K2 Discovery of Young Eclipsing Binaries in Upper Scorpius: Direct Mass and Radius Determinations for the Lowest Mass Stars and Initial Characterization of an Eclipsing Brown Dwarf Binary

We report the discovery of three low-mass double-lined eclipsing binaries in the pre-main sequence Upper Scorpius association, revealed by K2 photometric monitoring of the region over ∼78 days. The orbital periods of all three systems are <5 days. We use the K2 photometry plus multiple Keck/HIRES radial velocities (RVs) and spectroscopic flux ratios to determine fundamental stellar parameters for both the primary and secondary components of each system, along with the orbital parameters. We present tentative evidence that EPIC 203868608 is a hierarchical triple system comprised of an eclipsing pair of ∼25 M_Jup brown dwarfs with a wide M-type companion. If confirmed, it would constitute only the second double-lined eclipsing brown dwarf binary system discovered to date. The double-lined system EPIC 203710387 is composed of nearly identical M4.5-M5 stars with fundamentally determined masses and radii measured to better than 3% precision ({M}₁=0.1183+/- 0.0028{M}_⊙ , {M}₂=0.1076+/- 0.0031{M}_⊙ and {R}₁=0.417+/- 0.010{R}_⊙ , {R}₂=0.450+/- 0.012{R}_⊙ ) from combination of the light curve and RV time series. These stars have the lowest masses of any stellar mass double-lined eclipsing binary to date. Comparing our derived stellar parameters with evolutionary models, we suggest an age of ∼10-11 Myr for this system, in contrast to the canonical age of 3-5 Myr for the association. Finally, EPIC 203476597 is a compact single-lined system with a G8-K0 primary and a likely mid-K secondary whose lines are revealed in spectral ratios. Continued measurement of RVs and spectroscopic flux ratios will better constrain fundamental parameters and should elevate the objects to benchmark status. We also present revised parameters for the double-lined eclipsing binary UScoCTIO 5 ({M}₁=0.3336+/- 0.0022{M}_⊙ , {M}₂=0.3200+/- 0.0022{M}_⊙ and {R}₁=0.862+/- 0.012, {R}₂=0.852+/- 0.013{R}_⊙ ), which are suggestive of a system age younger than previously reported. We discuss the implications of our results on these ∼0.1-1.5 {M}_⊙ stars for pre-main-sequence evolutionary models.