The scatter of the M dwarf mass-radius relationship

2018
M dwarfsare prime targets in the hunt for habitable worlds around other stars. This is due to their abundance as well as their small radii and low massesand temperatures, which facilitate the detection of temperate, rocky planets in orbit around them. However, the fundamental properties of M dwarfsare difficult to constrain, often limiting our ability to characterize the planets they host. Here we test several theoretical relationships for M dwarfsby measuring 23 high-precision, model-independent masses and radii for M dwarfsin binaries with white dwarfs. We find a large scatter in the radii of these low-massstars, with 25 per cent having radii consistent with theoretical models while the rest are up to 12 per cent overinflated. This scatter is seen in both partially and fully convective M dwarfs. No clear trend is seen between the overinflation and age or metallicity, but there are indications that the radii of slowly rotating M dwarfsare more consistent with predictions, albeit with a similar amount of scatter in the measurements compared to more rapidly rotating M dwarfs. The sample of M dwarfsin close binaries with white dwarfsappears indistinguishable from other M dwarfsamples, implying that common envelopeevolution has a negligible impact on their structure. We conclude that theoretical and empirical mass–radius relationships lack the precision and accuracy required to measure the fundamental parameters of M dwarfswell enough to determine the internal structure and bulk composition of the planets they host.
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