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BM UMa: A middle shallow contact binary at pre-transition stage of evolution from W-type to A-type

Abstract : Abstract In this study, all unpublished time series photometric data of BM UMa (q ∼ 2.0, P = 0.2712 d) from available archives were re-investigated together with new data taken from the 2.4-m Thai National Telescope of the Thai National Observatory. Based on period analysis, there is a short-term variation superimposed on the long-term period decrease. The trend of period change can be fitted with a downward parabolic curve indicating a period decrease at a rate of dP/dt = −3.36(±0.02) × 10−8 d yr−1. This long-term period decrease can be explained by mass transfer from the more massive component (M2 ∼ 0.79M⊙) to the less-massive one (M1 ∼ 0.39M⊙), in combination with angular momentum loss. For photometric study, we found that the binary consists of K0 V stars and at the intermediate shallow of contact configuration with evolution of fill-out factor from 8.8% (in 2007) to 23.2% (in 2020). Those results suggest that the binary is at the pre-transition stage of evolution from W-type to A-type, agreeing with the results of statistical study of W-type contact binaries. The mass of M2 will be decreased close to or below M1 and the mass ratio will be decreased (q < 1.0). In this way, the binary will evolve into an A-type as a deeper normal over-contact system with period increase. Finally, the binary will become a merger or a rapid-rotating single star when the mass ratio meets the critical value (q < 0.094), and produce a red nova.
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Submitted on : Tuesday, November 2, 2021 - 11:53:17 AM
Last modification on : Wednesday, November 3, 2021 - 3:52:58 AM

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Thawicharat Sarotsakulchai, Boonrucksar Soonthornthum, Saran Poshyachinda, Christophe Buisset, Thierry Lépine, et al.. BM UMa: A middle shallow contact binary at pre-transition stage of evolution from W-type to A-type. Publications of the Astronomical Society of Japan, Oxford university press, 2021, ⟨10.1093/pasj/psab090⟩. ⟨ujm-03411451⟩



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