Discovery of the pre-main-sequence eclipsing binary MML 48

¹ Universidad Nacional Autónoma de México, Instituto de Astronomía, Ciudad Universitaria, 04510 Ciudad de México, Mexico
² Department of Physics, Hobart and William Smith Colleges, Geneva, New York 14456, USA
³ Department of Physics & Astronomy, Uppsala University, Box 516 SE-75120 Uppsala, Sweden
⁴ Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA
⁵ ASTRAVEO LLC, PO Box 1668 Gloucester, MA 01931, USA
⁶ Applied Materials Inc., 35 Dory Road, Gloucester, MA 01930, USA
⁷ Department of Physics & Astronomy, University of North Georgia, 82 College Circle, Dahlonega, GA 30597, USA
⁸ Universidad Nacional de Córdoba, Observatorio Astronómico de Córdoba, Laprida 854, X5000BGR Córdoba, Argentina
⁹ Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Godoy Cruz 2290, CABA CPC 1425FQB, Argentina
¹⁰ Las Cumbres Observatory Global Telescope Network, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA
¹¹ Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK
¹² École Normale Supérieure, CRAL (UMR CNRS 5574), Université de Lyon, Lyon, France
¹³ W. M. Keck Observatory, Kamuela, HI 96743, USA
¹⁴ Institute for Space, Space Park Leicester, 92 Corporation Rd, Leicester LE4 5SP, UK
¹⁵ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
¹⁶ Department of Physics and Astronomy, Vanderbilt University, Nashville TN 37235, USA

^⋆ Corresponding author: ygmc@astro.unam.mx

Received: 21 May 2025
Accepted: 16 July 2025

Abstract

Aims. We present the discovery of the eclipsing binary MML 48, which is a member of Upper Centaurus Lupus, has an associated age of 16 Myr, and is composed of two young, low-mass stars.

Methods. We used space- and ground-based observations to characterize the system with both time-series photometry and spectroscopy. Given the extreme mass ratio between the stars, q_EB = 0.209 ± 0.014, we modeled a single-lined spectroscopic and eclipsing binary system.

Results. The orbital period, 2.0171068 ± 0.0000004 d, is measured from the highest precision light curves. We derive a primary mass of 1.2 ± 0.07 M_⊙ using stellar models, and with radial velocities we measured a secondary mass of 0.2509 ± 0.0078 M_⊙. The radii are large, as expected for pre-main-sequence stars, and are measured as 1.574 ± 0.026 ± 0.050 R_⊙ and 0.587 ± 0.0095 ± 0.050 R_⊙, for the primary and secondary stars, respectively.

Conclusions. MML 48 joins the short list of known low-mass, pre-main-sequence eclipsing binaries (EBs), being one of only five systems with intermediate ages (15−25 Myr), and the system with the most extreme mass ratio. The primary star is currently at the “fusion bump”, undergoing an over-production of energy in the core due to the build-up of ³He before reaching its equilibrium abundance set by the proton-proton (p-p) I chain. MML 48 A is the first young star in an eclipsing system that has been found during its fusion bump. MML 48 is thus an important benchmark for low-mass stellar evolution at a time when the stars are rapidly changing, which allows for a tight constraint on the corresponding isochrone given the uneven mass ratio.