Optical and Near-infrared Spectra of σ Orionis Isolated Planetary-mass Objects

• Published in: The Astrophysical journal Vol. 842; no. 1; p. 65
• Main Authors: Zapatero Osorio, M. R., Béjar, V. J. S., Ramírez, K. Peña
• Format: Journal Article
• Language: English
• Published: Philadelphia IOP Publishing 10.06.2017
• Subjects: Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPARATIVE EVALUATIONS; Confidence intervals; DWARF STARS; EXTRAPOLATION; Extrasolar planets; GRAVITATION; Infrared spectra; Low mass stars; LUMINOSITY; MASS; METALLICITY; NEAR INFRARED RADIATION; Planets; RESOLUTION; Sigma Orionis; Solar neighborhood; SPECTRA; STAR CLUSTERS; STARS; SURFACES; VISIBLE RADIATION
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa70ec

Abstract We have obtained low-resolution optical (0.7–0.98 μ m) and near-infrared (1.11–1.34 μ m and 0.8–2.5 μ m) spectra of 12 isolated planetary-mass candidates ( J  = 18.2–19.9 mag) of the 3 Myr σ Orionis star cluster with the aim of determining the spectroscopic properties of very young, substellar dwarfs and assembling a complete cluster mass function. We have classified our targets by visual comparison with high- and low-gravity standards and by measuring newly defined spectroscopic indices. We derived L0–L4.5 and M9–L2.5 using high- and low-gravity standards, respectively. Our targets reveal clear signposts of youth, thus corroborating their cluster membership and planetary masses (6–13 M Jup ). These observations complete the σ Orionis mass function by spectroscopically confirming the planetary-mass domain to a confidence level of ∼75%. The comparison of our spectra with BT-Settl solar metallicity model atmospheres yields a temperature scale of 2350–1800 K and a low surface gravity of log g  ≈ 4.0 [cm s −2 ], as would be expected for young planetary-mass objects. We discuss the properties of the cluster’s least-massive population as a function of spectral type. We have also obtained the first optical spectrum of S Ori 70, a T dwarf in the direction of σ Orionis. Our data provide reference optical and near-infrared spectra of very young L dwarfs and a mass function that may be used as templates for future studies of low-mass substellar objects and exoplanets. The extrapolation of the σ Orionis mass function to the solar neighborhood may indicate that isolated planetary-mass objects with temperatures of ∼200–300 K and masses in the interval 6–13 M Jup may be as numerous as very low-mass stars.