Rediscussion of Eclipsing Binaries. Paper XXX. The Slightly Evolved F-type System BK Pegasi

Abstract

BK Peg is a double-lined detached eclipsing binary containing two late-F stars in an orbit with small eccentricity. We use light curves from the Transiting Exoplanet Survey Satellite (TESS) and spectroscopic measurements from previous studies to measure the physical properties of the companions to a high precision. We obtain masses of $1.411 \pm 0.004$\Msun\ and $1.254 \pm 0.004$\Msun, and radii of $1.990 \pm 0.004$\Rsun\ and $1.460 \pm 0.004$\Rsun, which are among the most precise measurements made for these quantities in normal stars. These properties match theoretical stellar evolutionary models for a solar chemical composition and an age of 2.65~Gyr. We also present an updated ephemeris of the system, as a result of our TESS measurements and a collection of mid-eclipse times from previous studies.

Figures (5)

• Figure 1: Top: TESS sector 57 photometry of BK Peg. The flux measurements have been converted to magnitude units, and the median was subtracted. Bottom: trimmed light curves of Sector 57 for further analysis.
• Figure 2: Best-fit model to the TESS Sector 57 light curves of BK Peg for the primary eclipse (left panels) and secondary eclipse (right panels), obtained using jktebop. The observed data are shown as black points, and the model fit is plotted as a red solid line. The residuals are displayed on an enlarged scale in the lower panels.
• Figure 3: Residuals of the primary minimum times from Demircan et al.demircan1994 (green), Ak et al.ak2003 (orange), Hübscher et al.hubscher2010 (purple), VarAstro (black) and TESS (yellow), as listed in Table \ref{['tab:tmin']}. Dashed black line represents the residuals of zero according to the linear ephemeris. Diamonds represent the photoelectric observations, while circles denote the CCD observations.
• Figure 4: RVs of BK Peg compared to the best fit from jktebop (solid blue lines). The RVs for star A are shown with dark red colour, and for star B with blue colour. The residuals are given in the lower panels separately for the two components. RVs from Popperpopper1983 are shown with closed circles and diamonds, and those from CL10 with open circles and diamonds.
• Figure 5: Hertzsprung-Russell diagram for the components of BK Peg (filled green circles) and the predictions of the parsec 1.2 models Bressan+12mn. The dashed blue line shows the zero-age main sequence for $Z=0.0170$. The dotted blue lines show evolutionary tracks for $Z=0.017$ and masses of 1.1$~{\rm M}_\odot$ to 1.5$~{\rm M}_\odot$ in steps of 0.1$~{\rm M}_\odot$ (from bottom-right to top-left). The solid red line shows an isochrone for $Z=0.017$ and an age of 2650 Myr; it is not a perfect match in this diagram because it was chosen as the best fit to the masses and radii of the components of BK Peg.