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

Abstract

Context. The Upper-Scorpius association (5-11 Myr) contains a unique population of low-mass (M ≤ 30 M Jup ) brown-dwarfs either free-floating, forming wide pairs, or on wide-orbits to solar-type and massive stars. The detailed relative characterization of their physical properties (mass, radius, temperature, composition, ongoing accretion) offers the opportunity to potentially explore their origin and their mechanisms of formation. Aims. In this study, we aim at characterizing 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 µm; R λ ∼ 3300), optical (0.55 − 1.02 µm; R λ ∼ 5400), and near-infrared (1.02 − 2.48 µm; R λ ∼ 4300) wavelengths. The spectra of six free-floating analogues 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 at looking and analyzing 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 µm range simultaneously. Performing a forward modelling 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 in using evolutionary models and a new analysis of the physical association is proposed in using the Gaia-DR2 astrometry.Results. The T eff and log(g) determined for our companions are compatible with those found for free floating analogues 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 non-reproducibilities 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 & 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 (Ṁ ≤ 10 −10.76 M /year). We confirm the × 4 over-luminosity of USco 161031.9-16191305 B down to 0.3 µm. It could be explained in part by the object activity and if the companion is an unresolved multiple system.

Strong Hα emission and signs of accretion in a circumbinary planetary mass companion from MUSE

“…Hα emission thought to be originating from chromospheric activity has been observed in a number of BDs. Recent examples include the more massive BD companion PZ TEL B (Musso Barcucci et al 2019) and a survey of Sco-Cen BDs by Petrus et al (2020), but typically with lower levels of emission than we find for Delorme 1 (AB)b. Manara et al (2013) obtained an analytical relation from observations of a number of young (<10 Myr) K7 -M9.5 stars and BDs for the purpose of estimating the chromospheric activity contribution as an accretion-luminosity noise.…”

“…In analogy to this process in young stars, young brown dwarfs (BDs) and planetary mass objects have been increasingly examined for accretion signatures (e.g. Bowler et al 2011;Joergens et al 2013;Zhou et al 2014;Manara et al 2015;Petrus et al 2020). With the advent of increasingly sophisticated adaptive optics (AO) systems, such studies can now be performed even for BDs and planetary companions at close separations from their parent stars (e.g.…”

Medium-resolution spectrum of the exoplanet HIP 65426 b

“…Our TExTRIS python analysis toolkit for IFS data was able to retrieve the star position outside the field of view and to optimize the data processing, which allowed us to extract the planet emission spectrum. We interpreted this spectrum following a Bayesian inference with self-consistent atmospheric forward models with the ForMoSA code (Petrus et al 2020) and compared the inferred atmospheric properties of the planet to those obtained from the recent molecular mapping technique based on cross correlation between the IFS data elements and grid of models. We summarize the main results as follows:…”

“…We used the ForMoSA code presented in Petrus et al (2020) to compare the synthetic spectra to the data following the forwardmodeling approach. ForMoSA relies on the nested sampling algorithm (Skilling 2006) to determine the posterior distribution function of a set of free parameters in the models.…”

“…the solutions in each posterior, they do not include possible systematical errors in models, which are difficult to quantify (see Petrus et al 2020). Whenever the posterior distribution did not show a maximum (e.g., C/O ratio), we defined an upper limit on the given parameter encompassing 68% of the posterior distribution starting from the posterior maximum.…”