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

• 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 (0.30-0.56 mu m; R-lambda similar to 3300), optical (0.55-1.02 mu m; R-lambda similar to 5400), and NIR (1.02-2.48 mu m; R-lambda similar to 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 0.3-2.5 mu m 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 alpha, H-beta, H-gamma, 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(circle dot)yr(-1)). We confirm the four-fold over-luminosity of USco 161031.9-16191305 B down to 0.3 mu m, which could be explained in part by the activity of this object and if the companion is an unresolved multiple system.

Ämnesord

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Nyckelord

brown dwarfs

stars: atmospheres

stars: fundamental parameters

stars: luminosity function

mass function

planetary systems

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