are conventional classical Be stars. They are encompassed by O-star+Wolf-Rayet systems towards higher mass, where no stable Be decretion discs exist, and by Be+sdO systems at lower mass, where the sdO winds may be too weak to cause the γ Cas phenomenon. In decreasing order of the helium-star mass, the descendants could be Be+black-hole, Be+NS, or Be+WD binaries. The interaction between the helium-star wind and the disc may provide new diagnostics of the outer disc.

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 Dates: 2020-01-08
 Publication Status: Published online
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 Identifiers: DOI: 10.1051/0004-6361/201936736
Other: LOCALID: 3213546
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Title: Astronomy and Astrophysics
  Other : Astron. Astrophys.
Source Genre: Journal
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Publ. Info: France : EDP Sciences S A
Pages: - Volume / Issue: 633 Sequence Number: A40 Start / End Page: - Identifier: ISSN: 1432-0746
CoNE: https://pure.mpg.de/cone/journals/resource/954922828219_1