Age Determination in Upper Scorpius with Eclipsing Binaries

TL;DR

This study uses eclipsing binaries in Upper Scorpius to precisely determine stellar ages and test evolutionary models, revealing a 5-7 Myr age and identifying a benchmark brown dwarf.

Contribution

It provides new measurements of stellar parameters in Upper Scorpius and evaluates stellar evolution models against empirical data, highlighting discrepancies.

Findings

Age of 5-7 Myr based on binary data

Systematic underestimation of low-mass star masses in models

Discovery of a transiting brown dwarf at 97.8 days period

Abstract

The Upper Scorpius OB association is the nearest region of recent massive star formation and thus an important benchmark for investigations concerning stellar evolution and planet formation timescales. We present nine EBs in Upper Scorpius, three of which are newly reported here and all of which were discovered from K2 photometry. Joint fitting of the eclipse photometry and radial velocities from newly acquired Keck-I/HIRES spectra yields precise masses and radii for those systems that are spectroscopically double-lined. The binary orbital periods in our sample range from 0.6-100 days, with total masses ranging from 0.2-8 $M_\odot$. At least 33% of the EBs reside in hierarchical multiples, including two triples and one quadruple. We use these EBs to develop an empirical mass-radius relation for pre-main-sequence stars, and to evaluate the predictions of widely-used stellar evolutionary models. We report evidence for an age of 5-7 Myr which is self-consistent in the mass range of 0.3-5 $M_\odot$ and based on the fundamentally-determined masses and radii of eclipsing binaries (EBs). Evolutionary models including the effects of magnetic fields imply an age of 9-10 Myr. Our results are consistent with previous studies that indicate many models systematically underestimate the masses of low-mass stars by 20-60% based on H-R diagram analyses. We also consider the dynamical states of several binaries and compare with expectations from tidal dissipation theories. Finally, we identify RIK 72 b as a long-period transiting brown dwarf ($M = 59.2 \pm 6.8\ M_\mathrm{Jup}$, $R = 3.10 \pm 0.31\ R_\mathrm{Jup}$, $P \approx 97.8$ days) and an ideal benchmark for brown dwarf cooling models at 5-10 Myr.