How fast is the fastest-rotating O-type star in the Galaxy?. LAMOST J040643.69+542347.8
Year: 2026
Type: article
Abstract: Rapid rotation distorts massive stars and produces latitude-dependent effective temperature and gravity. Stellar parameters inferred from a single spherical atmosphere convolved with a rotational profile can therefore be apparent inclination-dependent quantities. LAMOST J040643.69+542347.8 is an extreme Galactic O-type runaway star for which these effects should be appreciable. We re-determined the stellar and rotational parameters of LAMOST J040643.69+542347.8 with an oblate gravity-darkened photospheric model and used selected transitions treated with tabulated Stark profiles. He ii We fitted continuum-normalised spectra over 4000--4600,Å with ZPEKTR . The local TLUSTY/SYNSPEC inputs are emergent flux spectra, and we introduced their angular contribution to the visible Roche surface through the limb-darkening treatment implemented in . The gravity darkening we employed follows the Espinosa-Lara & Rieutord model, and for the selected transitions, we used the Schöning--Butler Stark profiles implemented in SYNSPEC. We masked interstellar and wind- or circumstellar-dominated features. ZPEKTR He ii The model reproduces the fitted łambda4200, łambda4471, and łambda4542 profiles. The parent non-rotating counterpart (PNRC) parameters are T_ He ii He i He ii ̊m eff ^ ̊m PNRC =38,716 =4.12 /v_ K and łog g^ ̊m PNRC and we obtained v_ ̊m e i=587 =0.93, v_ ̨ps. The central grid solution has Ω/Ω_ ̊m c ̊m e ̊m c =0.77, i=65 /R_ and R_ ̊m e ̊m p =1.25. The corresponding ratio of centrifugal to gravitational acceleration at the equator is η_ ̊m e =0.50, whereas η_ ̊m e =1 and R_ ̊m e /R_ ̊m p =1.5 at Roche critical rotation. Thus, the angular rotation rate is high, but the central solution is not critical in the equatorial force-balance sense. The projected rotational velocity is higher than but statistically consistent with the value previously obtained from a spherical rotational-convolution analysis. LAMOST J040643.69+542347.8 is an exceptionally rapid O-type rotator with a markedly oblate gravity-darkened photosphere. The ratios Ω/Ω_̊m c, v_ ̊m e /v_ ̊m c , and η_̊m e should be quoted together when discussing its proximity to critical rotation. A controlled spherical fit with the same atmosphere grid would be required to quantitatively isolate the bias introduced by neglecting rotational deformation and gravity darkening.
Source: Astronomy and Astrophysics
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W7223132269