A Strict Test of Stellar Evolution Models: The Absolute Dimensions of Massive Benchmark Eclipsing Binary V578 Mon

Abstract

We determine the absolute dimensions of the eclipsing binary V578 Mon, a detached system of two early B-type stars (B0V + B1V, P$=$2.40848 d) in the star-forming region NGC 2244 of the Rosette Nebula. From the light curve analysis of 40 yr of photometry and the analysis of HERMES spectra, we find radii of $5.41\pm0.04$ Rsun and $4.29\pm 0.05$ Rsun, and temperatures of $ 30000\pm 500$~K and $ 25750\pm 435$ K respectively. We find that our disentangled component spectra for V578 Mon agree well previous spectral disentangling from the literature. We also reconfirm the previous spectroscopic orbit of V578 Mon finding that masses of $ 14.54\pm 0.08$ Msun and $ 10.29\pm 0.06$ Msun are fully compatible with the new analysis. We compare the absolute dimensions to the rotating models of the Geneva and Utrecht groups and the models of Granada group. We find all three sets of models marginally reproduce the absolute dimensions of both stars with a common age within uncertainty for gravity-effective temperature isochrones. However - there are some apparent age discrepancies for the corresponding mass-radius isochrones. Models with larger convective overshoot $>0.35$ worked best. Combined with our previously determined apsidal motion of $0.07089^{+0.00021}_{-0.00013}$ deg cycle$^{-1}$, we compute the internal structure constants (tidal Love number) for the newtonian and general relativistic contribution to the apsidal motion, $\log{k_2}=-1.975\pm0.017$ and $\log{k_2}=-3.412\pm0.018$ respectively. We find the relativistic contribution to the apsidal motion of be small $<4\%$. We find that the prediction of $\log{k_{\rm 2,theo}}=-2.005\pm0.025$ of the Granada models fully agrees with our observed $\log{k_2}$.