The aim of the project is to characterise both components of the nearest brown dwarf sytem to the Sun, WISE J104915.57-531906.1 (=Luhman16AB) at optical and near-infrared wavelengths. We obtained high signal-to-noise intermediate-resolution (R~6000-1
1000) optical (600-1000 nm) and near-infrared (1000-2480nm) spectra of each component of Luhman16AB, the closest brown dwarf binary to the Sun, with the X-Shooter instrument on the Very Large Telescope. We classify the primary and secondary of the Luhman16 system as L6-L7.5 and T0+/-1, respectively, in agreement with previous measurements published in the literature. We present measurements of the lithium pseudo-equivalent widths, which appears of similar strength on both components (8.2+/-1.0 Angstroms and 8.4+/-1.5 Angstroms for the L and T components, respectively). The presence of lithium (Lithium 7) in both components imply masses below 0.06 Msun while comparison with models suggests lower limits of 0.04 Msun. The detection of lithium in the T component is the first of its kind. Similarly, we assess the strength of other alkali lines (e.g. pseudo-equivalent widths of 6-7 Angstroms for RbI and 4-7 Angstroms for CsI) present in the optical and near-infrared regions and compare with estimates for L and T dwarfs. We also derive effective temperatures and luminosities of each component of the binary: -4.66+/-0.08 dex and 1305(+180)(-135) for the L dwarf and -4.68+/-0.13 dex and 1320(+185)(-135) for the T dwarf, respectively. Using our radial velocity determinations, the binary does not appear to belong to any of the well-known moving group. Our preliminary theoretical analysis of the optical and J-band spectra indicates that the L- and T-type spectra can be reproduced with a single temperature and gravity but different relative chemical abundances which impact strongly the spectral energy distribution of L/T transition objects.
The aim of the project is to search for lithium in absorption at 6707.8 Angstroms to constrain the nature and the mass of the brightest low-metallicity L-type dwarfs (refered to as L subdwarfs) identified in large-scale surveys. We obtained low- to
intermediate-resolution (R~2500-9000) optical (~560-770 nm) spectra of two mid-L subdwarfs, SDSSJ125637.13-022452.4 (SDSS1256; sdL3.5) and 2MASSJ162620.14+392519.5 (2MASS1626; sdL4) with spectrographs on the European Southern Observatory Very Large Telescope and the Gran Telescopio de Canarias. We report the presence of a feature at the nominal position of the lithium absorption doublet at 6707.8 Angstroms in the spectrum of SDSS1256, with an equivalent width of 66+/-27 Angstroms at 2.4 sigma, which we identify as arising from a CaH molecular transition based on atmosphere models. We do not see any feature at the position of the lithium feature in the spectrum of 2MASS1626. The existence of overlapping molecular absorption sets a confusion detection limit of [Li/H]=-3 for equivalently-typed L subdwarfs. We provided improved radial velocity measurements of -126+/-10 km/s and -239+/-12 km/s for SDSS1256 and 2MASS1626, respectively, as well as revised Galactic orbits. We implemented adjusting factors for the CaH molecule in combination with the NextGen atmosphere models to fit the optical spectrum of SDSS1256 in the 6200-7300 Angstroms range. We also estimate the expected Li abundance from interstellar accretion ([Li/H]=-5), place limits on circumstellar accretion (10^9 g/yr), and discuss the prospects of Li searches in cooler L and T subdwarfs.
We present the results of photometric, astrometric, and spectroscopic follow-up of L dwarf candidates identified in the Hyades cluster by Hogan et al. (2008). We obtained low-resolution optical spectroscopy with the OSIRIS spectrograph on the Gran Te
lescopio de Canarias for all 12 L dwarf candidates as well as new J-band imaging for a subsample of eight to confirm their proper motion. We also present mid-infrared photometry from the Wise Field Infrared Survey Explorer (WISE) for the Hyades L and T dwarf candidates and estimate their spectroscopic distances, effective temperatures, and masses. We confirm the cool nature of several L dwarf candidates and confirm astrometrically their membership, bridging the gap between the coolest M dwarfs and the two T dwarfs previously reported in the Hyades cluster. These members represent valuable spectral templates at an age of 625 Myr and slightly super solar metallicity (Fe/H=+0.13). We update the Hyades mass function across the hydrogen-burning limit and in the substellar regime. We confirm a small number numbers of very-low-mass members below ~0.1 Msun belonging to the Hyades cluster.
The aim of the project is to improve our knowledge on the multiplicity of planet-host stars at wide physical separations. We cross-matched approximately 6200 square degree area of the Southern sky imaged by the Visible Infrared Survey Telescope for
Astronomy (VISTA) Hemisphere Survey (VHS) with the Two Micron All Sky Survey (2MASS) to look for wide common proper motion companions to known planet-host stars. We complemented our astrometric search with photometric criteria. We confirmed spectroscopically the co-moving nature of seven sources out of 16 companion candidates and discarded eight, while the remaining one stays as a candidate. Among these new wide companions to planet-host stars, we discovered a T4.5 dwarf companion at 6.3 arcmin (~9000 au) from HIP70849, a K7V star which hosts a 9 Jupiter mass planet with an eccentric orbit. We also report two new stellar M dwarf companions to one G and one metal-rich K star. We infer stellar and substellar binary frequencies for our complete sample of 37 targets of 5.4+/-3.8% and 2.7+/-2.7% (1 sigma confidence level), respectively, for projected physical separations larger than ~60-160 au assuming the range of distances of planet-host stars (24-75 pc). These values are comparable to the frequencies of non planet-host stars. We find that the period-eccentricity trend holds with a lack of multiple systems with planets at large eccentricities (e > 0.2) for periods less than 40 days. However, the lack of planets more massive than 2.5 Jupiter masses and short periods (<40 days) orbiting single stars is not so obvious due to recent discoveries by ground-based transit surveys and space missions.
We present the results of a deep ZYJ near-infrared survey of 13.5 square degrees in the Upper Scorpius (USco) OB association. We photometrically selected ~100 cluster member candidates with masses in the range 30-5 Jupiters, according to state-of-the
-art evolutionary models. We identified 67 ZYJ candidates as bona-fide members, based on complementary photometry and astrometry. We also extracted five candidates detected with VISTA at YJ-only. One is excluded using deep optical z-band imaging, while two are likely non-members, and three remain as potential members. We conclude that the USco mass function is more likely decreasing in the planetary-mass regime (although a flat mass function cannot yet be discarded), consistent with surveys in other regions.
We analysed the wide-field near-infrared survey of the Praesepe cluster carried out by the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS) and released by the Data Release 9 (DR9). We compare our Praesepe mass function (MF) wit
h the ones of the Pleiades, alpha Per, and the Hyades. We also present preliminary results of a spectroscopic follow-up for the low mass members (M=<0.1Msol) in Praesepe, alpha Per and Pleiades using the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) mounted on the 10.4m Gran Telescopio Canarias (GTC). We also present the optical spectrum of the first L dwarf in Praesepe.
We have obtained a low-resolution optical spectrum for one of the faintest cluster member candidates in Praesepe with the Optical System for Imaging and low Resolution Integrated Spectroscopy mounted on the 10.4m Gran Telescopio de Canarias. We confi
rm spectroscopically the first L dwarf member in Praesepe, UGCS J084510.66+214817.1. We derived an optical spectral type of L0.3+/-0.4 and estimated its effective temperature to Teff=2279+/-371 K and a mass of 71.1+/-23.0M_Jup, according to state-of-the-art models, placing it at the hydrogen-burning boundary. We measured the equivalent width of the gravity-sensitive sodium doublet at 8182/8194 A, which adds credit to the membership of this new L dwarf to Praesepe. We also derived a probability of ~20.5% that our candidate would be a field L0 dwarf. We conclude that this object is likely to be a true member of Praesepe, with evidence of being a binary system.
We present new z- and H-band photometry and proper motion measurements for the five candidate very-low-mass T-type objects we recently proposed to be members of the nearest OB association to the Sun, Upper Scorpius. These new data fail to corroborate
our prior conclusions regarding their spectral types and affiliation with the Upper Scorpius population. We conclude that we may be in presence of a turnover in the mass function of Upper Sco taking place below 10-4 Jupiter masses, depending on the age assigned to Upper Sco and the models used.
The aim of the project is to improve our current knowledge of the density of T dwarfs and the shape of the substellar initial mass function by identifying a magnitude-limited sample of T dwarfs in the full southern sky. We present the results of a
photometric search aimed at discovering cool brown dwarfs in the Southern sky imaged at infrared wavelengths by the Visible and Infrared Survey Telescope for Astronomy (VISTA) and the Wide Infrared Survey Explorer (WISE) satellite mission. We combined the first data release (DR1) of the VISTA Hemisphere Survey (VHS) and the WISE preliminary data release to extract candidates with red mid-infrared colours and near- to mid-infrared colours characteristics of cool brown dwarfs. The VHS DR1 vs WISE search returned tens of T dwarf candidates, 13 of which are presented here, including two previously published in the literature and five new ones confirmed spectroscopically with spectral types between T4.5 and T8. We estimate that the two T6 dwarfs lie within 16 pc and the T4.5 within 25 pc. The remaining three are 30-50 pc distant. The only T7 dwarf in our sample is the faintest of its spectral class with J=19.28 mag. The other six T dwarf candidates remain without spectroscopic follow-up. We also improve our knowledge on the proper motion accuracy for three bright T dwarfs by combining multi-epoch data from public databases (DENIS, 2MASS, VHS, WISE, Spitzer).
Over the past decades open clusters have been the subject of many studies. Such studies are crucial considering that the universality of the Initial Mass Function is still a subject of current investigations. Praesepe is an interesting open cluster f
or the study of the stellar and substellar mass function (MF), considering its intermediate age and its nearby distance. Here we present the results of a wide field, near-infrared study of Praesepe using the Data Release 9 (DR9) of the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS). We obtained cluster candidates of Praesepe based on a 3sigma astrometric and 5 band photometric selection. We derived a binary frequency for Praesepe of 25.6+/-3.0% in the 0.2-0.45Msol mass range, 19.6+/-3.0% for 0.1-0.2Msol, and 23.2+/-5.6% for 0.07-0.1Msol. We also studied the variability of the cluster candidates of Praesepe and we conclude that seven objects could be variable. We inferred the luminosity function of Praesepe in the Z- and J- bands and derived its MF. We observe that our determination of the MF of Praesepe differs from previous studies: while previous MFs present an increase from 0.6 to 0.1Msol, our MF shows a decrease. We looked at the MF of Praesepe in two different regions of the cluster, i.e. within and beyond 1.25deg, and we observed that both regions present a MF which decrease to lower masses. We compared our results with the Hyades, the Pleiades and alpha Per MF in the mass range of 0.072-0.6Msol and showed that the Praesepe MF is more similar to alpha Per although they are respectively aged ~85 and ~600Myr. Even though of similar age, the Praesepe remains different than the Hyades, with a decrease in the MF of only ~0.2 dex from 0.6 down to 0.1Msol, compared to ~1 dex for the Hyades.
