Exploring the M-dwarf Luminosity--Temperature--Radius Relationships using Gaia DR2

There has been growing evidence in the literature that M-dwarf stars suffer radius inflation compared to theoretical models, suggesting that models are missing some key physics required to completely describe stars at effective temperatures less than about 4000K. We presented evidence at the previous Cool Stars meeting that this problem is evident in pre-main sequence populations within stellar clusters. With the advent of Gaia DR2, we have been able to generalise our novel SED fitting methodology, which relies only upon multiband photometry and geometric distances, to measure the radii of >15,000 nearby main-sequence field M-dwarf stars, and show that radius inflation persists onto the main sequence. From this sample, we have determined that M-dwarfs show an inflation of 3 - 7 per cent from the purely theoretical models, with no more than 1 - 2 per cent intrinsic spread in the inflated sequence. We show that this measurement technique is able to measure M-dwarf radii to an accuracy of 2.4 per cent, however we determined that this is limited by the precision of metallicity measurements; which contribute 1.7 per cent to measurement uncertainty. We also present evidence that stellar magnetism is currently unable to explain the radius inflation in M-dwarfs.