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

Abstract

Context. About 20% of all B-type stars are classical Be stars – stars whose spectra imply the presence of a circumstellar decretion disk. The disk phenomenon is strongly correlated with rapid rotation, the origin of which remains unclear. It may be rooted in single- or binary-star evolution. In the framework of the binary channel, the initially more massive star transfers mass and angular momentum to the original secondary, which becomes a Be star. The system then evolves into a Be binary with a post-main-sequence companion, which, depending on the companion mass, may later be disrupted in a supernova event. Hence, if the binary channel dominates the formation of Be stars, one may expect a strong lack of close Be binaries with main sequence (MS) companions. Aims. We want to test the prediction of the binary channel. Through an extensive, star-by-star review of the literature of a magnitude-limited sample of Galactic early-type Be stars, we investigate whether Be binaries with MS companions are known to exist. Methods. Our sample is constructed from the BeSS database and cross-matched with all available literature on the individual stars. Archival and amateur spectra are used to verify the existing literature when conflicting reports are found. Results. Out of an initial list of 505 Be stars, we compile a final sample of 287 Galactic Be stars earlier than B1.5 with V ≤ 12 mag. Out of those, 13 objects were reported as Be binaries with known post-MS companions (i.e., compact objects or helium stars) and 11 as binaries with unknown, uncertain or debated companions. We find no confirmed reports of Be binaries with MS companions. For the remaining 263 targets, no significant reports of multiplicity exist in the literature, implying that they are either Be binaries with faint companions, or truly single. Conclusions. The clear lack of reported MS companions to Be stars, which stands in contrast to the high number of detected B+B MS binaries, strongly supports the hypothesis that early-type Be stars are binary interaction products that spun up after mass and angular momentum transfer from a companion star. Taken at face value, our results may suggest that a large majority of the early-type Be stars have formed through binary mass-transfer.

Velocity monitoring of γ Cas stars reveals their binarity status

“…Furthermore, the table only presents systems with well-defined orbital solutions. This differs from the choices of Bodensteiner et al (2020) who examined Be stars with spectral type B1.5e or earlier and split the "systems" depending on the companion's nature (post-MS/unknown). These authors considered some ambiguous indirect information as secure binarity evidence 5 while missing some secure binary cases such as the well-known Per system.…”

“…It comprises a Be star and a B1-3 companion in a highly eccentric orbit of period of 10.8 yrs, but longer-term velocity variations indicate the presence of a third star in the system. In parallel, the spectrum of HD 93683 indicates not only short-term binary motion linked to an inner O9V+B0V binary with ∼ 18 d, but also long-term velocity shifts in the H line, suggesting the presence of an outer Be star with a period of about 400 d (Bodensteiner et al 2020). Interferometric and spectroscopic data further indicate that Gem comprises an inner B+B binary with period 54 d orbited in 19 yr by an outer Be star (Gardner et al 2021;Klement et al 2021).…”

“…This is valid for the Cas stars as for the other Be stars. Regarding the secondary mass, seven early Be have companion with 5 In Bodensteiner et al (2020), HD 161103 (=V3892 Sgr) is categorized as having a "known post-MS companion" (their class i) while the provided references only offer a general discussion on the origin of the Cas phenomenon, with the magnetic and WD hypotheses highlighted: none of the provided references provide a direct, secure detection of a companion. Our velocity monitoring can only attribute it a "binary candidate" status.…”

Asteroseismology of High-Mass Stars: New Insights of Stellar Interiors With Space Telescopes

“…The Be stars are rapid rotators with circumstellar decretion disks, and their near-critical rotation rates are thought to be related to their evolutionary history. More specifically, Be stars may have accreted mass from a companion or are the result of a stellar merger, which is supported by the relative rarity of Be stars with main-sequence companions (Bodensteiner et al, 2020). Many Be stars show evidence of pulsations and experience outbursts of material thought to be driven by pulsations (Rivinius et al, 2003;Huat et al, 2009;Kurtz et al, 2015).…”

Discovery and Characterization of a Rare Magnetic Hybrid β Cephei Slowly Pulsating B-type Star in an Eclipsing Binary in the Young Open Cluster NGC 6193

“…OB type stars are observed to have high projected rotational velocities and are expected to rotate quickly at birth (Huang & Gies 2006a;Daflon et al 2007;Garmany et al 2015). Some 20% of B stars are observed to exhibit emission in their line profiles and are classed as Be stars (Rivinius et al 2013), a phenomenon which is associated with near-critical rotation (Porter & Rivinius 2003), non-radial pulsations (Semaan et al 2018), and/or binarity (Bodensteiner et al 2020). In the absence of the transient Be phenomenon, B stars are expected to only show rotational modulation in the event that surface features are introduced by magnetic fields or chemical inhomogeneities.…”