International audienc

Aims. We investigate the mass function in the substellar domain down to a few Jupiter masses in the young σ Orionis open cluster (3 ± 2 Ma, d = 360 +70 −60 pc). Methods. We have performed a deep IJ-band search, covering an area of 790 arcmin 2 close to the cluster centre. This survey was complemented with an infrared follow-up in the HK s -and Spitzer 3.6-8.0 µm-bands. Using colour-magnitude diagrams, we have selected 49 candidate cluster members in the magnitude interval 16.1 mag < I < 23.0 mag. Results. Accounting for flux excesses at 8.0 µm and previously known spectral features of youth, we identify 30 objects as bona fide cluster members. Four are first identified from our optical-near infrared data. Eleven have most probable masses below the deuterium burning limit which we therefore classify as candidate planetary-mass objects. The slope of the substellar mass spectrum (∆N/∆M ≈ aM −α ) in the mass interval 0.11 M < M < 0.006 M is α = +0.6 ± 0.2. Any mass limit to formation via opacity-limited fragmentation must lie below 0.006 M . The frequency of σ Orionis brown dwarfs with circumsubstellar discs is 47±9 %. Conclusions. The continuity in the mass function and in the frequency of discs suggests that very low-mass stars and substellar objects, even below the deuterium-burning mass limit, share the same formation mechanism.

Abstract. We present a detailed and uniform study of C, S, Zn and Cu abundances in a large set of planet host stars, as well as in a homogeneous comparison sample of solar-type dwarfs with no known planetary-mass companions. Carbon abundances were derived by EW measurement of two C  optical lines, while spectral syntheses were performed for S, Zn and Cu. We investigated possible differences in the behaviours of the volatiles C, S and Zn and in the refractory Cu in targets with and without known planets in order to check possible anomalies due to the presence of planets. We found that the abundance distributions in stars with exoplanets are the high [Fe/H]

Aims. We present a search for low-mass brown dwarfs in the Pleiades open cluster. The identification of Pleiades members fainter and cooler than those currently known allows us to constrain evolutionary models for L dwarfs and to extend the study of the cluster mass function to lower masses. Methods. We conducted a 1.8 deg 2 near-infrared J-band survey at the 3.5 m Calar Alto Telescope, with completeness J cpl ∼ 19.0. The detected sources were correlated with those of previously available optical I-band images (I cpl ∼ 22). Using a J versus I − J colour-magnitude diagram, we identified 18 faint red L-type candidates, with magnitudes 17.4 < J < 19.7 and colours I − J > 3.2. If Pleiades members, their masses would span ∼0.040−0.020 M . We performed follow-up HK s -band imaging to further confirm their cluster membership by photometry and proper motion. Results. Out of 11 IJ candidates with proper motion measurements, we find six cluster members, two non-members and three whose membership is uncertain and depends on the intrinsic velocity dispersion of Pleiades brown dwarfs. This dispersion (>4 mas yr −1 ) is at least four times that of cluster stars with masses > ∼ 1 M . Five of the seven other IJ candidates are discarded because their J − K s colours are bluer than those of confirmed members. Our least massive proper motion members are BRB 28 and 29 (∼25 M Jup ). The J versus I − J sequence of the L-type candidates at J > 18 is not as red as theoretical models predict; it rather follows the field L-dwarf sequence translated to the cluster distance. This sequence overlapping, also observed in the J versus J−H and J−K diagrams, suggests that Pleiades and field L dwarfs may have similar spectral energy distributions and luminosities, and thus possibly similar radii. Also, we find α = 0.5 ± 0.2 for a power-law approximation dN/dM ∝ M −α of the survey mass spectrum in the mass range 0.5−0.026 M . This value is similar to that of much younger clusters, indicating no significant differential evaporation of low-mass Pleiades members relative to more massive ones.