L. Valdivielso

Follow-up observations of binary ultra-cool dwarfs

Context. Astrometric observations of resolved binaries provide estimates of orbital periods and will eventually lead to measurement of dynamical masses. Only a few very low mass star and brown dwarf masses have been measured to date, and the mass-luminosity relation still needs to be calibrated. Aims. We have monitored 14 very low mass multiple systems for several years to confirm their multiplicity and, for those with a short period, derive accurate orbital parameters and dynamical mass estimates. Methods. We have used high spatial resolution images obtained at the Paranal, Lick and HST observatories to obtain astrometric and photometric measurements of the multiple systems at several epochs. The targets have periods ranging from 5 to 200 years, and spectral types in the range M7.5–T5.5. Results. All of our 14 multiple systems are confirmed as common proper motion pairs. One system (2MASSW J0920122+351742) is not resolved in our new images, probably because the discovery images were taken near maximum elongation. Six systems have periods short enough to allow dynamical mass measurements within the next 15 to 20 years.We estimate that only 8% of the ultracool dwarfs in the solar neighborhood are binaries with separations large enough to be resolved, and yet periods short enough to derive astrometric orbital fits over a reasonable time frame with current instrumentation. A survey that doubles the number of ultracool dwarfs observed with high angular resolution is called for to discover enough binaries for a first attempt to derive the mass-luminosity relationship for very low-mass stars and brown dwarfs.

An IPHAS-based search for accreting very low-mass objects using VO tools

Context. The main goal of this paper is to prove that accreting very low-mass stars and brown dwarfs can be identified in IPHAS, an Hα emission survey of the northern Milky Way. Full exploitation of the IPHAS database and a future extension of it in the southern hemisphere will be useful in identifying very low-mass accreting objects near to and far from well-known star-forming regions. Aims. We have used Virtual Observatory tools to cross-match the IPHAS catalogue with the 2MASS catalogue. We defined photometric criteria to identify Hα emission sources with near-infrared colours similar to those of known young very low-mass stars and brown dwarfs. 4000 candidates were identified that met our criteria over an area of 1600 square degrees. We present low-resolution optical spectra of 113 candidates. Spectral types have been derived for the 33 candidates that have spectroscopically confirmed Hα emission, negligible reddening and spectral class M. We have also measured Hα emission and investigated the NaI doublet (818.3 nm, 819.5 nm) in these 33 objects. Methods. We confirm that 33 IPHAS candidates have strong Hα emission indicative of disc accretion for their spectral type. Twenty-three of them have spectral class M4 or later, of which ten have classes in the range M5.5–M7.0 and could thus be very young brown dwarfs. Many objects also have a weak NaI doublet, an indication of low surface gravity. Results. IPHAS provides a very valuable database for identifying accreting very low-mass stars and brown dwarfs. Virtual Observatory tools provide an efficient method for identifying these objects over large areas of the sky. Based on our success rate of 23 Hα emission objects with spectral type in the range M4–M7 out of 113 candidates with spectroscopic follow-up, we estimate that there could be hundreds of such objects in the full IPHAS survey.

A New Gas Cell for High-precision Doppler Measurements in the Near-infrared

High-resolution spectroscopy in the near-infrared could become the leading method for discovering extra-solar planets around very low mass stars and brown dwarfs. In order to help to achieve an accuracy of {approx}m s{sup -1}, we are developing a gas cell which consists of a mixture of gases whose absorption spectral lines span all over the near-infrared region. We present the most promising mixture, made of acetylene, nitrous oxide, ammonia, chloromethanes, and hydrocarbons. The mixture is contained in a small size 13 cm long gas cell and covers most of the H and K bands. It also shows small absorptions in the J band, but they are few and not sharp enough for near-infrared wavelength calibration. We describe the working method and experiments, and compare our results with the state of the art for near-infrared gas cells.

The Least Massive (Sub)Stellar Component of the Milky Way

This review presents a panorama of the research topics that are currently being developed by our strategic research group at the IAC in the field of very low-mass stars, brown dwarfs, extrasolar giant planets and the solar system. Our main goal is to investigate the cosmogony of the least massive stellar and substellar component of the Milky Way. We are using multiwavelength observations and theoretical modeling to provide constrains to different scenarios of star and planet formation. We present summaries of the following results: (1) the study of a deeply-embedded low-mass protostar in the B59 molecular cloud; (2) the discovery of accreting very low-mass objects in the IPHAS survey; (3) the identification of faint planetary-mass candidates in the cores of young open clusters using multi-conjugate adaptive optics; (4) the discovery of a widely separated companion of a young brown dwarf; (5) the search for reflected light from hot Jupiters; and (6) the spectroscopic study of relics of the formation of our Solar System.

Gas Cell Development for Infrared Spectra Calibration

NAHUAL is a high-resolution near-infrared echelle spectrograph of high stability on preliminary phase development for GTC (Gran Telescopio de Canarias). Its natural location is a Nasmyth focus. One of the principal scientific aims is to carry out high precision radial velocity measurements (from 1 to 10 m/s) in the near infrared. To achieve high stability on radial velocity measurements, NAHUAL needs a calibration unit that uses a mixture of gases whose absorption spectra must be as homogeneous as possible between 0.95 and 2.4 μm. We report on the measurements done to date with potentially active gas mixtures as acetylene, methane, nitrous oxide or hydrocarbons.