A brown dwarf donor and an optically thin accretion disc with a complex stream impact region in the period-bouncer candidate BW Sculptoris

TL;DR

This study characterizes the low-mass donor and accretion disc in the period-bouncer candidate BW Scl, revealing a brown dwarf donor, a low-luminosity disc, and complex stream impact regions through multi-epoch spectroscopic and photometric analysis.

Contribution

It provides the first detailed measurements of the donor and white dwarf in BW Scl, confirming the donor as a brown dwarf and analyzing the accretion disc's structure and luminosity.

Findings

White dwarf mass is 0.85 solar masses.

Donor mass is 0.051 solar masses, below the hydrogen-burning limit.

Accretion disc has very low luminosity and mass accretion rate.

Abstract

We present an analysis of multi-epoch spectroscopic and photometric observations of the WZ Sge-type dwarf nova BW Scl, a period-bouncer candidate. 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 lines Mgii 4481 and Caii K 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 0.85$\pm$0.04 M$_\odot$, while the donor is a low-mass object with a mass of 0.051$\pm$0.006 M$_\odot$, well below the hydrogen-burning limit. Using NIR data, we put an upper limit on the effective temperature of the donor to be $\lesssim$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$7 $\times 10^{-13}$ 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. Based on the measured system parameters, we discuss the evolutionary status of the system.