How fast is the fastest-rotating O-type star in the Galaxy?

Abstract

{Rapid rotation distorts massive stars, making parameters from spherical models inclination-dependent. We studied these effects in the extreme Galactic O-type runaway LAMOST J040643.69+542347.8.} {We re-determined its stellar and rotational parameters with an oblate, gravity-darkened photospheric model and tabulated Stark profiles for selected \ion{He}{ii} lines.} {We fitted continuum-normalised spectra over 4000--4600\,Å with \texttt{ZPEKTR}, using TLUSTY/SYNSPEC emergent fluxes, limb darkening to weight their angular contribution over the visible Roche surface, Espinosa-Lara \& Rieutord gravity darkening, and Schöning--Butler Stark profiles. Interstellar and wind- or circumstellar-dominated features were masked.} {The model reproduces \ion{He}{ii}~$\lambda4200$, \ion{He}{i}~$\lambda4471$, and \ion{He}{ii}~$\lambda4542$. We obtained parent non-rotating counterpart (PNRC) parameters $T_{\rm eff}^{\rm PNRC}=38\,716\pm1041$~K, $\log g^{\rm PNRC}=4.12\pm0.12$, and $v_{\rm e}\sin i=587\pm73$~\kps. The central grid solution gives $Ω/Ω_{\rm c}=0.93$, $v_{\rm e}/v_{\rm c}=0.77$, $i=65^\circ$, $R_{\rm e}/R_{\rm p}=1.25$, and an equatorial centrifugal-to-gravitational acceleration ratio $η_{\rm e}=0.50$. Roche critical rotation requires $η_{\rm e}=1$ and $R_{\rm e}/R_{\rm p}=1.5$: despite the high angular rate, the central solution is not critical in equatorial force balance. The projected velocity exceeds, but is statistically consistent with, the previous spherical-fit value.} {\object{LAMOST J040643.69+542347.8} has a markedly oblate, gravity-darkened photosphere. Its proximity to critical rotation should be assessed using $Ω/Ω_{\rm c}$, $v_{\rm e}/v_{\rm c}$, and $η_{\rm e}$ together. A controlled spherical fit with the same atmosphere grid is needed to isolate the bias from neglecting deformation and gravity darkening.}