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}~$\lambda 4200$, \ion{He}{i}~$\lambda 4471$, and \ion{He}{ii}~$\lambda 4542$. We obtained parent non-rotating counterpart (PNRC) parameters $T_{\mathrm{eff}}^{\mathrm{PNRC}}=38716\pm 1041$~K, $\log g^{\mathrm{PNRC}}=4.12\pm 0.12$, and $v_{\mathrm{e}}\sin i=587\pm 73$~\kps. The central grid solution gives $\Omega /{\Omega }_{\mathrm{c}}=0.93$, $v_{\mathrm{e}}/v_{\mathrm{c}}=0.77$, $i=65^{\circ }$, $R_{\mathrm{e}}/R_{\mathrm{p}}=1.25$, and an equatorial centrifugal-to-gravitational acceleration ratio ${\eta }_{\mathrm{e}}=0.50$. Roche critical rotation requires ${\eta }_{\mathrm{e}}=1$ and $R_{\mathrm{e}}/R_{\mathrm{p}}=1.5$: 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 $\Omega /{\Omega }_{\mathrm{c}}$, $v_{\mathrm{e}}/v_{\mathrm{c}}$, and ${\eta }_{\mathrm{e}}$ together. A controlled spherical fit with the same atmosphere grid is needed to isolate the bias from neglecting deformation and gravity darkening.}

arXiv comment6 pages, 1 figure and 2 tables, A&A Accepted