Fundamental effective temperature measurements for eclipsing binary stars - VII. The solar twin in LL Aquarii

TL;DR

This work delivers high-precision fundamental parameters for the eclipsing binary LL Aqr, including masses $M_A=1.1947\pm0.0009\,M_\odot$, $M_B=1.0334\pm0.0006\,M_\odot$ and radii $R_A=1.3180\pm0.0013\,R_\odot$, $R_B=0.9927\pm0.0008\,R_\odot$, via TESS photometry and VLTI/GRAVITY-based orbital solutions. It directly measures effective temperatures $T_{\rm eff,A}=6242\pm50$ K and $T_{\rm eff,B}=5839\pm44$ K using bolometric fluxes, angular diameters, and parallax, with a 9 K CALSPEC flux-scale systematic. Interstellar reddening $E(B-V)=0.040\pm0.015$ mag is incorporated, and a solar-calibrated GARSTEC model grid yields system ages around $2.7-3.0$ Gyr, supporting LL Aqr B as a solar-twin benchmark. Magnetic-field upper limits ($\langle B\rangle_A<78$ G, $\langle B\rangle_B<96$ G) indicate very weak activity, consistent with low Rossby numbers and non-detection of activity indicators. The results solidify LL Aqr as a valuable benchmark for testing stellar evolution and spectrophotometric methods in solar-type stars.

Abstract

The eclipsing binary LL Aqr is a bright V = 9.32, detached system consisting of two solar-type stars in an eccentric orbit (P = 20.2 d). The secondary component, LL Aqr B, was previously found to have physical and atmospheric parameters very similar to the Sun. Using high-precision photometry from TESS along with previously published orbital solutions, we obtain updated model-independent stellar radii and masses: $R_A = 1.3180 \pm 0.0013~R_{\odot}$, $R_B = 0.9927 \pm 0.0008~R_{\odot}$, $M_A = 1.1947 \pm 0.0009~M_{\odot}$, and $M_B = 1.0334 \pm 0.0006~M_{\odot}$. We derive the bolometric flux and fundamental effective temperature for each component using observed magnitudes, flux ratios from light curves in multiple bands and angular diameters derived from the radii and parallax from long baseline interferometry, measuring the following values: $T_{\rm eff,A} = 6242 \pm 50$ K, $T_{\rm eff,B} = 5839 \pm 44$ K, with an additional 9 K systematic error from the flux scale zero-point. We confirm that LL Aqr displays low stellar activity by obtaining $2σ$ upper limits on the mean surface magnetic field strengths of 78 G and 96 G. Our results suggest an age of 2.67 - 3.01 Gyr, which is consistent with previous studies. LL Aqr now joins an ever-growing sample of well-characterised benchmark stars that can be used to test and calibrate a wide variety of methods and techniques in stellar astrophysics.

Figures (7)

• Figure 1: TESS sector 70 phase-folded light curve of the primary (left) and secondary (right) eclipses of LL Aqr, along with the best-fitting jktebop model following 1000 Monte-Carlo simulations. Residuals and binned residuals to the fit are given in the lower panels.
• Figure 2: Phase-folded $UBV$ light curves for the primary (left) and secondary (right) eclipses, plotted alongside the best jktebop fits. The residuals are shown in the lower panels, with $\pm0.25$ mag offsets from zero for $U$ and $V$.
• Figure 3: The HARPS spectra used in the estimation of $E(B-V)$ in the region of the NaI D lines. The interstellar lines are clearly visible and labelled.
• Figure 4: Upper panel: The SED of LL Aqr. The best-fit combined SED is plotted as a line and the mean SED $\pm1-\sigma$ 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 with green and orange lines. Lower panel: Same as the upper panel but with fluxes plotted on a logarithmic scale. Filters used to measure flux ratios are also plotted here.
• Figure 5: Hertzsprung-Russell diagram showing best-fitting GARSTEC stellar evolution tracks for all three model grids available in bagemass for LL Aqr, along with observed quantities.