A new free-floating planet in the Upper Scorpius association

¹ Instituto de Astrofísica. Pontificia Universidad Católica de Chile (IA-PUC), 7820436 Santiago, Chile
e-mail: kpena@astro.puc.cl
² Millennium Institute of Astrophysics, Casilla 360, Santiago, Chile
³ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁴ Universidad de La Laguna, 38207 Tenerife, Spain
⁵ Centro de Astrobiología (CSIC-INTA), 28850 Torrejón de Ardoz, Madrid, Spain

Received: 22 September 2015
Accepted: 17 November 2015

Abstract

We report on a deep photometric survey covering an area of 1.17 deg² in the young Upper Scorpius stellar association using VIMOS Iz and UKIDSS ZJHK data that was taken with several years in between. The search for the least massive population of Upper Scorpius (~5–10 Myr, 145 pc) is performed on the basis of various optical and infrared color−color and color−magnitude diagrams, including WISE photometry, in the magnitude interval J = 14.5−19 mag (completeness), which corresponds to substellar masses from 0.028 through 0.004  M_⊙ at the age and distance of Upper Scorpius. We also present the proper motion analysis of the photometric candidates, finding that two objects successfully pass all photometric and astrometric criteria for membership in the young stellar association. One of them, USco J155150.2−213457, is a new discovery. We obtained low resolution, near-infrared spectroscopy (R ~ 450, 0.85–2.35 μm) of this new finding using the FIRE instrument. We confirmed its low-gravity atmosphere expected for an Upper Scorpius member (weak alkaline lines, strong VO absorption, peaked H-band pseudocontinuum). By comparison with spectroscopic standards, we derive a spectral type of L6 ± 1, and estimate a mass of ≈0.008–0.010  M_⊙ for USco J155150.2−213457. The colors and spectral slope of this object resemble those of other young, cool members of Upper Scorpius and σ Orionis (~3 Myr) and field, high gravity dwarfs of related classification in contrast with the very red indices of field, low gravity, L-type dwarfs of intermediate age. USco J155150.2−213457, which does not show infrared flux excesses up to 4.5 μm, becomes one of the least massive and latest type objects known in the entire Upper Scorpius stellar association.