Two New Rapidly Rotating ON Stars Found with LAMOST

Abstract

The ON stars are a rare subtype of O stars of uncertain origin. We report two new, rapidly rotating ON stars found in data acquired with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope. LS I +61 28 is an ON8.5 Vn dwarf with a projected equatorial rotational velocity of {V}_{{e}}\sin I≃ 298 {km} {{{s}}}^-1, while HDE 236672 is an ON9 IVn subgiant with {V}_{{e}}\sin I≃ 253 {km} {{{s}}}^-1. The former is the first rapidly rotating ON dwarf to be found, and the latter is only the third ON subgiant. The luminosity classes of non-supergiant ON stars appear to be influenced by the axial inclination angle I: the rapidly rotating giants are close to equator-on, while ON dwarfs with lower {V}_{{e}}\sin I values are viewed more nearly pole-on. Combining parallaxes and proper motions from Gaia Data Release 2 with radial-velocity measurements, we investigate the kinematics of non-supergiant ON stars and infer that the dynamics, rapid rotation, and surface nitrogen characteristics may all be consequences of binary interaction.