Fundamental properties of the pre-main sequence eclipsing stars of MML 53 and the mass of the tertiary^⋆

¹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Ciudad de México, Mexico
e-mail: ygmc@astro.unam.mx
² Department of Physics, Hobart and William Smith Colleges, Geneva, New York 14456, USA
³ Department of Physics & Astronomy, Uppsala University, Box 516 75120 Uppsala, Sweden
⁴ National Optical Astronomy Observatory, 950 N. Cherry Ave, Tucson, Arizona 85719, USA
⁵ Department of Physics, Vanderbilt University, Nashville, TN, USA
⁶ Department of Physics, Fisk University, Nashville, TN, USA
⁷ INAF-Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁸ University of Warwick, Department of Physics, Gibbet Hill Road, Coventry CV4 7AL, UK
⁹ Las Cumbres Observatory Global Telescope Network, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA
¹⁰ Physics Department, State University of New York at Cortland, Cortland, New York 13045, USA
¹¹ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK

Received: 24 April 2018
Accepted: 20 December 2018

Abstract

We present the most comprehensive analysis to date of the Upper Centaurus Lupus eclipsing binary MML 53 (with P_EB = 2.097892 d), and for the first time, confirm the bound-nature of the third star (in a P₃ ∼ 9 yr orbit) by constraining its mass dynamically. Our analysis is based on new and archival spectra and time-series photometry, spanning 80% of one orbit of the outer component. From the spectroscopic analysis, we determined the temperature of the primary star to be 4880 ± 100 K. The study of the close binary incorporated treatment of spots and dilution by the tertiary in the light curves, allowing for the robust measurement of the masses of the eclipsing components within 1% (M₁ = 1.0400 ± 0.0067 M_⊙ and M₂ = 0.8907 ± 0.0058 M_⊙), their radii within 4.5% (R₁ = 1.283 ± 0.043 R_⊙ and R₂ = 1.107 ± 0.049 R_⊙), and the temperature of the secondary star (T_eff, 2 = 4379 ± 100 K). From the analysis of the eclipse timings, and the change in systemic velocity of the eclipsing binary and the radial velocities of the third star, we measured the mass of the outer companion to be 0.7 M_⊙ (with a 20% uncertainty). The age we derived from the evolution of the temperature ratio between the eclipsing components is fully consistent with previous, independent estimates of the age of Upper Centaurus Lupus (16 ± 2 Myr). At this age, the tightening of the MML 53 eclipsing binary has already occurred, thus supporting close-binary formation mechanisms that act early in the stars’ evolution. The eclipsing components of MML 53 roughly follow the same theoretical isochrone, but appear to be inflated in radius (by 20% for the primary and 10% for the secondary) with respect to recent evolutionary models. However, our radius measurement of the 1.04 M_⊙ primary star of MML 53 is in full agreement with the independent measurement of the secondary of NP Per which has the same mass and a similar age. The eclipsing stars of MML 53 are found to be larger but not cooler than predicted by non-magnetic models, it is not clear what is the mechanism that is causing the radius inflation given that activity, spots and/or magnetic fields slowing their contraction, require the inflated stars to be cooler to remain in thermal equilibrium.