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

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 diskstar 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; 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 disk 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 disks 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 disk may provide new diagnostics of the outer disk.

A naked-eye triple system with a nonaccreting black hole in the inner binary

“…However, 66 Oph is a fourth hierarchical triple with an outer Be star (but without a BH, Štefl et al 2004). Such a channel for the formation of Be stars (for a description of other channels, see Langer et al 2020a) may be due to angular momentum distribution during fragmentation in massive-star formation (Bodenheimer 1978;Kratter & Matzner 2006). If these outer Be stars become unbound, they may have above-average space velocities, but probably not at the top of the range observed in a fair fraction of Be stars (Berger & Gies 2001).…”

“…Alternatively, if the Be star is itself a binary, it might have been rejuvenated as well and by more than the B3 III star. This is not completely implausible: If Be stars owe their rapid rotation to spin-up by mass overflow from a now underluminous companion (e.g., Langer et al 2020a, and references therein) a resulting rejuvenation of the outer Be star is one possible explanation. Some blue stragglers in open clusters have been reported to show Hα in emission and may be classical Be stars (Aidelman et al 2018;Bodensteiner et al 2020).…”

Investigating the lack of main-sequence companions to massive Be stars

“…Despite being one of the brightest stars in the night sky, the nature of the companion remains under debate. Even though the majority of models suggest that the companion is a non-MS companion such as a hot He star or a compact object (e.g., Postnov et al 2017;Langer et al 2020b), a solar-type companion cannot be fully ruled out. Similar objects include the γ Cas analogue π Aqr and the suggested Be+sdO binary HD 161306 (Koubský et al 2014).…”

“…Kobulnicky et al (2014) report a biascorrected fraction of 55% in the period range of 1 < P < 5000 d and the mass-ratio range of 0.2 < q < 1. The fraction of reported binaries is fully dominated by MS+MS binaries, since OB+NS binaries will typically have q 0.2, and OB+BH binaries are very rare (e.g., Langer et al 2020b). We therefore assume that the fraction of MS+MS binaries in the aforementioned parameter domain among an unbiased sample of B stars is larger than 50%.…”

“…The nature of this companion is, however, not well constrained. The companion was speculated to be a white dwarf (Haberl 1995), a NS (e.g., Postnov et al 2017), or a He star (e.g., Langer et al 2020b). A recent study by Borre et al (2020) studied the long-term variations of the Hα profile and found indications that a spiral structure in the circumstellar decretion disk is in phase with the companion.…”