The optical+infrared L dwarf spectral sequence of young planetary-mass objects in the Upper Scorpius association

Abstract

We present the results of photometric and spectroscopic follow-ups of the lowest mass member candidates in the nearest OB association, Upper Scorpius (5-10 Myr; 145+/-17 pc), with the Gran Telescopio de Canarias (GTC) and European Southern Observatory (ESO) Very Large Telescope (VLT). We confirm the membership of the large majority (>80%) of the candidates selected originally photometrically and astrometrically based on their spectroscopic features, weak equivalent widths of gravity-sensitive doublets, and radial velocities. Confirmed members follow a sequence over a wide magnitude range (J=17.0-19.3 mag) in several colour-magnitude diagrams with optical, near-, and mid-infrared photometry, and have near-infrared spectral types in the L1-L7 interval with likely masses below 15 Jupiter masses. We find that optical spectral types tend to be earlier than near-infrared spectral types by a few subclasses for spectral types later than M9. We investigate the behaviour of spectral indices defined in the literature as a function of spectral type and gravity by comparison with values reported in the literature for young and old dwarfs. We also derive effective temperatures in the 1900-1600K from fits of synthetic model-atmosphere spectra to the observed photometry but we caution the procedure carries large uncertainties. We determine bolometric corrections for young L dwarfs with ages of ~5-10 Myr (Upper Sco association) and find them similar in the J-band but larger by 0.1-0.4 mag in the K-band with respect to field L dwarfs. Finally, we discovered two faint young L dwarfs, VISTAJ1607-2146 (L4.5) and VISTAJ1611-2215 (L5) that have H$\alpha$ emission and possible flux excesses at 4.5 microns, pointing towards the presence of accretion from a disk onto the central objects of mass below ~15 Jupiter masses at the age of 5-10 Myr.

40 pages, 10 Tables, 22 figures, accepted for publication to MNRAS, pending changes by Journal before appearance

References in corpus (44)

• The UKIRT Infrared Deep Sky Survey (UKIDSS)
• New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit
• Molecfit: A general tool for telluric absorption correction. I. Method and application to ESO instruments
• Constraining the Age-Activity Relation for Cool Stars: The SDSS DR5 Low-Mass Star Spectroscopic Sample
• VOSA: Virtual Observatory SED Analyzer. An application to the Collinder 69 open cluster
• Molecfit: A general tool for telluric absorption correction II. Quantitative evaluation on ESO-VLT X-Shooter spectra
• The WFCAM Science Archive
• Stellar SEDs from 0.3-2.5 Microns: Tracing the Stellar Locus and Searching for Color Outliers in SDSS and 2MASS
• The UKIRT Wide Field Camera ZYJHK Photometric System: Calibration from 2MASS
• Pulkovo Compilation of Radial Velocities for 35495 Hipparcos Stars in a Common System
• Young L Dwarfs Identified in the Field: A Preliminary Low-Gravity, Optical Spectral Sequence from L0 to L5
• Exploring Exoplanet Populations with NASA's Kepler Mission
• The Hawaii Infrared Parallax Program. II. Young Ultracool Field Dwarfs
• Direct Imaging and Spectroscopy of a Planetary Mass Candidate Companion to a Young Solar Analog
• Characterizing Young Brown Dwarfs using Low Resolution Near-IR Spectra
• Mapping the Shores of the Brown Dwarf Desert I.: Upper Scorpius
• Discovery of a young planetary mass companion to the nearby M dwarf VHS J125601.92-125723.9
• A Large-Area Search for Low Mass Objects in Upper Scorpius I: The Photometric Campaign and New Brown Dwarfs
• A Large-Area Search for Low-Mass Objects in Upper Scorpius II: Age and Mass Distributions
• Advances in exoplanet science from Kepler
• A Surprising Reversal of Temperatures in the Brown-Dwarf Eclipsing Binary 2MASS J05352184-0546085
• Discovery of a wide companion near the deuterium burning mass limit in the Upper Scorpius association
• New brown dwarfs in Upper Sco using UKIDSS Galactic Cluster Survey science verification data
• Brown dwarfs and very low mass stars in the Hyades cluster : a dynamically evolved mass function
• A Survey for New Members of the Taurus Star-Forming Region with the Sloan Digital Sky Survey
• Measuring tiny mass accretion rates onto young brown dwarfs
• The NIRSPEC Brown Dwarf Spectroscopic Survey II: High-Resolution J-Band Spectra of M, L and T Dwarfs
• Disks around Brown Dwarfs in the Sigma Orionis Cluster
• Substellar Objects in Nearby Young Clusters (SONYC) VI: The planetary-mass domain of NGC1333
• Spectroscopy of brown dwarf candidates in IC 348 and the determination of its substellar IMF down to planetary masses
• Infrared Spectroscopy and Analysis of Brown Dwarf and Planetary Mass Objects in the Orion Nebula Cluster
• Substellar Objects in Nearby Young Clusters (SONYC): The bottom of the Initial Mass Function in NGC1333
• SPITZER: Accretion in Low Mass Stars and Brown Dwarfs in the Lambda Orionis Cluster
• Self-consistent evolution of accreting low-mass stars and brown dwarfs
• Trigonometric parallaxes of young field L dwarfs
• Proper motions of field L and T dwarfs
• New members in the Upper Sco association from the UKIDSS Early Data Release
• Oph 1622-2405: Not a Planetary-Mass Binary
• New constraints on the membership of the T dwarf S Ori 70 in the sigma Orionis cluster
• Substellar Objects in Nearby Young Clusters (SONYC) VIII: Substellar population in Lupus 3
• Near Infrared Spectroscopy of Young Brown Dwarfs in Upper Scorpius
• Age spreads and the temperature dependence of age estimates in Upper Sco
• Spectral energy distribution simulations of a possible ring structure around the young, red brown dwarf G196-3B
• Characterization of the known T type dwarfs towards the Sigma Orionis cluster

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• Refining the Census of the Upper Scorpius Association with Gaia
• A rich population of free-floating planets in the Upper Scorpius young stellar association
• A Survey for New Members of Taurus from Stellar to Planetary Masses
• A medium-resolution spectrum of the exoplanet HIP 65426 b
• The B-Star Exoplanet Abundance Study: a co-moving 16-25 Mjup companion to the young binary system HIP 79098
• A new take on the low-mass brown dwarf companions on wide-orbits in Upper-Scorpius
• EMIR, the near-infrared camera and multi-object spectrograph for the GTC
• A Novel Survey for Young Substellar Objects with the W-band Filter. I. Filter Design and New Discoveries in Ophiuchus and Perseus
• The origin of free-floating planets
• Spectral library of age-benchmark low-mass stars and brown dwarfs
• Improved infrared photometry and a preliminary parallax measurement for the extremely cold brown dwarf CWISEP J144606.62$-$231717.8
• Exploring the planetary-mass population in the Upper Scorpius association
• USco1621 B and USco1556 B: Two wide companions at the deuterium-burning mass limit in Upper Scorpius
• Disks around young planetary-mass objects: Ultradeep Spitzer imaging of NGC1333
• Measurement of the angular momenta of pre-main-sequence stars: early evolution of slow and fast rotators and empirical constraints on spin-down torque mechanisms
• New Candidates for Planetary-mass Brown Dwarfs in IC 348
• ALMA observations of the early stages of substellar formation in the Lupus 1 and 3 molecular clouds
• Strong H$\alpha$ emission in the young planetary mass companion 2MASS J0249-0557 c
• Search for the sub-stellar lithium depletion boundary in the open star cluster Coma Berenices
• Fast Photometric Variability of Very Low Mass Stars in IC 348: Detection of Superflare in an M-dwarf
• Low-mass and sub-stellar eclipsing binaries in stellar clusters
• Two close binaries across the hydrogen-burning limit in the Praesepe open cluster