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

Abstract

In this study, all unpublished time series photometric data of BM UMa ($q\sim$ 2.0, P = 0.2712\,d) from available archives were re-investigated together with new data taken from the TNT-2.4m of the Thai National Observatory (TNO). 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 $\mathrm{d}P/\mathrm{d}t=-3.36(\pm 0.02)\times 10^{-8}$ d ${\text{yr}}^{-1}$. This long-term period decrease can be explained by mass transfer from the more massive component ($M_2\sim 0.79M_{\odot }$) to the less massive one ($M_1\sim 0.39M_{\odot }$), combination with AML. For photometric study, we found that the binary consists of K0\,V stars and at the middle shallow contact phase with evolution of fill-out factor from 8.8\,\% (in 2007) to 23.2\,\% (in 2020). Those results suggest that the binary is at pre-transition stage of evolution from W-type to A-type, agreeing to the results of statistical study of W-type contact binaries. The mass of $M_2$ will be decreased close to or below $M_1$ and the mass ratio will be decreased ($q<1.0$). By this way, the binary will evolve into A-type as a deeper normal over-contact system with period increase. Finally the binary will end as a merger or a rapid-rotating single star when the mass ratio meet the critical value ($q<0.094$), as well as produce a red nova.

27 pages, 11 figures

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