Ultraviolet Spectropolarimetry: on the origin of rapidly rotating B stars

UV spectroscopy and spectropolarimetry hold the key to understanding certain aspects of massive stars that are largely inaccessible with optical or longer wavelength observations. This is especially true for rapidly-rotating Be Bn stars, owing their high temperatures, geometric asymmetries, binary properties, evolutionary history. spectropolarimetric observations extremely sensitive photospheric consequences rapid rotation (i.e. oblateness, temperature, surface gravity gradients). Our polarized radiative-transfer modelling predicts low-resolution covering 120 -- 300 nm inclination angle a rotator can be determined within 5 degrees, rate 1%. The origin in Be/n explained by either single-star evolution, but relative importance unknown. Some have hot sub-luminous (sdO) companions, which at an earlier phase transferred envelope (and it mass angular momentum) present-day rotator. Through spectral wide range simulated Be/n+sdO configurations, we demonstrate high-resolution high-SNR detect sdO star even when $\sim$1,000 times fainter than its companion. degree sensitivity needed more fully explore parameter space binaries, so far has been limited about dozen systems relatively luminous stars. We suggest survey next step forward this population. Such dataset would, combined population synthesis models, allow determination possible pathways traversed these also crucial future evolution fate.