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, LAMOST. LS I +61 28 is an ON8.5 Vn dwarf with a projected equatorial rotational velocity of $v_{\text{e}}\sin i\simeq 298$ kms${}^{-1}$, while HDE 236672 is an ON9 IVn subgiant with $v_{\text{e}}\sin i\simeq 253$ kms${}^{-1}$ 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 $v_{\text{e}}\sin i$ values are viewed more nearly pole-on. Combining parallaxes and proper motions from Gaia DR2 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.

arXiv comment8 pages, 4 figures, 2 tables

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Cited by in corpus (3)

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• Galactic O-type stars in LAMOST data
• LAMOST J040643.69+542347.8: the fastest Rotator in the Galaxy