Two New Rapidly-Rotating ON Stars Found With LAMOST

TL;DR

This paper reports the discovery of two new rapidly-rotating ON stars using LAMOST data, highlighting their unique rotational velocities and potential binary interaction origins, and discusses how inclination affects their luminosity classes.

Contribution

First identification of rapidly-rotating ON dwarf and subgiant stars, expanding understanding of ON star properties and their possible binary evolution origins.

Findings

LS I +61 28 is the first rapidly-rotating ON dwarf discovered.

HDE 236672 is only the third ON subgiant identified.

Rapid rotation and kinematics suggest binary interaction influences ON star characteristics.

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 $v_{\textrm e}\sin{i} \simeq 298$ km$\;$s$^{-1}$, while HDE 236672 is an ON9 IVn subgiant with $v_{\textrm e}\sin{i} \simeq 253$ 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 $v_{\textrm e}\sin{i}$ 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.