USco1621 B and USco1556 B: Two wide companions at the deuterium-burning mass limit in Upper Scorpius

¹ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
e-mail: pcg@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
³ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁴ Janusz Gil Institute of Astronomy, University of Zielona Góra, Lubuska 2, 65-265 Zielona Góra, Poland
⁵ Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain
⁷ Departamento de Física Aplicada, Universidad Politécnica de Cartagena, Campus Muralla del Mar, 30202 Cartagena, Murcia, Spain
⁸ W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, HI 96743, USA

Received: 18 June 2019
Accepted: 29 November 2019

Abstract

Aims. Our objective is to identify analogues of gas giant planets, but located as companions at wide separations of very young stars. The main purpose is to characterise the binarity frequency and the properties of these substellar objects, and to elucidate their early evolutionary stages.

Methods. To identify these objects, we cross correlated the Visible and Infrared Survey Telescope for Astronomy Hemisphere Survey and the United Kingdom Infrared Telescope Infrared Deep Sky Survey Galactic Clusters Survey catalogues to search for common proper motion companions to 1195 already known members of Upper Scorpius (USco; age ~5–10 Myr, distance ~145 pc). We present the discovery and spectroscopic characterisation of two very wide substellar companions of two early-M stars in Upper Scorpius: USco1621 B and USco1556 B. We obtained optical and near-infrared low-resolution spectroscopy of the candidates to characterise their spectral energy distribution and confirm their youth and membership to the association. We also acquired adaptive optics images of the primaries and secondaries to search for signs of binarity and close companions.

Results. By comparison with field dwarfs and other young members of USco, we determined a spectral type of M8.5 in the optical for both companions, along with L0 and L0.5 in the near-infrared for USco1621 B and USco1556 B, respectively. The spectra of the two companions show evident markers of youth, such as weak alkaline Na I and K I lines, along with the triangular shape of the H-band. The comparison with theoretical evolutionary models gives estimated masses of 0.015 ± 0.002 and 0.014 ± 0.002 M_⊙, with temperatures of 2270 ± 90 and 2240 ± 100 K, respectively. The physical separations between the components of both systems are 2880 ± 20 and 3500 ± 40 AU for USco1621 and USco1556 systems, respectively. We did not find any additional close companion in the adaptive optics images. The probability that the two secondaries are physically bound to their respective primaries, and not chance alignments of USco members, is 86%, and the probability that none of them are physically related is 1.0%.