A new take on the low-mass brown dwarf companions on wide orbits in Upper-Scorpius^★

¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: simon.petrus@univ-grenoble-alpes.fr
² Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU UMI 3386 and Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
³ Instituto de Física y Astronomía, Universidad de Valparaíso, Valparaíso, Chile
⁴ Núcleo Milenio de Formación Planetaria (NPF), Valparaiso, Chile
⁵ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
⁶ Institute for Astronomy, The University of Edinburgh Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁷ W. M. Keck Observatory 65-1120 Mamalahoa Hwy. Kamuela, HI 96743, USA
⁸ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
⁹ School of Earth & Space Exploration, Arizona State University, Tempe AZ 85287, USA
¹⁰ Ural Federal University, Yekaterinburg 620002, Russia

Received: 18 April 2019
Accepted: 27 September 2019

Abstract

Context. The Upper-Scorpius association (5–11 Myr) contains a unique population of low-mass (M ≤ 30M_Jup) brown dwarfs either free-floating, forming wide pairs, or on wide orbits around solar-type and massive stars. The detailed relative characterization of their physical properties (mass, radius, temperature, composition, and ongoing accretion) offers the opportunity to potentially explore their origin and mechanisms of formation.

Aims. In this study, we aim to characterize the chemical and physical properties of three young, late-M brown dwarfs claimed to be companions of the Upper-Scorpius stars USco 161031.9-16191305, HIP 77900, and HIP 78530 using medium-resolution spectroscopy at UV (0.30−0.56μm; R_λ ~ 3300), optical (0.55−1.02μm; R_λ ~ 5400), and NIR (1.02−2.48μm; R_λ ~ 4300) wavelengths. The spectra of six free-floating analogs from the same association are analyzed for comparison and to explore the potential physical differences between these substellar objects found in different configurations. We also aim to examine and analyze hydrogen emission lines at UV and optical wavelengths to investigate the presence of ongoing accretion processes.

Methods. The X-shooter spectrograph at VLT was used to obtain the spectra of the nine young brown dwarfs over the 0.3−2.5μm range simultaneously. Performing a forward modeling of the observed spectra with the ForMoSA code, we infer the T_eff, log (g), and radius of our objects. The code compares here the BT-SETTL15 models to the observed spectra using the Nested Sampling Bayesian inference method. Mass is determined using evolutionary models, and a new analysis of the physical association is presented based on Gaia-DR2 astrometry.

Results. The T_eff and log (g) determined for our companions are compatible with those found for free-floating analogs of the Upper-Scorpius association and with evolutionary model predictions at the age of the association. However the final accuracy on the T_eff estimates is strongly limited by nonreproducibility of the BT-SETTL15 models in the range of T_eff corresponding to the M8–M9 spectral types. We identified H_α, H_β, H_γ, and Ca II H and K emission lines in the spectrum of several objects. We attribute these lines to chromospheric activity except for the free-floating object USco 1608-2315 for which they are indicative of active accretion (M^˙ ≤ 10^−10.76 M_⊙ yr⁻¹). We confirm the four-fold over-luminosity of USco 161031.9-16191305 B down to 0.3 μm, which could be explained in part by the activity of this object and if the companion is an unresolved multiple system.