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

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 ∼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 M 0.1183 0.0028from 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 M 0.3336 0.0022, 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.

New Low-mass Eclipsing Binary Systems in Praesepe Discovered by K2

“…Accordingly, in Figure 20 we show the PHOENIX v2 model fits to the observed broadband magnitudes of ADs 3814, 2615, and 1508 reported in Table 1 (for AD 3116, we show the BT-SETTL fit as the PHOENIX models do not extend to low enough temperatures to explain the secondary brown dwarf component). The T eff and distance values derived from our SED-fitting procedure with both the BT-SETTL and PHOENIX v2 models are reported in Table 5 along with the empirical relation predictions of Mann et al (2015) and David et al (2016b). We discuss the effective temperature and distance estimates in the following two sections.…”

A Continuum of Accretion Burst Behavior in Young Stars Observed by K2

“…They may thus represent "normal" discrete accretion variations and bursts, in contrast to the FU Ori and EX Lup outbursts described above. The NASA K2 mission Campaign 2 observations included the young ρ Ophiuchus molecular cloud region at <1-2 Myr, and the adjacent Upper Scorpius OB association, which is debated from analysis of HR diagrams to be either ∼3-5 Myr based on the low mass stellar population (e.g., Preibisch et al 2002;Herczeg & Hillenbrand 2015) or ∼11 Myr based on the solar and super-solar mass population (Pecaut et al 2012); the latter age is beginning to be favored by results on eclipsing binaries David et al 2016) and asteroseismology (Ripepi et al 2015). By sampling stars with ages comparable to and extending to much older than NGC 2264 (in ρ Oph and Upper Sco, respectively), the K2 time series data can be used to compare accretion burst behavior as a function of age and therefore presumably disk properties which are expected to evolve with time.…”

An M Dwarf Companion to an F-Type Star in a Young Main-Sequence Binary

“…For these stars, stellar models also show systematic discrepancies in the observed mass-radius relations, but on a larger scale. Over 30 eclipsing very low-mass stars (VLMSs) with masses below 0.3 M e and radii known to better than 10% have been observed so far (e.g., Parsons et al 2012;Pyrzas et al 2012;Nefs et al 2013;Gómez Maqueo Chew et al 2014;Zhou et al 2014;Kraus et al 2015;David et al 2016). However, only eight have radii known to a precision better than 2%.…”