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

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


Authors:

Lodieu et al

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 Hα 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.

Dippers and Dusty Disks Edges: A Unified Model

Dippers and Dusty Disks Edges: A Unified Model

Authors:

Bodman et al

Abstract:

A search for dips in observed stellar flux in the Upper Scorpius and ρ Ophiuchus star formation regions with the Kepler mission by Ansdell et al. primarily identified young, low mass stars (dippers) with low accretion rates and hosting moderately evolved dusty circumstellar disks. These young stars likely exhibit rotating star spots that cause quasi-periodic photometric variations. However, a separate period associated with the dips is not evident in spectrograms constructed from the light curves. The material causing the dips in most of these light curves must be approximately corotating with the star. We find that disk temperatures computed at the disk corotation radius are cool enough that dust should not sublimate. Dippers are preferentially associated with young, low mass stars as they have low enough luminosities to allow dust to survive within a few stellar radii. Crude estimates for stellar magnetic field strengths and accretion rates are consistent with magnetospheric truncation near the corotation radius. Magnetospheric truncation models can explain why dusty material is lifted out of the midplane to obscure the star causing the light curve dips and why so many young low mass stars are dippers. We propose that variations in disk orientation angle, stellar magnetic field dipole tilt axis, and disk accretion rate are underlying parameters accounting for differences in the dipper light curves.

ALMA Observations of Circumstellar Disks in the Upper Scorpius OB Association

ALMA Observations of Circumstellar Disks in the Upper Scorpius OB Association

Authors:

Barenfeld et al

Abstract:

We present ALMA observations of 106 G-, K-, and M-type stars in the Upper Scorpius OB Association hosting circumstellar disks. With these data, we measure the 0.88 mm continuum and 12CO J=3−2 line fluxes of disks around low mass (0.14−1.66 M⊙) stars at an age of 5-11 Myr. Of the 75 primordial disks in the sample, 53 are detected in the dust continuum and 26 in CO. Of the 31 disks classified as debris/evolved transitional disks, 5 are detected in the continuum and none in CO. The lack of CO emission in approximately half of the disks with detected continuum emission can be explained if CO is optically thick but has a compact emitting area (≲40 AU), or if the CO is heavily depleted by a factor of at least ∼1000 relative to interstellar medium abundances and is optically thin. The continuum measurements are used to estimate the dust mass of the disks. We find a correlation between disk dust mass and stellar host mass consistent with a power-law relation of Mdust∝M1.67±0.37∗. Disk dust masses in Upper Sco are compared to those measured in the younger Taurus star forming region to constrain the evolution of disk dust mass. We find that the difference in the mean of log(Mdust/M∗) between Taurus and Upper Sco is 0.64±0.09, such that Mdust/M∗ is lower in Upper Sco by a factor of ∼4.5.

EPIC 205117205b: a Neptune Class Exoplanet Orbiting a pre Main Sequence Star

Zodiacal Exoplanets in Time (ZEIT) III: A Neptune-sized planet orbiting a pre-main-sequence star in the Upper Scorpius OB Association

Authors:

Mann et al

Abstract:

We confirm and characterize a close-in (Porb = 5.425 days), super-Neptune sized (5.04+0.34−0.37 Earth radii) planet transiting EPIC 205117205 (2MASS J16101473-1919095), a late-type (M3) pre-main sequence (≃11 Myr-old) star in the Upper Scorpius subgroup of the Scorpius-Centaurus OB association. The host star has the kinematics of a member of the Upper Scorpius OB association, and its spectrum contains lithium absorption, an unambiguous sign of youth (less than 20 Myr) in late-type dwarfs. We combine photometry from K2 and the ground-based MEarth project to refine the planet's properties and constrain the average stellar density. We determine EPIC 205117205's bolometric flux and effective temperature from moderate resolution spectra. By utilizing isochrones that include the effects of magnetic fields, we derive a precise (6-7%) radius and mass for the host star, and a stellar age consistent with the established value for Upper Scorpius. Follow-up high-resolution imaging and Doppler spectroscopy confirm that the transiting object is not a stellar companion or a background eclipsing binary blended with the target. The shape of the transit, the constancy of the transit depth and periodicity over 1.5 years, and the independence with wavelength rules out stellar variability, or a dust cloud or debris disk partially occulting the star as the source of the signal; we conclude it must instead be planetary in origin. The existence of EPIC 205117205b suggests close-in planets can form in situ or migrate within ≃10 Myr, e.g., via interactions with a disk, and that long-timescale dynamical migration such as by Lidov-Kozai or planet-planet scattering is not responsible for all short-period planets.

UScoJ155150.2−213457: A new Free-floating ExoPlanet (Brown Dwarf?) in the Upper Scorpius association

A new free-floating planet in the Upper Scorpius association

Authors:

Ramírez et al

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 taking several years apart. 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, UScoJ155150.2−213457, 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 UScoJ155150.2−213457. 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. UScoJ155150.2−213457, 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.