Search

Ultracool dwarfs (UCDs) encompass a wide variety of compact stellar-like objects with spectra classified as late-M, L, T, and Y. Most of them have been discovered using wide-field imaging surveys. The Virtual Observatory (VO) has proven to be of great utility to efficiently exploit these astronomical resources. We aim to validate a VO methodology designed to discover and characterize UCDs in deep extragalactic surveys like Advance Large Homogeneous Area Medium-Band Redshift Astronomical (ALHAMBRA) and Cosmological Evolution Survey (COSMOS). Three complimentary searches based on parallaxes, proper motions and colours, respectively, were carried out. A total of 897 candidate UCDs were found, with only 16 previously reported in SIMBAD. Most of the new UCDs reported here are likely late-M and L dwarfs because of the limitations imposed by the utilization of optical (Gaia DR2 and r -band) data. We complement ALHAMBRA and COSMOS photometry with other catalogues in the optical and infrared using VOSA, a VO tool that estimates effective temperatures from the spectral energy distribution fitting to collections of theoretical models. The agreement between the number of UCDs found in the COSMOS field and theoretical estimations together with the low false-negative rate (known UCDs not discovered in our search) validates the methodology proposed in this work, which will be used in the forthcoming wide and deep surveys provided by the Euclid space mission. Simulations of Euclid number counts for UCDs detectable in different photometric passbands are presented for a wide survey area of 15 000 deg2, and the limitations of applicability of Euclid data to detect UCDs using the methods employed in this paper are discussed. [ABSTRACT FROM AUTHOR]

We present the first release of GTC OSIRIS broad-band data archive. This is an effort conducted in the framework of the Spanish Virtual Observatory to help optimize science from the Gran Telescopio Canarias Archive. Data Release 1 includes 6788 broad-band images in the Sloan griz filters obtained between 2009 April and 2014 January and the associated catalogue with roughly 6.23 million detections of more than 630 000 unique sources. The catalogue contains standard PSF and Kron aperture photometry with a mean accuracy better than 0.09 and 0.15 mag, respectively. The relative astrometric residuals are always better than 30 mas and better than 15 mas in most cases. The absolute astrometric uncertainty of the catalogue is of 0.12 arcsec. In this paper we describe the procedure followed to build the image archive and the associated catalogue, as well as the quality tests carried out for validation. To illustrate some of the scientific potential of the catalogue, we also provide two examples of its scientific exploitation: discovery and identification of asteroids and cool dwarfs. [ABSTRACT FROM AUTHOR]

We used the OSIRIS camera at the 10.4 m Gran Telescopio Canarias (GTC) to monitor the astrometric motion of the L4.5 dwarf 2M1821+14 over 17 months. The astrometric residuals of 11 epochs have an rms dispersion of 0.4 mas, which is larger than the average precision of 0.23 mas per epoch and hints towards an additional signal or excess noise. Comparison of the pointspread functions in OSIRIS and FORS2/VLT images reveals no differences critical for highprecision astrometry, despite the GTC's segmented primary mirror. We attribute the excess noise to an unknown effect that may be uncovered with additional data. For 2M1821+14, we measured a relative parallax of 106.15 ± 0.18 mas and determined a correction of 0.50 ± 0.05 mas to absolute parallax, leading to a distance of 9.38 ± 0.03 pc. We excluded at 3s confidence the presence of a companion to 2M1821+14 down to a mass ratio of 0.1 (≈5MJupiter) with a period of 50-1000 d and a separation of 0.1-0.7 au. The accurate parallax allowed us to estimate the age and mass of 2M1821+14 of 120-700 Myr and 0.049-0.024+0.014M☉, thus confirming its intermediate age and substellar mass. We complement our study with a parallax and proper motion catalogue of 587 stars (i' ≃ 15.5-22) close to 2M1821+14, used as astrometric references. This study demonstrates submas astrometry with the GTC, a capability applicable for a variety of science cases including the search for extrasolar planets and relevant for future astrometric observations with E-ELT and TMT. [ABSTRACT FROM AUTHOR]

ABSTRACT NGC 6823 is a young open cluster that lies at a distance of ∼2 kpc in the Vulpecula OB1 association. Previous studies using CCD photometry and spectroscopy have identified a Trapezium system of bright O- and B-type stars at its centre, along with several massive O-, B- and A-type stars in the cluster. We present optical VRI and near-infrared JHK photometric observations, complemented with Spitzer/Infrared Array Camera archival data, with an aim to identify the young low-mass population and the disc candidates in this region. Our survey reaches down to I∼ 22 mag and Ks∼ 18 mag. There is significant differential reddening within the cluster. We find a bimodal distribution for A V, with a peak at ∼3 mag and a broader peak at ∼10 mag. We have classified the sources based on the [4.5] − [8] colour, which is least affected by extinction. We find a ∼20 per cent fraction of Class I/Class II young stellar objects (YSOs) in the cluster, while a large 80 per cent fraction of the sources have a Class III classification. We have made use of the INT Photometric Hα Survey (IPHAS) in order to probe the strength in Hα emission for this large population of Class III sources. Nearly all of the Class III objects have photospheric ( r′−Hα) colours, implying an absence of Hα in emission. This large population of Class III sources is thus likely the extinct field star population rather than the discless YSOs in the cluster. There is a higher concentration of the Class I/II systems in the eastern region of the cluster and close to the central Trapezium. The western part of the cluster mostly contains Class III/field stars and seems devoid of disc sources. We find evidence of a pre-main-sequence population in NGC 6823, in addition to an upper main-sequence population. The pre-main-sequence population mainly consists of young disc sources with ages between ∼1 and 5 Myr, and at lower masses of ∼0.1-0.4 M⊙. There may be a possible mass-dependent age spread in the cluster, with the older stars being more massive than the younger ones. The presence of young disc sources in NGC 6823 indicates similar star formation properties in the outer regions of the Galaxy as observed for young clusters in the solar neighbourhood. [ABSTRACT FROM AUTHOR]