The dynamical mass and evolutionary status of the type-II Cepheid in the eclipsing binary system OGLE-LMC-T2CEP-211 with a double-ring disk

Abstract

We present the analysis of a peculiar W~Virginis (pWVir) type-II Cepheid, OGLE-LMC-T2CEP-211 ($P_{puls}=9.393d$), in a double-lined binary system ($P_{orb}=242d$), which shed light on virtually unknown evolutionary status and structure of pWVir stars. The dynamical mass of the Cepheid (first ever for a type-II Cepheid) is $0.64\pm 0.02M_{\odot }$ and the radius $R=25.1\pm 0.3R_{\odot }$. The companion is a massive ($5.67M_{\odot }$) main-sequence star obscured by a disk. Such configuration suggests a mass transfer in the system history. We found that originally the system ($P_{orb}^{init}=12d$) was composed of $3.5$ and $2.8M_{\odot }$ stars, with the current Cepheid being more massive. The system age is now $\sim$200 My, and the Cepheid is almost completely stripped of hydrogen, with helium mass of $\sim 92\%$ of the total mass. It finished transferring the mass 2.5 My ago and is evolving towards lower temperatures passing through the instability strip. Comparison with observations indicate a reasonable $2.7\cdot 10^{-8}{M}_{\odot }/y$ mass loss from the Cepheid. The companion is most probably a Be main-sequence star with $T=22000K$ and $R=2.5R_{\odot }$. Our results yield a good agreement with a pulsation theory model for a hydrogen-deficient pulsator, confirming the described evolutionary scenario. We detected a two-ring disk ($R_{disk}\sim 116R_{\odot }$) and a shell ($R_{shell}\sim 9R_{\odot }$) around the companion, that is probably a combination of the matter from the past mass transfer, the mass being lost by the Cepheid due to wind and pulsations, and a decretion disk around a rapidly rotating secondary. Our study together with observational properties of pWVir stars suggests that their majority are products of a similar binary evolution interaction.

arXiv comment21 pages, 14 figures, 6 tables, accepted for publication in ApJ

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