A census of very-low-mass stars and brown dwarfs in the sigma Orionis cluster
N. Lodieu, M. R. Zapatero Osorio, R. Rebolo, E. L. Martin, N. C. Hambly
备注：
16 pages, 10 figures, 4 Tables, and appendix containing 6 tables including tables in electronic format only. Revised version corrected for english. Table 4 has been updated and corrected
(ABRIDGED) We have analysed the near-infrared photometric data from the Fourth Data Release (DR4) of the UKIRT Infrared Deep Sky Suvey (UKIDSS) Galactic Clusters Survey (GCS) to derive the cluster luminosity and mass functions, evaluate the extent of the cluster, and study the distribution and variability of low-mass stars and brown dwarfs down to the deuterium-burning limit. We have recovered most of the previously published members and found a total of 287 candidate members within the central 30 arcmin in the 0.5-0.009 Msun mass range, including new objects not previously reported in the literature. This new catalogue represents a homogeneous dataset of brown dwarf member candidates over the central 30 arcmin of the cluster. The expected photometric contamination by field objects with similar magnitudes and colours to sigma Orionis members is ~15%. We present evidence of variability at the 99.5% confidence level over ~yearly timescales in 10 member candidates that exhibit signs of youth and the presence of disks. The level of variability is low (<0.3 mag) and does not impact the derivation of the cluster luminosity and mass functions. Furthermore, we find a possible dearth of brown dwarfs within the central five arcmin of the cluster, which is not caused by a lower level of photometric sensitivity around the massive, O-type multiple star sigma Ori in the GCS survey. Using state-of-the-art theoretical models, we derived the luminosity and mass functions within the central 30 arcmin from the cluster centre, with completeness down to J = 19 mag, corresponding to masses ranging from 0.5 Msun down to the deuterium-burning mass boundary (~0.013 Msun). The mass function of sigma Orionis in this mass interval shows a power law index alpha = 0.5+/-0.2.
We present a spectroscopic survey of the stellar population of the Sigma Orionis cluster. We have obtained spectral types for 340 stars. Spectroscopic data for spectral typing come from several spectrographs with similar spectroscopic coverage and resolution. More than a half of stars of our sample are members confirmed by the presence of lithium in absorption, strong H$α$ in emission or weak gravity-sensitive features. In addition, we have obtained high resolution (R~34000) spectra in the H$α$ region for 169 stars in the region. Radial velocities were calculated from this data set. The radial…
Context. Although many studies have been performed so far, there are still dozens of low-mass stars and brown dwarfs in the young sigma Orionis open cluster without detailed spectroscopic characterisation. Aims. We look for unknown strong accretors and disc hosts that were undetected in previous surveys. Methods. We collected low-resolution spectroscopy (R ~ 700) of ten low-mass stars and brown dwarfs in sigma Orionis with OSIRIS at the Gran Telescopio Canarias under very poor weather conditions. These objects display variability in the optical, infrared, Halpha, and/or X-rays on time scales o…
Whilst there is a generally accepted evolutionary scheme for the formation of low-mass stars, the analogous processes when moving down in mass to the brown dwarfs regime are not yet well understood. In this first paper we try to build the most complete and unbiased spectroscopically confirmed census of the population of Collinder 69, the central cluster of the Lambda Orionis star forming region, as a first step in addressing the question of how brown dwarfs and planetary mass objects form. We have studied age dependent features in optical and near-infrared spectra of candidate members to the c…
We present a study of 15 new brown dwarfs belonging to the $\sim7$ Myr old 25 Orionis group and Orion OB1a sub-association with spectral types between M6 and M9 and estimated masses between $\sim0.07$M$_\odot$ and $\sim0.01$ M$_\odot$. By comparing them through a Bayesian method with low mass stars ($0.8\lesssim$ M/M$_\odot\lesssim0.1$) from previous works in the 25 Orionis group, we found statistically significant differences in the number fraction of classical T Tauri stars, weak T Tauri stars, class II, evolved discs and purely photospheric emitters at both sides of the sub-stellar mass lim…
The 32 Orionis group was discovered almost a decade ago and despite the fact that it represents the first northern, young (age ~ 25 Myr) stellar aggregate within 100 pc of the Sun ($d \simeq 93$ pc), a comprehensive survey for members and detailed characterisation of the group has yet to be performed. We present the first large-scale spectroscopic survey for new (predominantly M-type) members of the group after combining kinematic and photometric data to select candidates with Galactic space motion and positions in colour-magnitude space consistent with membership. We identify 30 new members, …
The continuous temporal coverage and high photometric precision afforded by space observatories has opened up new opportunities for the study of variability processes in young stellar cluster members. Of particular interest is the phenomenon of deuterium-burning pulsation in brown dwarfs and very-low-mass stars, whose existence on 1-4 hours timescales has been proposed but not yet borne out by observations. To investigate short-timescale variability in young, low-mass objects, we carried out high-precision, high-cadence time series monitoring with the Warm Spitzer mission on 14 low mass stars …
Aims: We investigated the X-ray emission from young stars and brown dwarfs in the sigma Orionis cluster (tau~3 Ma, d~385 pc) and its relation to mass, presence of circumstellar discs, and separation to the cluster centre by taking advantage of the superb spatial resolution of the Chandra X-ray Observatory. Methods: We used public HRC-I/Chandra data from a 97.6 ks pointing towards the cluster centre and complemented them with X-ray data from IPC/Einstein, HRI/ROSAT, EPIC/XMM-Newton, and ACIS-S/Chandra together with optical and infrared photometry and spectroscopy from the literature and public …
Context. Most observational studies so far point towards brown dwarfs sharing a similar formation mechanism as the one accepted for low mass stars. However, larger databases and more systematic studies are needed before strong conclusions can be reached. Aims. In this second paper of a series devoted to the study of the spectroscopic properties of the members of the Lambda Orionis Star Forming Region, we study accretion, activity and rotation for a wide set of spectroscopically confirmed members of the central star cluster Collinder 69 to draw analogies and/or differences between the brown dwa…
We present the results of a survey of the low mass star and brown dwarf population of the 25 Orionis group. Using optical photometry from the CIDA Deep Survey of Orion, near IR photometry from the Visible and Infrared Survey Telescope for Astronomy and low resolution spectroscopy obtained with Hectospec at the MMT, we selected 1246 photometric candidates to low mass stars and brown dwarfs with estimated masses within $0.02 \lesssim M/M_\odot \lesssim 0.8$ and spectroscopically confirmed a sample of 77 low mass stars as new members of the cluster with a mean age of $\sim$7 Myr. We have obtained…
The sigma Orionis cluster is to date the star-forming region with the largest number of confirmed brown dwarfs and substellar objects below the deuterium burning mass limit. The most massive star, sigma Ori Aa, just in the cluster centre, is the \sim20Msol-mass O9.5V star that illuminates the Horsehead Nebula, while the least massive object yet reported, S Ori 70, is only around 3 MJup. In the middle, there is a continuum of stars and substellar objects of all types (including magnetically active B2Vp stars, Herbig-Haro objects, FU Ori stars or T Tauri brown dwarfs) that makes the cluster a co…
Context. Only a few open clusters are as important for the study of stellar and substellar objects, and their formation and evolution, as the young sigma Orionis cluster. However, a complete spectroscopic characterisation of its whole stellar population is still missing. Aims. We filled most of that gap with a large spectroscopic and astrometric survey of targets towards sigma Orionis. Eventually, it will be one of the open clusters with the lowest proportion of interlopers and the largest proportion of confirmed cluster members with known uncontrovertible youth features. Methods. We acquired …
The 32 Orionis group is a co-moving group of roughly 20 young (24 Myr) M3-B5 stars 100 pc from the Sun. Here we report the discovery of its first substellar member, WISE J052857.69+090104.2. This source was previously reported to be an M giant star based on its unusual near-infrared spectrum and lack of measurable proper motion. We re-analyze previous data and new moderate-resolution spectroscopy from Magellan/FIRE to demonstrate that this source is a young near-infrared L1 brown dwarf with very low surface gravity features. Spectral model fits indicate T$_{eff}$ = 1880$^{+150}_{-70}$ K and $\…
Aims: I seek to fully know the stellar and substellar populations in the young sigma Orionis open cluster, which is a benchmark for star-forming studies. Because of the very low proper motion of the cluster, late-type dwarfs with appreciable proper motion in the foreground of sigma Orionis can be easily discarded for expensive spectroscopic follow-up. Methods: I use the Aladin sky atlas, USNO-B1, public astrometric catalogues, and photographic plate digitisations to identify stars with proper motions inconsistent with cluster membership in a circular area of radius 30 arcmin centred on the ear…
Deep optical imaging is used to study time-domain properties of young brown dwarfs in sigma Orionis over typical rotational time-scales and to search for new substellar and planetary-mass cluster members. We used VIMOS at the VLT to monitor a 24 arcmin x 16 arcmin field in the I-band. Using the individual images from this run we investigated the photometric time series of nine substellar cluster members with masses from 10 to 60 Mj. The deep stacked image shows cluster members down to $\approx$ 5 Mj. We search for new planetary mass objects by combining our deep I-band photometry with public J…
We report on our analysis of the VISTA Orion ZYJHKs photometric data (completeness magnitudes Z=22.6 and J=21.0mag) focusing on a circular area of 2798.4 arcmin^2 around the young sigma Orionis star cluster (~3Myr, ~352pc, solar metallicity). The combination of the VISTA photometry with optical, WISE and Spitzer data allows us to identify a total of 210 cluster member candidates with masses in the interval 0.25-0.004Msun, 23 of which are new planetary-mass object findings. These discoveries double the number of cluster planetary-mass candidates known so far. One object has colors compatible wi…
The Virtual Observatory is useful. I summarise some of my works where I extensively use the Aladin sky atlas. Topics cover from the search and common proper motion confirmation of very low-mass stars and brown dwarfs in wide (rho > 1000 AU) binaries and multiple systems, to the identification and characterisation of stellar and substellar populations in young open clusters and OB associations at heliocentric distances of up to 1000 pc. I present three practical examples of what one can do with Aladin in one morning: a fruitful proper-motion search of objects with available ugrizJHKs photometry…
We have conducted a near-infrared (NIR) proper motion survey, the Sondage Infrarouge de Mouvement Propre, in order to discover field ultracool dwarfs (UCD) in the solar neighborhood. The survey was conducted by imaging $\sim28\%$ of the sky with the Caméra PAnoramique Proche-InfraRouge both in the southern hemisphere at the Cerro Tololo Inter-American Observatory 1.5 m telescope, and in the northern hemisphere at the Observatoire du Mont-Mégantic 1.6 m telescope and comparing the source positions from these observations with the Two Micron All-Sky Survey Point Source Catalog (2MASS PSC). Addit…
We present IRAC/MIPS Spitzer observations of intermediate-mass stars in the 5 Myr old Lambda Orionis cluster. In a representative sample of stars earlier than F5 (29 stars), we find a population of 9 stars with a varying degree of moderate 24um excess comparable to those produced by debris disks in older stellar groups. As expected in debris disks systems, those stars do not exhibit emission lines in their optical spectra. We also include in our study the star HD 245185, a known Herbig Ae object which displays excesses in all Spitzer bands and shows emission lines in its spectrum. We compare t…
We are performing a series of observations with ground-based telescopes toward Planck Galactic cold clumps (PGCCs) in the $λ$ Orionis complex in order to systematically investigate the effects of stellar feedback. In the particular case of PGCC G192.32-11.88, we discovered an extremely young Class 0 protostellar object (G192N) and a proto-brown dwarf candidate (G192S). G192N and G192S are located in a gravitationally bound bright-rimmed clump. The velocity and temperature gradients seen in line emission of CO isotopologues indicate that PGCC G192.32-11.88 is externally heated and compressed. G…
We present high-precision photometry on 107 variable low-mass stars and brown dwarfs in the ~3 Myr Sigma Orionis open cluster. We have carried out I-band photometric monitoring within two fields, encompassing 153 confirmed or candidate members of the low-mass cluster population, from 0.02 to 0.5 M_\odot. We are sensitive to brightness changes on time scales from 10 minutes to two weeks with amplitudes as low as 0.004 magnitudes, and find variability on these time scales in nearly 70% of cluster members. We identify both periodic and aperiodic modes of variability, as well as semi-periodic rapi…
We present a deep I,Z photometric survey covering a total area of 1.12 deg^{2} of the Sigma Orionis cluster (Icompl=22 and Zcompl=21.5mag). From I, I-Z color-magnitude diagrams we have selected 153 candidates that fit the previously known sequence of the cluster. Using J-band photometry, we find that 124 of the 151 candidates follow the previously known infrared photometric sequence of the cluster and are probably members. We have studied the spatial distribution of these candidates and found that there are objects located at distances greater than 30 arcmin to the north and west of Sigma Orio…
SONYC -- Substellar Objects in Nearby Young Clusters -- is a program to investigate the frequency and properties of young substellar objects with masses down to a few times that of Jupiter. Here we present a census of very low mass objects in the ~1 Myr old cluster NGC1333. We analyze near-infrared spectra taken with FMOS/Subaru for 100 candidates from our deep, wide-field survey and find 10 new likely brown dwarfs with spectral types of M6 or later. Among them, there are three with >~M9 and one with early L spectral type, corresponding to masses of 0.006 to <~0.02 Msol, so far the lowest mass…
Over the past years, the lambda-Orionis cluster has been a prime location for the study of young very low mass stars, substellar and isolated planetary mass objects and the determination of the initial mass function and other properties of low mass cluster members. In the continuity of our previous studies of young associations cores, we search for ultracool members and new multiple systems within the central 5.3' (~0.6pc) of the cluster. We obtained deep seeing limited J, Ks-band images of the 5.3' central part of the cluster with NTT/SofI and H-band images with CAHA/Omega2000. These images w…
The Orion OB1a sub-association is a rich low mass star (LMS) region. Previous spectroscopic studies have confirmed 160 LMSs in the 25 Orionis stellar group (25 Ori), which is the most prominent overdensity of Orion OB1a. Nonetheless, the current census of the 25 Ori members is estimated to be less than 50% complete, leaving a large number of members to be still confirmed. We retrieved 172 low-resolution stellar spectra in Orion OB1a observed as ancillary science in the SDSS-III/BOSS survey, for which we classified their spectral types and determined physical parameters. To determine membership…
SONYC, short for "Substellar Objects in Nearby Young Clusters", is a survey program to provide a census of the substellar population in nearby star forming regions. We have conducted deep optical and near-infrared photometry in five young regions (NGC1333, rho Ophiuchi, Chamaeleon-I, Upper Sco, and Lupus-3), combined with proper motions, and followed by extensive spectroscopic campaigns with Subaru and VLT, in which we have obtained more than 700 spectra of candidate low-mass objects. We have identified and characterized more than 60 new substellar objects, among them a handful of objects with…
We report optical and infrared imaging spectroscopy observations of the young binary object Sigma Orionis IRS1 A/B. The brighter component (Sigma Ori IRS1 A) of this binary system has M1 spectral type and a mass in the range of 0.3 to 0.8 solar masses. The fainter component (Sigma Ori IRS1 B) has a unique morphology and spectrum. The unresolved stellar object is surrounded by an extended envelope that is slightly offset from the position of this star. The envelope's spectrum shows strong emission lines of H and HeI but no shock-excited emission from molecular hydrogen or from [FeII]. The embed…
To improve the census of the Upper Sco association (~11 Myr, ~145 pc), we have identified candidate members using parallaxes, proper motions, and color-magnitude diagrams from several wide-field imaging surveys and have obtained optical and infrared spectra of several hundred candidates to measure their spectral types and assess their membership. We also have performed spectroscopy on a smaller sample of previously known or suspected members to refine their spectral types and evidence of membership. We have classified 530 targets as members of Upper Sco, 377 of which lack previous spectroscopy…
The reflection nebula NGC 7129 has long been known to be a site of recent star formation as evidenced, e.g., by the presence of deeply embedded protostars and HH objects. However, studies of the stellar population produced in the star formation process have remained rudimentary. At a presumed age of ~3 Myr, NGC7129 is in the critical range where disks around young stars disappear. We make use of Chandra X-ray and Spitzer and 2MASS IR imaging observations to identify the pre-main sequence stars in NGC7129. We define a sample of Young Stellar Objects based on color-color diagrams composed from I…
The nearest, youngest groups of stars to the Sun provide important samples of age-dated stars for studying circumstellar disk evolution, imaged exoplanets, and brown dwarfs. I briefly comment on the status of the known stellar groups within 100 pc: $β$ Pic, AB Dor, UMa, Car-Near, Tuc-Hor and $β$ Tuc nucleus, Hyades, Col, TW Hya, Car, Coma Ber, 32 Ori, $η$ Cha, and $χ^1$ For. I also discuss some poorly characterized groups and "non-groups." Grades for 2015 of Pass, Satisfactory, or Fail are assigned to the groups for the purposes of age-dating stars and brown dwarfs. I speculate that Tuc-Hor co…
We report the discovery of a new Herbig-Haro jet, HH 1165, in SOAR narrow-band imaging of the vicinity of the sigma Orionis cluster. HH 1165 shows a spectacular extended and collimated spatial structure, with a projected length of 0.26 pc, a bent C-shaped morphology, multiple knots, and fragmented bow-shocks at the apparent ends of the flow. The Halpha image shows a bright halo with a clumpy distribution of material seen around the driving source, and curved reflection nebulosity tracing the outflow cavities. The driving source of HH 1165 is a Class I proto-brown dwarf, Mayrit 1701117 (M170111…
This chapter reviews our current knowledge of metal-poor ultracool dwarfs with spectral types later than M7. The current census of M, L, and T subdwarfs is explored. The main colour trends of subdwarfs from the optical to the mid-infrared are described and their spectral features presented, which led to a preliminary and tentative spectral classification subject to important changes in the future when more of these metal-poor objects are discovered. Their multiplicity and the determination of their physical parameters (effective temperature, gravity, metallicity, and mass) are discussed. Final…
Surveys with the Spitzer and Herschel space observatories are now enabling the discovery and characterization of large samples of protostars in nearby molecular clouds, providing the observational basis for a detailed understanding of star formation in diverse environments. We are pursuing this goal with the Herschel Orion Protostar Survey (HOPS), which targets 328 Spitzer-identified protostars in the Orion molecular clouds, the largest star-forming region in the nearest 500 pc. The sample encompasses all phases of protostellar evolution and a wide range of formation environments, from dense c…
In 2005, Palla & Baraffe proposed that brown dwarfs (BDs) and very low mass stars (VLMSs; <0.1 solar masses) may be unstable to radial oscillations during the pre-main-sequence deuterium burning phase. With associated periods of 1-4 hours, this potentially new class of pulsation offers unprecedented opportunities to probe the interiors and evolution of low-mass objects in the 1-15 million year age range. Following up on reports of short-period variability in young clusters, we designed a high-cadence photometric monitoring campaign to search for deuterium-burning pulsation among a sample of 34…
We have obtained optical and near-infrared spectra of candidate members of the star-forming clusters IC 348 and NGC 1333. We classify 100 and 42 candidates as new members of the clusters, respectively, which brings the total numbers of known members to 478 and 203. We also have performed spectroscopy on a large majority of the previously known members of NGC 1333 in order to provide spectral classifications that are measured with the same scheme that has been applied to IC 348 in previous studies. The new census of members is nearly complete for Ks<16.8 at Aj<1.5 in IC 348 and for Ks<16.2 at A…
In this paper we present a catalogue of 11,745 brown dwarfs with spectral types ranging from L0 to T9, photometrically classified using data from the Dark Energy Survey (DES) year 3 release matched to the Vista Hemisphere Survey (VHS) DR3 and Wide-field Infrared Survey Explorer (WISE) data, covering approx 2,400 deg2 up to i_AB=22. The classification method follows the same photo-type method previously applied to SDSS-UKIDSS-WISE data. The most significant difference comes from the use of DES data instead of SDSS, which allow us to classify almost an order of magnitude more brown dwarfs than a…
We present the first observations of a probable brown dwarf, obtained with the new spectrograph X-shooter mounted on the UT2@VLT. The target (2MASS J053825.4-024241) is a 0.06 Msun object in the star-formation region sigma Orionis. The X-shooter spectrum covers simultaneously the whole range from UV to NIR (300-2500 nm). The J053825.4-024241 spectrum is rich in emission lines that are typical of accreting young object and clearly shows the Balmer jump. Moreover, many photospheric atomic and molecular absorption lines yield the spectral type and confirm that the object is young. We compute the …
The census of solar neighbours is still complemented by new discoveries, mainly of very low-mass, faint dwarfs, close to or within the substellar domain. These discoveries contribute to a better understanding of the field population; its origin in terms of Galactic dynamics and (sub)stellar formation and evolution. Also, the nearest stars and brown dwarfs at any given age allow the most precise direct characterization, including the search for planetary companions. We aim to further assess the substellar census on the Galactic plane. We projected the 136 stars and 26 brown dwarfs known at <6.5…
Pulsation powered by deuterium burning in brown dwarfs and very low mass stars has been put forth (Palla & Baraffe 2005) as a novel probe of the interiors of these objects in the 1-15 Myr age range. Previous observations have hinted at variability on the expected timescales of a few hours, suggesting but not confirming that the phenomenon is at work in young brown dwarfs. We have recently carried out a dedicated campaign to search for this putative class of pulsators among known low-mass members of five young star clusters. Our survey achieves sensitivity to periodic oscillations with photomet…
Context. Brown dwarfs represent a sizable fraction of the stellar content of our Galaxy and populate the transition between the stellar and planetary mass regime. There is however no agreement on the processes responsible for their formation. Aims. We have conducted a large survey of the young, nearby cluster IC 348, to uncover its low-mass brown dwarf population and study the cluster properties in the substellar regime. Methods. Deep optical and near-IR images taken with MegaCam and WIRCam at the Canada-France-Hawaii Telescope (CFHT) were used to select photometric candidate members. A spectr…
Measurement of the substellar initial mass function (IMF) in very young clusters is hampered by the possibility of the age spread of cluster members. This is particularly serious for candidate planetary mass objects (PMOs), which have a very similar location to older and more massive brown dwarfs on the Hertzsprung-Russell Diagram (HRD). This degeneracy can be lifted by the measurement of gravity-sensitive spectral features. To this end we have obtained medium resolution (R~5000) Near-infrared Integral Field Spectrometer (NIFS) K band spectra of a sample of late M- / early L-type dwarfs. The s…
We present a census of the disk population for UKIDSS selected brown dwarfs in the 5-10 Myr old Upper Scorpius OB association. For 116 objects originally identified in UKIDSS, the majority of them not studied in previous publications, we obtain photometry from the WISE database. The resulting colour-magnitude and colour-colour plots clearly show two separate populations of objects, interpreted as brown dwarfs with disks (class II) and without disks (class III). We identify 27 class II brown dwarfs, 14 of them not previously known. This disk fraction (27 out of 116 or 23%) among brown dwarfs wa…
Context. The second Gaia data release (Gaia DR2) contains high precision positions, parallaxes and proper motions for 1.3 billion sources. The resulting Hertzsprung-Russel diagram reveals fine structures all over the mass range. Aims. This paper aims to investigate the content of Gaia DR2 at the low-mass end and to characterize ultra-cool and brown dwarfs. Methods. We first retrieve the sample of spectroscopically confirmed ultra-cool and brown dwarfs in Gaia DR2. We use their locus in the precise Hertzsprung-Russel diagram to select new candidates, and to investigate their properties. Results…
Recent wide-field near-infrared surveys have uncovered a large number of cool brown dwarfs, extending the temperature sequence down to less than 500 K and constraining the faint end of the luminosity function. One interesting implication of the derived luminosity function is that the brown dwarf census in the immediate (<10 pc) solar neighborhood is still largely incomplete, and some bright (J<16) brown dwarfs remain to be identified in existing surveys. These objects are especially interesting as they are the ones that can be studied in most detail, especially with techniques that require lar…
We present Spitzer 3.6$μ$m and 4.5$μ$m follow-up of 170 candidate extremely cool brown dwarfs newly discovered via the combination of WISE and NEOWISE imaging at 3$-$5$μ$m. CatWISE, a joint analysis of archival WISE and NEOWISE data, has improved upon the motion measurements of AllWISE by leveraging a $>$10$\times$ time baseline enhancement, from 0.5 years (AllWISE) to 6.5 years (CatWISE). As a result, CatWISE motion selection has yielded a large sample of previously unrecognized brown dwarf candidates, many of which have archival detections exclusively in the WISE 4.6$μ$m (W2) channel, sugges…
We study the properties of the coronae of the low-mass stars in the young (~2-3Myr), nearby (~310pc) open cluster IC348 combining X-ray and optical/infrared data. The four existing Chandra observations of IC348 are merged, thus providing a deeper and spatially more complete X-ray view than previous X-ray studies of the cluster. We have compiled a comprehensive catalog of IC348 members taking into account recent updates to the cluster census. Our data collection comprises fundamental stellar parameters, infrared excess indicating the presence of disks, Halpha emission as a tracer of chromospher…
AIMS: We contribute to improving the census of cool brown dwarfs (late-T and Y dwarfs) in the immediate solar neighbourhood. METHODS: By combining near-infrared (NIR) data of UKIDSS with mid-infrared WISE and other available NIR (2MASS) and red optical (SDSS $z$-band) multi-epoch data we detect high proper motion (HPM) objects with colours typical of late spectral types ($>$T5). We use NIR low-resolution spectroscopy for the classification of new candidates. RESULTS: We determined new proper motions for 14 known T5.5-Y0 dwarfs, many of them being significantly ($>$2-10 times) more accurate tha…
The origin of the lowest mass free-floating objects -- brown dwarfs and planetary-mass objects -- is one of the major unsolved problems in star formation. Establishing a census of young substellar objects is a fundamental prerequisite for distinguishing between competing theoretical scenarios. Such a census allows us to probe the initial mass function (IMF), binary statistics, and properties of accretion disks. Our SONYC (Substellar Objects in Nearby Young Clusters) survey relies on extremely deep wide-field optical and near-infrared imaging, with follow-up spectroscopy, in combination with Sp…
The nearest stars provide a fundamental constraint for our understanding of stellar physics and the Galaxy. The nearby sample serves as an anchor where all objects can be seen and understood with precise data. This work is triggered by the most recent data release of the astrometric space mission Gaia and uses its unprecedented high precision parallax measurements to review the census of objects within 10 pc. The first aim of this work was to compile all stars and brown dwarfs within 10 pc observable by Gaia, and compare it with the Gaia Catalogue of Nearby Stars as a quality assurance test. W…
We present ground-based spectroscopic verification of six Y dwarfs (see Cushing et al), eighty-nine T dwarfs, eight L dwarfs, and one M dwarf identified by the Wide-field Infrared Survey Explorer (WISE). Eighty of these are cold brown dwarfs with spectral types greater than or equal to T6, six of which have been announced earlier in Mainzer et al and Burgasser et al. We present color-color and color-type diagrams showing the locus of M, L, T, and Y dwarfs in WISE color space. Near-infrared classifications as late as early Y are presented and objects with peculiar spectra are discussed. After d…
We present an extensive search in the literature and Gaia DR2 for visual co-moving binary companions to stars hosting exoplanets and brown dwarfs within 200 pc. We found 218 planet hosts out of 938 to be part of multiple-star systems, with 10 newly discovered binaries and 2 new tertiary stellar components. This represents an overall raw multiplicity rate of 23.2$\pm$1.6% for hosts to exoplanets across all spectral types, with multi-planet systems found to have a lower duplicity frequency at the 2.2$σ$ level. We found that more massive hosts are more often in binary configurations, and that pla…
We confirm 66 low-mass stellar and brown dwarf systems (K7-M9) plus 19 visual or spectroscopic companions of the beta Pictoris Moving Group (BPMG). Of these, 41 are new discoveries, increasing the known low-mass members by 45%. We also add four objects to the 14 known with masses predicted to be less than 0.07 Msun. Our efficient photometric+kinematic selection process identified 104 low-mass candidates which we observed with ground-based spectroscopy. We collected infrared observations of the latest spectral types (>M5) to search for low gravity objects. These and all <M5 candidates were obse…
We present the discovery of another seven Y dwarfs from the Wide-field Infrared Survey Explorer (WISE). Using these objects, as well as the first six WISE Y dwarf discoveries from Cushing et al., we further explore the transition between spectral types T and Y. We find that the T/Y boundary roughly coincides with the spot where the J-H colors of brown dwarfs, as predicted by models, turn back to the red. Moreover, we use preliminary trigonometric parallax measurements to show that the T/Y boundary may also correspond to the point at which the absolute H (1.6 um) and W2 (4.6 um) magnitudes plum…
Stellar multiplicity properties have been studied for much of the range from the lowest to the highest stellar masses, but intermediate-mass stars from F-type to late A-type have received relatively little attention. Here we report on a Gemini/NICI snapshot imaging survey of 138 such stars in the young Scorpius-Centaurus (Sco-Cen) region, for the purpose of studying multiplicity with sensitivity down to planetary masses at wide separations. In addition to two brown dwarfs and a companion straddling the hydrogen burning limit we reported previously, here we present 26 new stellar companions and…
A search for young substellar objects in the rho Oph cloud core region has been made using the deep-integration Combined Calibration Scan images of the 2MASS extended mission in J, H and Ks bands, and Spitzer IRAC images at 3.6, 4.5, 5.8 and 8.0 microns. The field of view of the combined observations was 1 deg x 9.3 arcmin, and the 5 sigma limiting magnitude at J was 20.5. Comparison of the observed SEDs with the predictions of the COND and DUSTY models, for an assumed age of 1 Myr, supports the identification of many of the sources with brown dwarfs, and enables the estimation of effective te…
Although many new brown dwarf (BD) neighbours have recently been discovered thanks to new sky surveys in the mid- and near-infrared (MIR, NIR), their numbers are still more than five times lower than those of stars in the same volume. Our aim is to detect and classify new BDs to eventually complete their census in the immediate Solar neighbourhood. We combined multi-epoch data from sky surveys at different wavelengths to detect BD neighbours of the Sun by their high proper motion (HPM). We concentrated at relatively bright MIR (w2<13.5) BD candidates from WISE expected to be so close to the Su…
