[Paper Review] The EBLM Project VI. The mass and radius of five low-mass stars in F+M binaries discovered by the WASP survey

Samuel Gill, P. F. L. Maxted+31 more|Edinburgh Research Explorer|Apr 29, 2019

Stellar, planetary, and galactic studies104 references9 citations

TL;DR

This study measures the masses and radii of five low-mass M-dwarf stars in F+M eclipsing binary systems discovered by the WASP survey using simultaneous radial velocity and photometric light curve modeling. It finds little evidence of stellar inflation, but highlights that uncertainties in limb-darkening laws, third-light contamination, and stellar evolution model parameters (like mixing-length and helium abundance) can introduce up to 5–8% systematic errors in M-dwarf mass and radius estimates.

ABSTRACT

Some M-dwarfs around F-/G-type stars have been measured to be hotter and larger than predicted by stellar evolution models. Inconsistencies between observations and models need addressing with more mass, radius and luminosity measurements of low-mass stars to test and refine evolutionary models. Our aim is to measure the masses, radii and ages of the stars in five low-mass eclipsing binary systems discovered by the WASP survey. We use WASP photometry to establish eclipse-time ephemerides and to obtain initial estimates for the transit depth and width. Radial velocity measurements were simultaneously fitted with follow-up photometry to find the best-fitting orbital solution. This solution was combined with measurements of atmospheric parameters to interpolate evolutionary models and estimate the mass of the primary star, and the mass and radius of the M-dwarf companion. We assess how the best fitting orbital solution changes if an alternative limb-darkening law is used and quantify the systematic effects of unresolved companions. We also gauge how the best-fitting evolutionary model changes if different values are used for the mixing length parameter and helium enhancement. We report the mass and radius of five M-dwarfs and find little evidence of inflation with respect to evolutionary models. The primary stars in two systems are near the ``blue hook'' stage of their post sequence evolution, resulting in two possible solutions for mass and age. We find that choices in helium enhancement and mixing-length parameter can introduce an additional 3-5\,\% uncertainty in measured M-dwarf mass. Unresolved companions can introduce an additional 3-8\% uncertainty in the radius of an M-dwarf, while the choice of limb-darkening law can introduce up to an additional 2\% uncertainty.

Motivation & Objective

• To measure precise masses, radii, and ages of five low-mass M-dwarf stars in F+M eclipsing binary systems discovered by the WASP survey.
• To assess systematic uncertainties in stellar parameters arising from choices in limb-darkening laws, third-light contamination, and stellar evolution model parameters.
• To test whether observed M-dwarfs are inflated relative to theoretical models, particularly in the context of known discrepancies in low-mass star evolution.
• To evaluate the impact of unresolved companions and orbital solution fitting on derived physical properties of the systems.
• To provide high-precision constraints on M-dwarf properties to refine and validate stellar evolution models.

Proposed method

• Simultaneous fitting of radial velocity curves and follow-up photometric light curves to derive the best-fitting orbital solution for each eclipsing binary system.
• Use of the Claret 4-parameter limb-darkening law and comparison with quadratic law using theoretical coefficients from the ldtk tool to assess limb-darkening sensitivity.
• Application of evolutionary model interpolation via the eblmmass code, using atmospheric parameters (Teff, [Fe/H], log g) to estimate masses, radii, and ages.
• Systematic variation of key model parameters—mixing-length parameter (αMLT) and helium enhancement (ΔY)—to quantify their impact on derived stellar properties.
• Incorporation of red-noise models (Matern-3/2 kernel) to account for correlated noise in follow-up photometry, particularly in systems like J2308−46.
• Assessment of third-light contamination by simulating 10% flux contributions from unresolved companions to quantify their effect on radius measurements.

Experimental results

Research questions

• RQ1 To what extent are the measured radii of M-dwarf companions in F+M binaries consistent with predictions from standard stellar evolution models?
• RQ2 How do uncertainties in limb-darkening laws affect the derived radii and masses of the binary components?
• RQ3 What is the impact of third-light contamination (e.g., unresolved companions) on the precision of M-dwarf radius measurements?
• RQ4 How do variations in the mixing-length parameter (αMLT) and helium abundance (ΔY) in stellar models affect the derived masses and ages of the M-dwarf companions?
• RQ5 Do the observed M-dwarfs show signs of inflation or deflation relative to theoretical models, and how robust are these conclusions under model and measurement uncertainties?

Key findings

• The M-dwarf companions in the five systems show little to no evidence of inflation; two systems (J2308−46 and J1847+39) show deflation at 2σ and 1.5σ significance, respectively.
• J0218−31 is consistent with models within 1σ, though moderate evidence suggests possible deflation.
• J2349−32 shows a 1σ radius inflation, while J1436−13 has a high impact parameter, making inflation assessment inconclusive.
• The choice of limb-darkening law introduces up to 2% additional uncertainty in M-dwarf radius measurements.
• Third-light contamination from unresolved companions can increase the derived M-dwarf radius by 3–8%, highlighting the need for flux deconvolution.
• Uncertainties in the mixing-length parameter (αMLT) and helium enhancement (ΔY) contribute up to 3–5% additional uncertainty in M-dwarf mass estimates.