2018A&A...619A.167D

Context. Gravity darkening (GD) and flattening are important consequences of stellar rotation. The precise characterization of these effects across the Hertzsprung-Russell (H-R) diagram is crucial to a deeper understanding of stellar structure and evolution.
Aims. We seek to characterize such important effects on Sargas (θ Scorpii), an evolved, fast-rotating, intermediate-mass (∼5 M_☉) star, located in a region of the H-R diagram where they have never been directly measured as far as we know.
Methods. We use our numerical model CHARRON to analyze interferometric (VLTI/PIONIER) and spectroscopic (VLT/UVES) observations through a MCMC model-fitting procedure. The visibilities and closure phases from the PIONIER data are particularly sensitive to rotational flattening and GD. Adopting the Roche approximation, we investigate two GD models: (1) the β-model (T_eff∝g_eff^β), which includes the classical von Zeipel's GD law, and (2) the ω-model, where the flux is assumed to be anti-parallel to g_eff.
Results. Using this approach we measure several physical parameters of Sargas, namely, equatorial radius, mass, equatorial rotation velocity, mean T_eff, inclination and position angle of the rotation axis, and β. In particular, we show that the measured β leads to a surface flux distribution equivalent to the one given by the ω-model. Thanks to our results, we also show that Sargas is most probably located in a rare and interesting region of the H-R diagram: within the Hertzsprung gap and over the hot edge of the instability strip (equatorial regions inside it and polar regions outside it because of GD).
Conclusions. These results show once more the power of optical/infrared long-baseline interferometry, combined with high-resolution spectroscopy, to directly measure fast-rotation effects and stellar parameters, in particular GD. As was the case for a few fast rotators previously studied by interferometry, the ω-model provides a physically more profound description of Sargas' GD, without the need of a β exponent. It will also be interesting to further investigate the implications of the singular location of such a fast rotator as Sargas in the H-R diagram.