Interferometric program on the multiplicity of Be stars: new detections & orbits of stripped subdwarf companions

Rapid rotation and nonradial pulsations enable Be stars to build decretion disks, where the characteristic line emission forms. A major but unconstrained fraction of Be stars owe their rapid rotation to mass and angular-momentum transfer in a binary. The faint, stripped companions can be helium-burning subdwarf OB-type stars (sdOBs), white dwarfs (WDs), or neutron stars. In this contribution, I will present the results from an ongoing interferometric campaign on the multiplicity of Be stars, which has thus far led to seven new published orbital orbital solutions for Be + sdOB binaries based on CHARA observations, including distance-dependent dynamical masses for both components in each binary. These results quadruple the number of anchor points for evolutionary models of close binary stars leading to the formation of Be stars. Based on more recent observations from CHARA and VLTI, we detected several more similar systems, as well as a few systems where the companion is in a short-lived, 'bloated pre-subdwarf' phase immediately following the mass transfer between the two components. On the other hand, we failed to detect any companion to the six observed Be stars with γ Cas-like X-ray emission, with sdOB and main-sequence companions of the expected spectroscopic mass being ruled out for the X-ray-prototypical stars gamma Cas and pi Aqr. This leaves the elusive WD companions as the most likely companions for these stars, as well as a likely explanation of the observed X-rays. No low-mass main-sequence close companions were identified in the entire sample, providing further support to the importance and possible prevalence of the binary formation channel for classical Be stars.