Proper motion measurements of the cool and ultracool populations in the Upper Scorpius OB association are crucial to confirm membership and to identify possible run-away objects.
We cross-match samples of photometrically selected and spectroscopically confirmed cool and ultracool (K5<SpT<M8.5) candidate members in the Upper Scorpius OB association using the literature and the USNO-B and the UCAC2 catalogues. 251 of these objects have a USNO-B and/or UCAC2 counterpart with proper motion measurements.
A significant fraction (19 objects, 7.6+-1.8%) of spectroscopically confirmed young objects show discrepant proper motion. They must either belong to unidentified coincident foreground associations, or originate from neighboring star forming regions or have recently experienced dynamical interactions within the association. The observed accretor and disc frequencies are lower among outliers, but with only 19 objects it is unreliable to draw firm statistical conclusions. Finally, we note that transverse velocities of very low mass members are indistinguishable from those of low mass members within 4km/s
We present parallax observations and a detailed model atmosphere analysis of 54 cool and ultracool ($T_{\rm eff}$ < 4000 K) white dwarfs (WDs) in the solar neighbourhood. For the first time, a large number of cool and ultracool WDs have distance and tangential velocities measurements available. Our targets have distances ranging from 21 pc to >100 pc, and include five stars within 30 pc. Contrary to expectations, all but two of them have tangential velocities smaller than 150 km s$^{-1}$ thus suggesting Galactic disc membership. The oldest WDs in this sample have WD cooling ages of 10 Gyr, pro…
Studies of cool white dwarfs in the solar neighbourhood have placed a limit on the age of the Galactic disk of 8-9 billion years. However, determining their cooling ages requires the knowledge of their effective temperatures, masses, radii, and atmospheric composition. So far, these parameters could only be inferred for a small number of ultracool white dwarfs for which an accurate distance is known, by fitting their spectral energy distributions (SEDs) in conjunction with a theoretical mass-radius relation. However, the mass-radius relation remains largely untested, and the derived cooling ag…
We present the first results of the ongoing Canada-France Brown Dwarfs Survey-InfraRed, hereafter CFBDSIR, a Near InfraRed extension to the optical wide-field survey CFBDS. Our final objectives are to constrain ultracool atmosphere physics by finding a statistically significant sample of objects cooler than 650K and to explore the ultracool brown dwarf mass function building on a well defined sample of such objects. Candidates are identified in CFHT/WIRCam J and CFHT/MegaCam z' images using optimised psf-fitting, and we follow them up with pointed near infrared imaging with SOFI at NTT. We fin…
A recent analysis of the 100 pc white dwarf sample in the SDSS footprint demonstrated for the first time the existence of a well defined ultracool -- or IR-faint -- white dwarf sequence in the Hertzsprung-Russell diagram. Here we take advantage of this discovery to enlarge the IR-faint white dwarf sample threefold. We expand our selection to the entire Pan-STARRS survey footprint as well as the Montreal White Dwarf Database 100 pc sample, and identify 37 candidates with strong flux deficits in the optical. We present follow-up Gemini optical spectroscopy of 30 of these systems, and confirm all…
We report the identification of LSR J0745+2627 in the United Kingdom InfraRed Telescope Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) as a cool white dwarf with kinematics and age compatible with the thick-disk/halo population. LSR J0745+2627 has a high proper motion (890 mas/yr) and a high reduced proper motion value in the J band (H_J=21.87). We show how the infrared-reduced proper motion diagram is useful for selecting a sample of cool white dwarfs with low contamination. LSR J0745+2627 is also detected in the Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Ex…
Context: A rich population of low-mass brown dwarfs and isolated planetary mass objects has been reported recently in the Upper Scorpius and Ophiuchus star forming complex. Aims: We investigate the membership, nature and properties of 17 of these isolated planetary mass candidates using low-resolution near-infrared spectra. Methods: We investigate the membership by looking for evidences of youth using four diagnostics: the slope of the continuum between the J and Ks band, the Hcont and TLI-g gravity sensitive indices, and by comparing the spectra to young and field (old) M and L-dwarf standard…
We present deep 1.2 millimeter photometry of 37 stars in the young (5 Myr) Upper Scorpius OB association, sensitive to ~4 x 10^-3 Mjup of cool millimeter dust. Disks around four low- and solar-mass stars are detected, as well as one debris disk around an intermediate mass star, with dust masses ranging from 3.6 x 10^-3 -- 1.0 x 10^-1 Mjup. The source with the most massive disk exhibits a transition-disk spectral energy distribution. Combining our results with previous studies, we find the millimeter-detection fraction of Class II sources has significantly decreased from younger ages, and compa…
We present the discovery of 61 wide (>5 arcsecond) separation, low-mass (stellar and substellar) companions to stars in the solar neighborhood identified from Pan-STARRS\,1 (PS1) data and the spectral classification of 27 previously known companions. Our companions represent a selective subsample of promising candidates and span a range in spectral type of K7-L9 with the addition of one DA white dwarf. These were identified primarily from a dedicated common proper motion search around nearby stars, along with a few as serendipitous discoveries from our Pan-STARRS1 brown dwarf search. Our disco…
When determining absolute ages of identifiably young stellar populations, results strongly depend on which stars are studied. Cooler (K, M) stars typically yield ages that are systematically younger than warmer (A, F, G) stars by a factor of two. I explore the possibility that these age discrepancies are the result of magnetic inhibition of convection in cool young stars by using magnetic stellar evolution isochrones to determine the age of the Upper Scorpius subgroup of the Scorpius-Centaurus OB Association. A median age of 10 Myr consistent across spectral types A through M is found, except …
We report on a deep photometric survey covering an area of 1.17 deg$^2$ 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 ($\sim$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$_\odot$ at the age and distance of Upper Scorpius. We also present the pro…
We aim at identifying very low-mass isolated planetary-mass member candidates in the nearest OB association to the Sun, Upper Scorpius (145 pc; 5-10 Myr), to constrain the form and shape of the luminosity function and mass spectrum in this regime. We conducted a deep multi-band ($Y$=21.2, $J$=20.5, $Z$=22.0 mag) photometric survey of six square degrees in the central region of Upper Scorpius. We extend the current sequence of astrometric and spectroscopic members by about two magnitudes in $Y$ and one magnitude in $J$, reaching potentially T-type free-floating members in the association with p…
We have identified a sample of cool field brown dwarf candidates using IRAC data from the Spitzer Deep, Wide-Field Survey (SDWFS). The candidates were selected from 400,000 SDWFS sources with [4.5] <= 18.5 mag and required to have [3.6]-[4.5] >= 1.5 and [4.5] - [8.0] <= 2.0 on the Vega system. The first color requirement selects objects redder than all but a handful of presently known brown dwarfs with spectral classes later than T7, while the second eliminates 14 probable reddened AGN. Optical detection of 4 of the remaining 18 sources implies they are likely also AGN, leaving 14 brown dwarf …
Context. Thanks to recent and ongoing large scale surveys, hundreds of brown dwarfs have been discovered in the last decade. The Canada-France Brown Dwarf Survey is a wide-field survey for cool brown dwarfs conducted with the MegaCam camera on the Canada-France-Hawaii Telescope telescope. Aims. Our objectives are to find ultracool brown dwarfs and to constrain the field brown-dwarf luminosity function and the mass function from a large and homogeneous sample of L and T dwarfs. Methods. We identify candidates in CFHT/MegaCam i' and z' images and follow them up with pointed near infrared (NIR) i…
We present the discovery of eight young M7-L2 dwarfs in the Taurus star-forming region and the Scorpius-Centaurus OB Association, serendipitously found during a wide-field search for L/T transition dwarfs using Pan-STARRS1 (optical) and WISE (mid-infrared) photometry. We identify PSO J060.3200+25.9644 (near-infrared spectral type L1) and PSO J077.1033+24.3809 (L2) as new members of Taurus based on their VL-G gravity classifications, the consistency of their photometry and proper motions with previously known Taurus objects, and the low probability of contamination by field objects. PSO J077.10…
Ultra-cool brown dwarfs offer a unique window into understanding substellar atmospheric physics and chemistry. Their strong molecular absorption bands at infrared wavelengths, Jupiter-like radii, cool temperatures, and lack of complicating stellar irradiation, make them ideal test-beds for understanding Jovian-like atmospheres. Here we report the findings of a uniform atmospheric retrieval analysis on a set of 14 Y and T-dwarfs observed with the Hubble Space Telescope Wide Field Camera 3 instrument. From our retrieval analysis, we find the temperature-structures to be largely consistent with r…
We report upper limits to the radio and X-ray emission from the newly discovered ultracool dwarf binary WISE J104915.57$-$531906.1 (Luhman 16AB). As the nearest ultracool dwarf binary (2 pc), its proximity offers a hefty advantage to studying plasma processes in ultracool dwarfs which are more similar in gross properties (radius, mass, temperature) to the solar system giant planets than stars. The radio and X-ray emission upper limits from the Australia Telescope Compact Array (ATCA) and Chandra observations, each spanning multiple rotation periods, provide the deepest fractional radio and X-r…
We present the results of a study to discover prospective new white dwarf-L dwarf binaries as identified by their near-infrared excesses in the UKIDSS catalogue. We obtained optical spectra to validate the white dwarf nature for 22 of the candidate primary stars, confirming ten as white dwarfs and determining their effective temperatures and gravities. For all ten white dwarfs we determined that the near-infrared excess was indeed indicative of a cool companion. Six of these are suggestive of late M dwarf companions, and three are candidate L dwarf companions, with one straddling the M$-$L bou…
The Upper Scorpius OB association is the nearest region of recent massive star formation and thus an important benchmark for investigations concerning stellar evolution and planet formation timescales. We present nine EBs in Upper Scorpius, three of which are newly reported here and all of which were discovered from K2 photometry. Joint fitting of the eclipse photometry and radial velocities from newly acquired Keck-I/HIRES spectra yields precise masses and radii for those systems that are spectroscopically double-lined. The binary orbital periods in our sample range from 0.6-100 days, with to…
DENIS J222644.3-750342 is a nearby, very-low-mass, M8.5 V-type star that has been repeatedly targeted by kinematic and activity studies. Thought to be an isolated star, it is actually a wide common-proper-companion at 265 arcsec from the bright G0 V Hipparcos star HD 212168. The third member in the trio is CD-75 1242, a poorly investigated K dwarf. We confirm the physical binding of the triple system, to which we call Koenigstuhl 5, by compiling common radial velocities and proper-motions and measuring constant angular separations and position angles between components A, B and C over very lon…
In this paper we present our project that aims at determining accurate distances and proper motions for the Y brown dwarf population using the Hubble Space Telescope. We validate the program with our first results, using a single new epoch of observations of the Y0pec dwarf WISE J163940.83$-$684738.6. These new data allowed us to refine its proper motion and improve the accuracy of its parallax by a factor of three compared to previous determinations, now constrained to $\varpi = 211.11 \pm 0.56$ mas. This newly derived absolute parallax corresponds to a distance of $4.737 \pm 0.013$ pc, an ex…
We present a new volume-limited sample of L0-T8 dwarfs out to 25 pc defined entirely by parallaxes, using our recent measurements from UKIRT/WFCAM along with Gaia DR2 and literature parallaxes. With 369 members, our sample is the largest parallax-defined volume-limited sample of L and T dwarfs to date, yielding the most precise space densities for such objects. We find the local L0-T8 dwarf population includes $5.5\%\pm1.3\%$ young objects ($\lesssim$200 Myr) and $2.6\%\pm1.6\%$ subdwarfs, as expected from recent studies favoring representative ages $\lesssim$4 Gyr for the ultracool field popu…
We present the first results from our high-precision infrared (IR) astrometry program at the Canada-France-Hawaii Telescope. We measure parallaxes for 83 ultracool dwarfs (spectral types M6--T9) in 49 systems, with a median uncertainty of 1.1 mas (2.3%) and as good as 0.7 mas (0.8%). We provide the first parallaxes for 48 objects in 29 systems, and for another 27 objects in 17 systems, we significantly improve upon published results, with a median (best) improvement of 1.7x (5x). Three systems show astrometric perturbations indicative of orbital motion; two are known binaries (2MASSJ0518-2828A…
A SPHEREx Pipeline and Spectral Library for Ultracool Dwarfs
Planétarium de Montréal, Espace pour la Vie（蒙特利尔天文馆，生命空间）；Trottier Institute for Research on Exoplanets, Université de Montréal, Département de Physique（特罗蒂埃系外行星研究所，蒙特利尔大学）；Department of Astrophysics, American Museum of Natural History（美国自然历史博物馆天体物理学部）；School of Physical Sciences, The Open University（开放大学物理科学学院）；United States Naval Observatory, Flagstaff Station（美国海军天文台弗拉格斯塔夫站）
The evolution of brown dwarfs from L to T spectral types is one of the least understood aspects of the ultracool population, partly for lack of a large, well-defined, and well-characterized sample in the L/T transition. To improve the existing census, we have searched $\approx$28,000 deg$^2$ using the Pan-STARRS1 and WISE surveys for L/T transition dwarfs within 25 pc. We present 130 ultracool dwarf discoveries with estimated distances $\approx9-130$ pc, including 21 that were independently discovered by other authors and 3 that were previously identified as photometric candidates. Seventy-nin…
Ultracool dwarfs (UCDs) comprise the lowest mass members of the stellar population and brown dwarfs, from M7 V to cooler objects with L, T, and Y spectral types. Most of them have been discovered using wide-field imaging surveys, for which the Virtual Observatory (VO) has proven to be of great utility. We aim to perform a search for UCDs in the entire Javalambre Photometric Local Universe Survey (J-PLUS) second data release (2176 deg$^2$) following a VO methodology. We also explore the ability to reproduce this search with a purely machine learning (ML)-based methodology that relies solely on …
We present near-infrared spectroscopy for 52 ultracool dwarfs, including two newly discovered late-M dwarfs, one new late-M subdwarf candidate, three new L and four new T dwarfs. We also present parallaxes and proper motions for 21 of them. Four of the targets presented here have previous parallax measurements, while all the others are new values. This allow us to populate further the spectral sequence at early types (L0-L4). Combining the astrometric parameters with the new near-infrared spectroscopy presented here, we are able to investigate further the nature of some of the objects. In part…
We have been using Keck laser guide star adaptive optics to monitor the orbits of ultracool binaries, providing dynamical masses at lower luminosities and temperatures than previously available and enabling strong tests of theoretical models. We have identified three specific problems with theory: (1) We find that model color-magnitude diagrams cannot be reliably used to infer masses as they do not accurately reproduce the colors of ultracool dwarfs of known mass. (2) Effective temperatures inferred from evolutionary model radii are typically inconsistent with temperatures derived from fitting…
We present optical and near-infrared adaptive optics (AO) imaging and spectroscopy of 13 ultracool (>M6) companions to late-type stars (K7-M4.5), most of which have recently been identified as candidate members of nearby young moving groups (YMGs; 8-120 Myr) in the literature. The inferred masses of the companions (~10-100 Mjup) are highly sensitive to the ages of the primary stars so we critically examine the kinematic and spectroscopic properties of each system to distinguish bona fide YMG members from old field interlopers. 2MASS J02155892-0929121 C is a new M7 substellar companion (40-60 M…
