arXiv subjects

J. Olivares

Publications and source records attributed to J. Olivares.

At least 19 recordsLinked to original sources

arXiv2026

Euclid Early Release Observations of the Barnard 30 dark cloud I. Brown dwarfs and planetary mass candidate members at the core of the association

This study aims to identify very low-mass members within the Barnard 30 dark cloud, with a particular focus on detecting objects in the substellar domain, including those within the planetary-mass regime. We employed deep photometric observations using data from the Euclid mission, incorporating advanced data processing techniques, in particular the DeNeb tool for optimized source detection. We also analyze multi-wavelength ancillary observations and perform a Spectral Energy Distribution analysis for each candidate member. In addition, low-resolution near-infrared spectroscopy was obtained for one candidate to further assess its nature and properties. Our initial photometric analysis yielded a initial sample of nearly one hundred candidate members in the substellar mass range. A subsample of 23 probable members, located close to the 3 Myr isochrone, has been identified. Thus, we have substantially expanded the known population of faint, cool sources associated with the region. Low resolution near-IR spectroscopic analysis of one candidate reveals an L2 spectral type with low gravity, consistent with a young ultra-cool dwarf. If its membership is confirmed, its estimated mass lies in the range 15-20 Mjup. These findings validate the reliability of our multi-wavelength photometric selection methodology. These results offer valuable insights into the low-mass end of the initial mass function (IMF) and demonstrate the effectiveness of Euclid in identifying brown dwarf and planetary-mass candidates in nearby, densely packed star-forming regions.

astro-ph.SR↗

arXiv2026

Chronos: Towards a self-consistent and absolute stellar age scale. I. A Bayesian hierarchical lithium-age model: Validation on the Pleiades cluster

Context. Establishing a self-consistent age scale for stellar populations requires physically well-calibrated chronometers. Among these, lithium-based diagnostics, particularly the lithium depletion boundary (LDB), provide one of the most robust age constraints for stellar populations in the low-mass regime. However, their application is limited by heterogeneous temperature scales and astrophysical dispersion, especially among FGK stars, where rotation can significantly affect lithium abundances. Aims. As a first step towards a self-consistent age scale, our aim has been to formulate Chronos, the first version of a Bayesian hierarchical lithium-based age-dating model combined with a neural network trained on stellar evolutionary models. Methods. We implemented a Bayesian hierarchical model that jointly infers stellar effective temperatures, lithium abundances, and the global age of a stellar association. The theoretical LDB is provided by a pre-trained multilayer perceptron based on BT--Settl evolutionary models. The model incorporates a temperature-dependent transition between fully convective ultra-cool dwarfs (UCDs) and FGKM dwarf stars, together with a two-component FGK mixture to account for rotation-induced lithium enhancement. We applied the method to the Pleiades cluster and performed a validation using synthetic datasets. Results. For the Pleiades cluster, Chronos yields a posterior age distribution centred at $\mathrm{Age}=124.53_{-2.70}^{+3.34}$ Myr, consistent with classical LDB estimates, while simultaneously constraining both global and stellar-level rotation parameters. Conclusions. This work demonstrates that lithium-based stellar chronology can be recast as a coherent hierarchical inference problem, providing a flexible and statistically robust framework for making age determinations for young (1--600 Myr) stellar populations.

astro-ph.SR↗

arXiv2026

Re-visiting the Canis Major star-forming region with Gaia data release 3 data

Context: The Canis Major (CMa) star-forming region, a remote molecular cloud complex within the recently discovered Radcliffe Wave, remains under-explored in the literature. Aims: We revisit the stellar census in the CMa region, characterizing its stellar population, kinematics, and age using recent astrometric and photometric data from the third data release of the Gaia space mission (Gaia DR3). Methods: We conducted a membership analysis of Gaia DR3 sources across a 16 deg$^2$ field encompassing the youngest subgroups in CMa. This new stellar census, combined with spectroscopic observations, allowed us to investigate the structure, kinematics, and age of this region. Results: We identified 1531 objects as members of the CMa region, confirming 401 previously known members and introducing 1130 new candidate members. These objects have magnitudes ranging from 10 to 18 mag in the G band from Gaia DR3. We identified two subgroups of CMa stars in our sample labelled as Cluster A and Cluster B. They are located at roughly the same distance ($d_{A} = 1150^{+79}_{-88}$ pc and $d_{B} = 1183^{+103}_{-108}$ pc) and exhibit similar space motions that can be derived thanks to the precise radial velocities obtained in this study. The subgroups have a mean isochronal age of about 2-3 Myr. However, based on infrared photometry we show that Cluster A has a higher fraction of disc-bearing stars suggesting that it could be somewhat younger than Cluster B. Conclusions: Our analysis provides new insights into the stellar population of the Canis Major region, by identifying new members, characterizing their kinematics, and assessing their evolutionary stages. Future studies incorporating additional data from upcoming Gaia data releases, multi-wavelength and high-resolution spectroscopic observations will be essential to further advance our understanding of the history of star formation in this region.

astro-ph.SR↗

arXiv2026

Multiplicity of young isolated planetary mass objects in Taurus and Upper Scorpius

Free-floating planetary mass objects--worlds that roam interstellar space untethered to a parent star--challenge conventional notions of planetary formation and migration, but also of star and brown dwarf formation. We focus on the multiplicity among free-floating planets. By virtue of their low binding energy (compared to other objects formed in these environments), these low-mass substellar binaries represent a most sensitive probe of the mechanisms at play during the star formation process. We use the HST and its WFC3 and the VLT and its ERIS AO facility to search for visual companions among a sample of 77 objects members of the USco and Taurus young nearby associations with estimated masses in the range between approximately 6-66 M$_{\rm Jup}$. We report the discovery of one companion candidate around a Taurus member with a separation of 111.9$\pm$0.4~mas, or $\sim$18~au assuming a distance of 160~pc, with an estimated primary mass in the range between 3--6~M$_{\rm Jup}$and a secondary mass between 2.6--5.2~M$_{\rm Jup}$ depending on the assumed age. This corresponds to an overall binary fraction of 1.8$^{+2.6}_{-1.3}$\% among low-mass brown dwarfs and free-floating planetary mass objects over the separation range $\ge$7~au. Despite the limitations of small-number statistics and variations in spatial resolution and sensitivity, our results, combined with previous high-spatial-resolution surveys, suggest a notable difference in the multiplicity properties of objects below $\sim$30--50~M$_{\rm Jup}$ between USco and Taurus. In Taurus, a binary fraction of $5.6^{+3.2}_{-2.3}$\% is found for objects with masses below 30M$_{\rm Jup}$, and of $7.8^{+3.0}_{-2.4}$\% for objects with masses below 50M$_{\rm Jup}$, whereas no binary were found among 80 objects over the matching luminosity range in USco, corresponding to an upper limit of $\le$1.2\%.

astro-ph.SR↗

arXiv2026

The internal kinematics of local young stellar associations. Identifying correlations among age, expansion, rotation, and shear

Context. The local (<200 pc away) young (<50 Myr old) stellar associations (LYSA) provide fundamental evidence for the study of the star formation process in the local neighbourhood. Aims. We aim at exploring robust statistical correlations in the internal kinematics of LYSAs and of these with age. Methods. We analyse a public data set containing the linear velocity field parameters and expansion ages of 18 LYSAs. We identify the most robust correlations using frequentist and Bayesian methods. Results. Among the 45 correlations, we identify only four that passed both frequentist and Bayesian criteria, with these four related to radial motions in the Galactic Z direction. We hypothesise several origins for these four correlations and identify the gravitational potential of the Galactic disk as the most likely driving element. It imprinted the observed motions in the parent molecular clouds, and once the stars were formed, it also damped these motions on a timescale shorter than the LYSAs' ages. Conclusions. The internal kinematics of local young stellar associations contain fundamental information about the star-formation process that is not fully addressed by star-formation theories, in particular, rotation and shear. Although the Galactic potential appears to be the driving force of these correlations, we urge the theoretical community to provide predictions about the internal motions of expansion, rotation, and shear of stellar associations.

astro-ph.GA↗

arXiv2025

Euclid: Quick Data Release (Q1) -- A photometric search for ultracool dwarfs in the Euclid Deep Fields

We present a catalogue of 5306 new ultracool dwarf (UCD) candidates in the three Euclid Deep Fields in the Q1 data release. They range from late M to late T dwarfs, and include 1200 L and T dwarfs. A total of 546 objects have been spectroscopically confirmed, including 329 L dwarfs and 26 T dwarfs. We also provide empirical Euclid colours as a function of spectral type. Our UCD selection criteria are based only on colour ($I_\mathrm{E}-Y_\mathrm{E}>2.5$). The combined requirement for optical detection and stringent signal-to-noise ratio threshold ensure a high purity of the sample, but at the expense of completeness, especially for T dwarfs. The detections range from magnitudes 19 and 24 in the near-infrared bands, and extend down to 26 in the optical band. We discuss Euclid's capability to identify UCD candidates based on its photometric passbands. The average surface density of detected UCDs on the sky is approximately 100 objects per $\mathrm{deg}^2$, including 20 L and T dwarfs per $\mathrm{deg}^2$. This leads to an expectation of at least 1.4 million ultracool dwarfs in the final data release of the Euclid Wide Survey, including at least $300\,000$ L dwarfs, and more than 2600 T dwarfs, using the strict selection criteria from this work.

astro-ph.SR↗

arXiv2025

Going Bayesian on the ages of nearby young stellar systems I. The expansion rate method

Context. Determining the ages of young stellar systems is fundamental to test and validate current star-formation theories. Aims. We aim at developing a Bayesian version of the expansion rate method that incorporates the a priori knowledge on the stellar system's age and solves some of the caveats of the traditional frequentist approach. Methods. We upgrade an existing Bayesian hierarchical model with additional parameter hierarchies that include, amongst others, the system's age. For this later, we propose prior distributions inspired by literature works. Results. We validate our method on a set of extensive simulations mimicking the properties of real stellar systems. In stellar associations between 10 and 40 Myr and up to 150 pc the errors are <10%. In star forming regions up to 400 pc, the error can be as large as 80% at 3 Myr but it rapidly decreases with increasing age. Conclusions. The Bayesian expansion rate methodology that we present here offers several advantages over the traditional frequentist version. In particular, the Bayesian age estimator is more robust and credible than the commonly used the frequentist ones. This new Bayesian expansion rate method is made publicly available as a module of the free and open-source code Kalkayotl.

astro-ph.SR↗

arXiv2025

Bayesian ages of local young stellar associations I. Through the expansion rate method

Context. Local young stellar associations (LYSAs <50 Myr and <150 pc) are important laboratories to test predictions from star-formation theories. Estimating their ages through various dating techniques with minimal biases is thus of paramount importance. Aims. We aim at determining the ages of LYSAs with the expansion rate dating technique. Methods. We estimate the ages of the LYSAs using literature membership lists, publicly available data (astrometry and radial velocities), and a recent open-source Bayesian code that implements the expansion rate method. This code in combination with simple statistical assumptions allow us to decontaminate, identify possible substructures or populations, and estimate expansion ages. Results. We derive the largest and most methodological homogeneous set of ages of LYSAs. We rediscover three and discover four associations hidden within the literature membership lists of the classical ones. Conclusions. The expansion ages we report here are compatible with literature age estimates. Moreover, our analysis shows that previous age tensions can be explained, in most cases, by the presence of unidentified populations or substructures.

astro-ph.SR↗

arXiv2025

Free-floating planetary mass objects in LDN 1495 from Euclid Early Release Observations

Context. Substellar objects, including brown dwarfs and free-floating planetary-mass objects, are a significant product of star formation. Their sensitivity to initial conditions and early dynamical evolution makes them especially valuable for studying planetary and stellar formation processes. Aims. We search for brown dwarfs and isolated planetary mass objects in a young star-forming region to better constrain their formation mechanisms. Methods. We took advantage of the Euclid unprecedented sensitivity, spatial resolution and wide field of view to search for brown dwarfs and free-floating planetary mass objects in the LDN 1495 region of the Taurus molecular clouds. We combined the recent Euclid Early Release Observations with older very deep ground-based images obtained over more than 20 yr to derive proper motions and multiwavelength photometry and to select members based on their morphology and their position in a proper motion diagram and in nine color-magnitude diagrams. Results. We identified 15 point sources whose proper motions, colors, and luminosity are consistent with being members of LDN 1495. Six of these objects were already known M9-L1 members. The remaining nine are newly identified sources whose spectral types might range from late-M to early-T types, with masses potentially as low as 1~2 MJup based on their luminosity and according to evolutionary models. However, follow-up observations are needed to confirm their nature, spectral type, and membership. When it is extrapolated to the entire Taurus star-forming region, this result suggests the potential presence of several dozen free-floating planetary mass objects.

astro-ph.SR↗

arXiv2024

Kalkayotl 2.0 Bayesian phase-space modelling of star-forming regions, stellar associations, and open clusters

Context: Star-forming regions, stellar associations, and open clusters are fundamental stellar systems where predictions from star-formation theories can be robustly contrasted with observations. Aims: We aim to provide the astrophysical community with a free and open-source code to infer the phase-space (i.e. positions and velocities) parameters of stellar systems with $\lesssim$1000 stars based on \textit{Gaia} astrometry and possibly observed radial velocities. Methods: We upgrade an existing Bayesian hierarchical model and extend it to model 3D (positions) and 6D (positions and velocities) stellar coordinates and system parameters with a flexible variety of statistical models, including a linear velocity field. This velocity field allows for the inference of internal kinematics, including expansion, contraction, and rotation. Results: We extensively validated our statistical models using realistic simulations that mimic the properties of the \textit{Gaia} Data Release 3. We applied \textit{Kalkayotl} to $β$-Pictoris, the Hyades, and Praesepe, recovering parameter values compatible with those from the literature. In particular, we found an expansion age of $19.1\pm1.0$ Myr for $β$-Pictoris and rotational signal of $32\!\pm\!11\,\rm{m\,s^{-1}\,pc^{-1}}$ for the Hyades and that Praesepe's rotation reported in the literature comes from its periphery. Conclusions: The robust and flexible Bayesian hierarchical model that we make publicly available here represents a step forward in the statistical modelling of stellar systems. The products it delivers, such as expansion, contraction, rotation, and velocity dispersions, can be directly contrasted with predictions from star-formation theories.

astro-ph.GA↗

arXiv2024

Dynamical traceback age of the Octans young stellar association

Octans is one of the most distant ($d\sim150$pc) young stellar associations of the solar neighbourhood. Its age is still poorly constrained in the literature and requires further investigation. We take advantage of the state-of-the-art astrometry delivered by the third data release of the Gaia space mission combined with radial velocity measurements obtained from high-resolution spectroscopy to compute the 3D positions and 3D spatial velocities of the stars and derive the dynamical traceback age of the association. We performed an extensive traceback analysis using different subsamples of stars, different metrics to define the size of the association, and different models for the Galactic potential to integrate the stellar orbits in the past. We derive a dynamical age of $34^{+2}_{-2}$Myr that is independent from stellar models and represents the most precise age estimate currently available for the Octans association. After correcting the radial velocity of the stars for the effect of gravitational redshift, we obtain a dynamical age of $33^{+3}_{-1}$Myr, which is in very good agreement with our first solution. This shows that the effect of gravitational redshift is small for such a distant young stellar association. Our result is also consistent with the less accurate age estimates obtained in previous studies from lithium depletion (30-40Myr) and isochrones (20-30Myr). By integrating the stellar orbits in time, we show that the members of Octans and Octans-Near had different locations in the past, which indicates that the two associations are unrelated despite the close proximity in the sky. Our results confirm that it is possible to derive precise dynamical ages via the traceback method for $\sim30$Myr old stellar clusters at about $\sim150$pc with the same precision level that has been achieved in other studies for young stellar groups within 50pc of the Sun.

astro-ph.GA↗

arXiv2024

Euclid: Early Release Observations -- A glance at free-floating new-born planets in the sigma Orionis cluster

We provide an early assessment of the imaging capabilities of the Euclid space mission to probe deeply into nearby star-forming regions and associated very young open clusters, and in particular to check to what extent it can shed light on the new-born free-floating planet population. This paper focuses on a low-reddening region observed in just one Euclid pointing where the dust and gas has been cleared out by the hot sigma Orionis star. One late-M and six known spectroscopically confirmed L-type substellar members in the sigma Orionis cluster are used as benchmarks to provide a high-purity procedure to select new candidate members with Euclid. The exquisite angular resolution and depth delivered by the Euclid instruments allow us to focus on bona-fide point sources. A cleaned sample of sigma Orionis cluster substellar members has been produced and the initial mass function (IMF) has been estimated by combining Euclid and Gaia data. Our sigma Orionis substellar IMF is consistent with a power-law distribution with no significant steepening at the planetary-mass end. No evidence of a low-mass cutoff is found down to about 4 Jupiter masses at the young age (3 Myr) of the sigma Orionis open cluster.

astro-ph.EP↗

arXiv2023

Young nearby open clusters and their luminosity functions

Context. Open clusters are groups of coeval stars sharing properties such as distance and metallicity, and they are key to understanding stellar evolution. Aims. Our main goal is to study the evolution of open clusters with a special focus on the universality of the luminosity function. Methods. We applied an upgraded version of the convergent point technique on about 50 open clusters. The selection of cluster members was based purely on the exquisite astrometry of the Gaia DR3 and Hipparcos catalogues in the five-dimensional or full six-dimensional space. Results. We present updated lists of bona fide members of ~50 open clusters within 500 pc and younger than 1 Gyr, exploiting the full depth of the third Gaia data release complemented by Hipparcos at the bright end, excluding regions in the Galactic plane. Our catalogues also are complemented by optical and infrared photometry from the major large-scale public surveys. All the data will be made available on a dedicated webpage with interactive plots and a direct link to Aladin and Vizier hosted at the Centre de Données de Strasbourg. We derived luminosity functions for all bound clusters and compared them in three age groups of ~50 Myr, ~150 Myr, and ~600 Myr, discussing similarities and differences to constrain their dynamical evolution. Conclusions. Luminosity functions of clusters at 50 Myr are more likely similar to each other and show a greater degree of similarity than older clusters. We explain this observation with the universal luminosity function within the volume of our sample (500 pc). Luminosity functions of clusters with ages similar to the Pleiades or Hyades are more diverse, perhaps due to internal dynamical evolution, but more work is needed to provide additional evidence.

astro-ph.SR↗

arXiv2023

The cosmic waltz of Coma Berenices and Latyshev 2 (Group X). Membership, phase-space structure, mass, and energy distributions

Context. Open clusters (OCs) are fundamental benchmarks where theories of star formation and stellar evolution can be tested and validated. Coma Ber and Latyshev 2 (Group X) are the second and third OCs closest to the Sun, making them excellent targets to search for low-mass stars and ultra-cool dwarfs. In addition, this pair will experience a flyby in 10-16 Myr which makes it a benchmark to test OCs pair interactions. Aims. We aim at analysing the membership, luminosity, mass, phase-space (i.e., positions and velocities), and energy distributions for Coma Ber and Latyshev 2 and test the hypothesis of the mixing of their populations at the encounter time. Methods. We develop a new phase-space membership methodology and apply it to Gaia data. With the recovered members we infer the phase-space, luminosity and mass distributions using publicly available Bayesian inference codes. Then, with a publicly available orbit integration code and members' positions and velocities, we integrate their orbits 20 Myr into the future. Results. In Coma Ber, we identify 302 candidate members distributed in the core and tidal tails. The tails are dynamically cold and asymmetrically populated. The stellar system called Group X is made of two structures: the disrupted OC Latyshev 2 (186 candidate members) and a loose stellar association called Mecayotl 1 (146 candidate members), both of them will fly by Coma Ber in $11.3\pm0.5$ Myr and $14.0\pm0.6$ Myr, respectively, and each other in $8.1\pm1.3$ Myr. Conclusions. We study the dynamical properties of the core and tails of Coma Ber and also confirm the existence of the OC Latyshev 2 and its neighbour stellar association Mecayotl 1. Although these three systems will experience encounters we find no evidence supporting the mixing of their populations.

astro-ph.SR↗

arXiv2022

The cosmic DANCe of Perseus I: Membership, phase-space structure, mass, and energy distributions

Context. Star-forming regions are excellent benchmarks for testing and validating theories of star formation and stellar evolution. The Perseus star-forming region being one of the youngest (<10 Myr), closest (280-320 pc), and most studied in the literature, is a fundamental benchmark. Aims. We aim to study the membership, phase-space structure, mass, and energy (kinetic plus potential) distribution of the Perseus star-forming region using public catalogues (Gaia, APOGEE, 2MASS, PanSTARRS). Methods. We use Bayesian methodologies accounting for extinction to identify the Perseus physical groups in the phase-space, retrieve their candidate members, derive their properties (age, mass, 3D positions, 3D velocities, and energy), and attempt to reconstruct their origin. Results. We identify 1052 candidate members in seven physical groups (one of them new) with ages between 3 and 10 Myr, dynamical super-virial states, and large fractions of energetically unbound stars. Their mass distributions are broadly compatible with that of Chabrier for masses >0.1 $M_\odot$ and do not show hints of over-abundance of low-mass stars in NGC1333 with respect to IC348. These groups' ages, spatial structure, and kinematics are compatible with at least three generations of stars. Future work is still needed to clarify if the formation of the youngest was triggered by the oldest. Conclusions. The exquisite Gaia data complemented with public archives and mined with comprehensive Bayesian methodologies allow us to identify 31% more members than in previous studies, discover a new physical group (Gorgophone: 7 Myr, 191 members, and 145 $M_\odot$), and confirm that the spatial, kinematic, and energy distributions of these groups support the hierarchical star-formation scenario.

astro-ph.SR↗

arXiv2022

The star formation history of Upper Scorpius and Ophiuchus

We aim to study spatial and kinematic substructures of the region encompassed by Upper Scorpius and Ophiuchus star-forming regions, determining dynamical traceback ages and studying the star formation history of the complex. We identified seven different groups in this region. Four groups (nu Sco, beta Sco, sigma Sco and delta Sco) are part of Upper Scorpius, two groups (rho Oph and alpha Sco) are in Ophiuchus, and another group (pi Sco) is a nearby young population. We found an age gradient from the rho Oph group (the youngest) to the delta Sco group (<~5 Myr), showing that star formation was a sequential process for the past 5 Myr. Our traceback analysis shows that Upper Scorpius and rho Oph groups share a common origin. The closer group of pi Sco is probably older, and the traceback analysis suggests that this group and the alpha Sco group have a different origin, likely related to other associations in the Sco-Cen complex. Our study shows that this region has a complex star formation history that goes beyond the current formation scenario, likely a result of stellar feedback from massive stars, supernovae explosions, and dynamic interactions between stellar groups and the molecular gas. In particular, we speculate that photo-ionisation from the massive delta Sco star could have triggered star formation first in the beta Sco group and then in the nu Sco group. The perturbations of stellar orbits due to stellar feedback and dynamical interactions could also be responsible for the 1-3 Myr difference that we found between dynamical traceback ages and isochronal ages.

astro-ph.GA↗

arXiv2022

Unraveling UBC 274: a morphological, kinematical and chemical analysis of a disrupting open cluster

We do a morphological, kinematic and chemical analysis of the disrupting cluster UBC 274 (2.5 Gyr, $d=1778$ pc) to study its global properties. We use HDBSCAN to obtain a new membership list up to 50 pc from its centre and up to magnitude $G=19$ using Gaia EDR3 data. We use high resolution and high signal-to-noise spectra to obtain atmospheric parameters of 6 giants and subgiants, and individual abundances of 18 chemical species. The cluster has a highly eccentric (0.93) component, tilted $\sim$10 deg with respect to the plane of the Galaxy, which is morphologically compatible with the result of a test-particle simulation of a disrupting cluster. Our abundance analysis shows that the cluster has a subsolar metallicity of [Fe/H]$=-0.08\pm0.02$. Its chemical pattern is compatible with that of Ruprecht 147, of similar age but located closer to the Sun, with the remarkable exception of neutron-capture elements, which present an overabundance of $[n\mathrm{/Fe]}\sim0.1$. The cluster's elongated morphology is associated with the internal part of its tidal tail, following the expected dynamical process of disruption. We find a significant sign of mass segregation where the most massive stars appear 1.5 times more concentrated than other stars. The cluster's overabundance of neutron-capture elements can be related to the metallicity dependence of the neutron-capture yields due to the secondary nature of these elements, predicted by some models. UBC 274 presents a high chemical homogeneity at the level of $0.03$ dex in the sampled region of its tidal tails.

astro-ph.GA↗

arXiv2022

Infrared Spectroscopy of free-floating planet candidates in Upper Scorpius and Ophiuchus

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 standards. Results: All the targets but one are confirmed as young ultracool objects, with spectral types between L0 and L6 and masses in the range 0.004-0.013 M according to evolutionary models. The status of the last target is unclear at this point. Conclusions: Only one possible contaminant has been identified among the 17 targets, suggesting that the contamination level of the original sample must be lower than 6%

astro-ph.SR↗