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

TL;DR

This study reports the discovery and detailed characterization of three low-mass eclipsing binaries in Upper Scorpius, including the first potential eclipsing brown dwarf binary, providing precise stellar parameters to test pre-main-sequence models.

Contribution

It presents the first detailed measurements of fundamental parameters for the lowest mass stars and brown dwarfs in eclipsing binaries, advancing understanding of stellar evolution in young associations.

Findings

Precise masses and radii for low-mass stars and brown dwarfs.

Identification of a potential eclipsing brown dwarf binary.

Revised parameters for known low-mass binaries.

Abstract

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 $\sim$ 78 days. The orbital periods of all three systems are $<$5 days. We use the $K2$ photometry plus multiple Keck/HIRES radial velocities 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 $\sim$25 $M_\mathrm{Jup}$ 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 ($M_1=0.1183\pm0.0028 M_\odot$, $M_2=0.1076\pm0.0031 M_\odot$ and $R_1=0.417\pm0.010 R_\odot$, $R_2=0.450\pm0.012 R_\odot$) from combination of the light curve and radial velocity time series. These stars have the lowest masses of any stellar mass double-lined eclipsing binary to date. Finally, EPIC 203476597 is a compact single-lined system with a G8-KO primary and a likely mid-K secondary whose line are revealed in spectral ratios. Continued measurement of radial velocities 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 ($M_1=0.3336\pm0.0022 M_\odot$, $M_2=0.3200\pm0.0022 M_\odot$ and $R_1=0.862\pm0.012$, $R_2=0.852\pm0.013 R_\odot$). We discuss the implications of our results on these $\sim$0.1-1.5 $M_\odot$ stars for pre-main-sequence evolutionary models.