Rediscussion of eclipsing binaries. Paper XXIX. The F-type twin system BS Draconis

Abstract

We present an analysis of BS Dra, a detached eclipsing binary containing two almost-identical F3 V stars in a 3.36-d circular orbit, based on 40 sectors of observations from the Transiting Exoplanet Survey Satellite (TESS) and published spectroscopic results. We measure masses of 1.305 +/- 0.015 Msun and 1.284 +/- 0.017 Msun, and radii of 1.409 +/- 0.006 Rsun and 1.400 +/- 0.006 Rsun, for the two components. The high quality of the TESS data allow -- for the first time -- a definitive identification of the primary eclipse, which is 0.007 mag deeper than the secondary. The primary star is the hotter, larger and more massive of the two: the ratios of the radii and surface brightnesses are both slightly but significantly below unity. We find a distance concordant with the Gaia DR3 parallax and, by comparison to theoretical models, an age of 1600 +/- 300 Myr and a slightly sub-solar chemical composition. Our mean times of primary eclipse, each representing all eclipses in one sector, have a scatter of only 0.37 s around a linear ephemeris: BS Dra may be useful as a celestial clock.

Figures (6)

• Figure 1: Light curve of BS Dra from TESS sector 83, chosen as it has very few data gaps. The flux measurements have been converted to magnitude units and the median subtracted.
• Figure 2: jktebop best fit to the light curves of BS Dra from TESS sector 83 for the primary eclipse (left panels) and secondary eclipse (right panels). The data are shown as filled red circles and the best fit as a light blue solid line. The residuals are shown on an enlarged scale in the lower panels.
• Figure 3: The best fit to selected photometric parameters of BS Dra from all TESS sectors. The times used on the abscissae are given in Table \ref{['tab:tmin']}. The errorbars are from the MC simulations.
• Figure 4: Residuals of the times of minimum light from Table \ref{['tab:tmin']} (red circles) versus the best-fitting ephemerides. The blue solid line indicates a residual of zero. The ordinate axis does indeed show only $\pm$1.5 seconds.
• Figure 5: RVs of BS Dra from M05 compared to the best fit from jktebop (solid blue lines). The RVs for star A are shown with filled circles, and for star B with open circles. The residuals are given in the lower panels separately for the two components.