Aims:The nature of S Ori 70 (S Ori J053810.1-023626), a faint mid-T type object found towards the direction of the young σ Orionis cluster, is still under debate. We intend to find out whether it is a field brown dwarf or a 3-Myr old planetary-mass member of the cluster. Methods: We report on near-infrared {JHK}s and mid-infrared [3.6] and [4.5]

Context: Multi-conjugate adaptive optics (MCAO) combine the advantages of both standard adaptive optics, which provide high contrast and high spatial resolution, and of wide field imaging (≈1'). Up to recently, MCAO for astronomy was limited to laboratory experiments. In this paper, we present the first scientific results obtained with the first

Aims:We study the contradictory magnetic field strength distributions retrieved from independent analyses of spectropolarimetric observations in the near-infrared (1.56 μm) and in the visible (630 nm) spectral ranges in internetwork regions. Methods: To solve this apparent controversy, we present simultaneous and co-spatial 1.56 μm and 630 nm

Aims:We study the properties and nature of extremely red galaxies (ERO, R-K≥5.6) found behind two lensing clusters and compare them with other known galaxy populations. Methods: New HST/ACS, Spitzer (IRAC and MIPS), and Chandra/ACIS observations of the two lensing clusters Abell 1835 and AC114 have been obtained, combined with our earlier optical

We report on new nearby L and late-M dwarfs (dphot = 3.0, later than M8.0) at low Galactic latitude (|b| 15°) over 4800 deg2 in the Deep Near Infrared Survey of the Southern Sky (DENIS) data base. We used late-M (>= M8.0), L and T dwarfs with accurate trigonometric parallaxes to calibrate the MJ versus I-J colour-luminosity relation. The resulting

Non-LTE effects affect the formation of many spectral features in the spectra of late-type stars. This thesis investigates the effect on carbon and oxygen abundances. The non-LTE calculations have been performed with the code MULTI, using MARCS atmospheric models with 4500