http://arxiv.org/abs/2212.03264
We present an analysis of multi-epoch spectroscopic and photometric observations of BW Scl, which is believed to be one of the best period-bouncer candidates. We detected multiple irradiation-induced emission lines from the donor star allowing the radial velocity variations to be measured with high accuracy. Also, using the absorption Mgii 4481 line originated in the photosphere of the accreting white dwarf (WD), we measured the radial velocity semi-amplitude of the WD and its gravitational redshift. We find that the WD has a mass of 0.92$\pm$0.04 M$\odot$, while the donor is a low-mass object with a mass of 0.054$\pm$0.008 M$\odot$, well below the hydrogen-burning limit. Using NIR data, we put an upper limit on the effective temperature of the donor to be $\leq$1600 K, corresponding to a brown dwarf of T spectral type. The optically thin accretion disc in BW Scl has a very low luminosity $\lesssim$4 $\times 10^{30}$ erg s$^{-1}$ which corresponds to a very low mass accretion rate of $\lesssim$6 $\times 10^{-13}$ M$_\odot$ year$^{-1}$. The outer parts of the disc have a low density allowing the stream to flow down to the inner disc regions. The brightest part of the hotspot is located close to the circularization radius of the disc. The hotspot is optically thick and has a complex, elongated structure. Despite the relatively high temperature of the WD (14750-15000 K), we suggest that BW Scl has already passed the minimum period and is now evolving back towards longer periods. Thus, BW Scl is a period bouncer.
V. Neustroev and I. Mäntynen
Thu, 8 Dec 22
20/63
Comments: 20 pages, 16 figures, and 3 pages of appendices
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