We present the discovery that UScoCTIO 5 , a known spectroscopic binary in the Upper Scorpius star-forming region ( P = 34 days , M _ { tot } \sin ( i ) = 0.64 M _ { \odot } ) , is an eclipsing system with both primary and secondary eclipses apparent in K2 light curves obtained during Campaign 2 .
We have simultaneously fit the eclipse profiles from the K2 light curves and the existing RV data to demonstrate that UScoCTIO 5 consists of a pair of nearly identical M4.5 stars with M _ { A } = 0.329 \pm 0.002 M _ { \odot } , R _ { A } = 0.834 \pm 0.006 R _ { \odot } , M _ { B } = 0.317 \pm 0.002 M _ { \odot } , and R _ { B } = 0.810 \pm 0.006 R _ { \odot } .
The radii are broadly consistent with pre-main sequence ages predicted by stellar evolutionary models , but none agree to within the uncertainties .
All models predict systematically incorrect masses at the 25–50 % level for the HR diagram position of these mid-M dwarfs , suggesting significant modifications to mass-dependent outcomes of star and planet formation .
The form of the discrepancy for most model sets is not that they predict luminosities that are too low , but rather that they predict temperatures that are too high , suggesting that the models do not fully encompass the physics of energy transport ( via convection and/or missing opacities ) and/or a miscalibration of the SpT- T _ { eff } scale .
The simplest modification to the models ( changing T _ { eff } to match observations ) would yield an older age for this system , in line with the recently proposed older age of Upper Scorpius ( \tau \sim 11 Myr ) .