2020ApJ...892L..26L

Rotation and binary interaction play important roles in understanding the nature of massive stars (>=8M_☉). Some interesting transients, such as the long-duration gamma-ray bursts, are thought to be originated from fast-rotating massive stars. Because the strong stellar wind can effectively spin down a metal-rich massive star with fast rotation, it is very hard to find single massive stars rotating critically in the Galaxy. In the present work reported is the discovery of the fastest rotator in the Galaxy, LAMOST J040643.69+542347.8, with a projected rotational velocity v_esini∼540 km s^–1, which is ∼100 km s^–1 faster than that of the previous record holder HD 191423. The star has a spectral type of O6.5 Vnnn(f)p. Its He I λ4471 absorption line is blueshifted and asymmetric, while its He II λ4686 and Hα have central absorption reversals in their emissions. It is also a runaway star, which implies an origin in a close binary interaction. Compared to VFTS 285 and VFTS 102 (their v_esini∼610 km s^–1) in the Large Magellanic Cloud, LAMOST J040643.69+542347.8 has its own peculiar spectral characteristics and earlier spectral type. Moreover, LAMOST J040643.69+542347.8 is bright (B ∼ 13.9 mag) enough to allow future high-resolution spectroscopic follow-up.