Fundamental effective temperature measurements for eclipsing binary stars -- VI. Improved methodology and application to the circumbinary planet host star BEBOP-3

TL;DR

BEBOP-3 tackles the challenge of obtaining precise fundamental parameters for solar-type stars in eclipsing binaries with very low-mass companions by delivering direct $T_{\rm eff}$ measurements from angular diameters and bolometric flux within a Bayesian SED framework. The authors implement an improved method with updated colour–$T_{\rm eff}$ relations, a photometric database, and robust handling of photometric systematics, then apply it to BEBOP-3 to derive $T_{\rm eff,1}=6065\pm44$ K and $T_{\rm eff,2}=3191\pm40$ K, along with radii and $\log g$ for both components. The study also reports near-zero reddening toward BEBOP-3 and confirms mass–radius consistency with models, while highlighting the sensitivity of the M-dwarf Teff to flux-ratio priors and metallicity, and advocating near-IR eclipse measurements for stronger VLMS constraints. Overall, BEBOP-3 serves as a crucial benchmark for validating spectroscopic and photometric Teff and $\log g$ scales and demonstrates a practical, extensible methodology for eclipsing-binary analyses.

Abstract

BEBOP-3 is detached eclipsing binary star that shows total eclipses of a faint M~dwarf every 13.2 days by a 9$^{\rm th}$-magnitude F9V star. High precision radial velocity measurements have recently shown that this binary star is orbited by a planet with an orbital period $\approx 550$ days. The extensive spectroscopy used to detect this circumbinary planet has also been used to directly measure the masses of the stars in the eclipsing binary. We have used light curves from the TESS mission combined with these mass measurements to directly measure the following radii and surface gravities for the stars in this system: $R_1 = 1.386 \pm 0.010\,R_{\odot}$, $\log g_1 = 4.190 \pm 0.004$, $R_2 = 0.274 \pm 0.002\,R_{\odot}$, $\log g_2 = 4.979 \pm 0.002$. We describe an improved version of our method to measure the effective temperatures (T$_{\rm eff}$) of stars in binary systems directly from their angular diameters and bolometric fluxes. We measure T$_{\rm eff,1} = 6065{\rm\,K} \pm 44\,{\rm K}$ and T$_{\rm eff,2} = 3191{\rm\,K} \pm 40\,{\rm K}$ for the stars in BEBOP-3 using this method. BEBOP-3 can be added to our growing sample of stars that can be used test the accuracy of spectroscopic and photometric methods to estimate T$_{\rm eff}$ and $\log g$ for solar-type stars.

Figures (5)

• Figure 1: Photometry obtained from TESS images of BEBOP-3 as a function of orbital phase plotted with the best-fit light curve model. The residuals from the best-fit model are shown offset vertically above the light curve data.
• Figure 2: Upper panel: The SED of BEBOP-3. The best-fit SED is plotted as a line and the mean SED $\pm 1-\sigma$ is plotted as a filled region. The observed fluxes are plotted as points with error bars and predicted fluxes for the best-fit SED integrated over the response functions shown are plotted with open circles. The SEDs of the two stars are also plotted but are barely distinguishable at this scale. Lower panel: Same as the upper panel but with fluxes plotted on a logarithmic scale. The response function of the TESS instrument is also plotted here.
• Figure 3: Pair-wise joint distributions and sample histograms for some parameters of interest from our analysis of BEBOP-3 plotted using cornercorner.
• Figure 4: Upper panel: primary component of BEBOP-3 in the mass -- radius plane (diamond with error bars). Middle panel: secondary component of BEBOP-3 in the mass -- radius plane (diamond with error bars). Lower panel: both components of BEBOP-3 in the Hertzsprung-Russell diagram (diamonds with error bars). The points on the isochrones corresponding to the observed mass of BEBOP-3 B are marked with "+" symbols. All panels show isochrones for an age of 5.9 Gyr assuming $[{\rm Fe/H}]=0.0$ from bagemass (red solid line), DSEP (blue dotted line) and MIST (green dashed line). Cyan error bars show stars in eclipsing binary systems taken from DEBCat 2015ASPC..496..164S. The values of T$_{\rm eff}$ and $\log L/L_{\odot}$ for the three M-dwarf companions in eclipsing binaries in Table \ref{['tab:teff_test']} have been used here. These stars are circled in the middle and lower panels.
• Figure 5: Empirical colour -- effective temperature (T$_{\rm eff}$) relations late-type stars. The mean and sample standard deviation in each T$_{\rm eff}$ bin are shown as a solid point with an error bar. These have been calculated from the points in the same T$_{\rm eff}$ bin plotted using dots.