One of the main results obtained by the large optical and infrared sky surveys (DENIS, SDSS, 2MASS, UKIDSS, WISE) that started at the end of the last century, has been the discovery of hundreds of ultracool dwarfs in the solar neighborhood. Spectroscopic characterization of these objects has led to the definition of 3 new spectral classes, namely L, T and Y, which have extended the systematic classification of atmospheres to ever decreasing effective temperatures. The diversity of physical properties in these objects provide a natural link between the very low-mass stars and the giant planets. The study of ultracool dwarfs provides constraints to the models of formation and evolution of stars and planets, it is also a testbed for atmospheric models which are needed to determine fundamental properties using spectral synthesis techniques, and it is a key ingredient to obtain a complete knowledge of the solar neighborhood and the low-mass tail of the stellar populations in the Milky Way.
To advance in the study of ultracool dwarfs, this project has the following objectives: 1) Exploitation of the large infrared surveys currently underway with VISTA to detect and characterize new ultracool dwarfs in the solar neighborhood.
2) Scientific preparation of the Euclid mission where we are poised to play a leading role in the field of ultracool dwarfs thanks to ESA support and this coordinated project. 3) Search and detailed characterization of subestelar companions to young nearby stars and of ultracool subdwarfs; and determination of the mass function in young clusters and the halo.
The method to be used for objective 1 will be based on previous programs which demonstrated a highly successful rate of confirmation of candidates with low-resolution reconnaissance spectroscopy. Improvement of the ultracool dwarf search tenchniques will be seeked and optimized to prepare for efficient exploitation of the Euclid survey (objective 2). To address objective 2 numerical simulations will be developed so that the number of different types of ultracool dwarfs to be detected with JPAS and Euclid can be estimated, not only as a function of the underlying properties of the populations (which will be studied in objective 3), but also as a function of the survey parameters. Reference catalogs will be prepared to be used as tools for survey calibrations. Particular attention will be devoted to the use of GAIA results. PSF modelization will be carried out to estimate the sensitivity of Euclid for the detection of close and faint companions to stars.
Objective 3 will be accomplished by using large surveys (2MASS, WISE, VHS) that will provide identification of high proper motion objects, substellar companions at large separations, ultracool subdwarfs, and planetary mass objects in young clusters. The basic physical properties of these objects will be studied by means of spectra to be obtained with OSIRIS and EMIR at GTC, LIRIS at WHT, SOFi at NTT and XShooter at the VLT. These data will provide characterization of benchmark ultracool dwarfs that belong to binary systems or clusters of known age, distance and metallicity. Particular attention will be paid to comparison between the properties of ultracool dwarfs and exoplanets in the range of overlapping temperatures.
