The EBLM project X. Benchmark masses, radii, and temperatures for two fully convective M-dwarfs using K2

Abstract

M-dwarfs are the most abundant stars in the galaxy and popular targets for exoplanet searches. However, their intrinsic faintness and complex spectra inhibit precise characterisation. We only know of dozens of M-dwarfs with fundamental parameters of mass, radius and effective temperature characterised to better than a few per cent. Eclipsing binaries remain the most robust means of stellar characterisation. Here we present two targets from the Eclipsing Binary Low Mass (EBLM) survey that were observed with K2: EBLM J0055-00 and EBLM J2217-04. Combined with HARPS and CORALIE spectroscopy, we measure M-dwarf masses with precisions better than 5%, radii better than 3% and effective temperatures on order 1%. However, our fits require invoking a model to derive parameters for the primary star and fitting the M-dwarf using the transit and radial velocity observations. By investigating three popular stellar models, we determine that the model uncertainty in the primary star is of similar magnitude to the statistical uncertainty in the model fits of the secondary M-dwarf. Therefore, whilst these can be considered benchmark M-dwarfs, we caution the community to consider model uncertainty when pushing the limits of precise stellar characterisation.

The EBLM Project—From False Positives to Benchmark Stars and Circumbinary Exoplanets

“…This is naturally a long-term project due to the long orbital periods of these planets, but the first successes from this project have been published [8,45] and a substantial observing programme is ongoing, so more discoveries are to be expected in the coming decade. [11,20,26,[30][31][32][33][34]36].…”

“…The evolution of main-sequence M dwarfs is negligible within the lifetime of the Universe, but an age constraint is nevertheless useful to rule out the possibility that the M dwarf is a pre-main sequence star that is contracting towards the main sequence. [11,17,26,[30][31][32][33][34]36,38,[96][97][98][99]101,. The data used to generate this figure are available in the Supplementary Materials, File S1.…”

“…It is interesting to test theories of radius inflation against observations of EBLMs because stars with masses 0.35 M lack a radiative core [21], so models that successfully reproduce the properties of stars with masses ∼ 0.6 M may not work for very-low-mass stars [22,23]. The EBLM project has published 10 papers over the past decade in a series entitled "The EBLM project", in which we have used follow-up observations to characterise the stars in these eclipsing binary star systems [11,[26][27][28][29][30][31][32][33][34]. The project is ongoing and will soon publish two more papers in this series [20,35].…”

“…Figure 8. Orbital eccentricity as a function of orbital period for EBLM systems[11,20,26,[30][31][32][33][34]36].…”

The EBLM project – VIII. First results for M-dwarf mass, radius, and effective temperature measurements using CHEOPS light curves

“…The EBLM project seeks to use this large source of identified eclipsing binaries to address a shortfall of accurate mass, radius and effective temperature measurements for low mass stars, further exploring apparent problems at the low-mass end of the HR diagram. The EBLM series has explored eclipsing binaries at different stellar limits (Triaud et al 2013;von Boetticher et al 2017), the impact of different models for primary stars (Duck et al 2023), potential radius inflation (von Boetticher et al 2019;Gill et al 2019) and in EBLM IV (Triaud et al 2017) derived masses from the spectroscopic orbits of over 100 M-dwarfs.…”