Pulsation modelling of the Cepheid Y Ophiuchi with RSP/MESA. Impact of the circumstellar envelope and a high projection factor on Baade-Wesselink method

Abstract

Y~Ophiuchi (Y~Oph) is a classical Cepheid reported to be as dim as a Cepheid of about half its pulsation period, and exhibits a low radial velocity and light-curves amplitude. Our objective is to conduct hydrodynamical pulsation modeling of Y~Oph to derive its distance and provide physical insight to its low amplitude and luminosity, constrained by an extensive set of observations. We first perform a linear analysis on a grid of models using hydrodynamical pulsation code \texttt{MESA-RSP} in order to find the combinations of mass, metallicity, effective temperature and luminosity resulting in linear excitation of pulsations with period of about 17$\,$days. Then, for the best combinations of stellar parameters, we perform non-linear computations to obtain the full-amplitude pulsations of these models. Last, we compare the results to a complete set of observations along the pulsation cycle. We adjust simultaneously the distance, the color excess and circumstellar envelope (CSE) model to fit the light curves and the angular diameter. We find that all pulsation models at high effective temperature are in remarkable agreement with the observations along the pulsation cycle. This result suggests that the low amplitude of Y~Oph can be explained by its location close to the blue edge of the instability strip. We also find that a pulsational mass of about 7-8$\,\mathrm{M}_\odot$ is consistent with a non-canonical evolutionary model with moderate overshooting, PL relation and \textit{Gaia} parallax. However, a much lower mass below 5$\,$M$_\odot$ is required to match Baade-Wesselink (BW) distance measurements from the literature. We show that the combination of the impact of the CSE on the photometry together with a projection factor of about 1.5 explains the discrepant distance and luminosity values obtained from BW methods.