2015A&A...574A.118P

The detailed study of T-type candidate members of the young σ Orionis cluster (∼3Myr, ∼352pc, solar metallicity) is fundamental to properly assess the objects' cluster membership and their contribution to the definition of the substellar mass function. A total of three T-type candidates (S Ori 70, S Ori 73, and S Ori J053804.65-021352.5) lying in the line of sight towards σ Orionis were characterized by means of near-infrared photometric, astrometric, and spectroscopic studies. H-band methane images were collected for all three sources and an additional sample of 15 field T-type dwarfs using the LIRIS instrument on the 4.2m William Herschel Telescope (WHT). J-band spectra of resolution of ∼500 were obtained for S Ori J053804.65-021352.5 with the ISAAC spectrograph on the 8m Very Large Telescope (VLT), and JH spectra of resolution of ∼50 acquired with the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST) were employed for the spectroscopic classification of S Ori 70 and 73. Accurate proper motions with a typical uncertainty of ±3 mas/yr and a time interval of ∼7-9yr were derived using old images and new data collected with ISAAC/VLT and WFC3/HST. Usingthe LIRIS observations of the field T dwarfs, we calibrated this imager for T spectral typing via methane photometry. The three S Ori objects were spectroscopically classified as T4.5±0.5 (S Ori 73), T5±0.5 (S Ori J053804.65-021352.5), and T7^+0.5_–1.0 (S Ori 70). These spectral types agree with the measured H-band methane colors. The similarity between the observed JH spectra and the methane colors and the data of field ultra-cool dwarfs of related classifications suggests that S Ori 70, 73, and S Ori J053804.65-021352.5 do not deviate significantly in surface gravity in relation to the field. Additionally, the detection of KI at ∼1.25µm in S Ori J053804.65-021352.5 points to a high-gravity atmosphere. Only the K-band reddish nature of S Ori 70 may be consistent with a low-gravity atmosphere. The proper motions of S Ori 70 and 73 are measurable and are larger than that of the cluster by >3.5σ. The proper motion of S Ori J053804.65-021352.5 is consistent with a null displacement. These observations suggest that none of the three T dwarfs is a likely σ Orionis member, and that either planetary-mass objects with masses below ∼4M_Jup may not exist free-floating in the cluster or they may lie at fainter near-infrared magnitudes than those of the targets (i.e., H>20.6mag), thus remaining unidentified to date. We determined the volume density of field T4-T7 dwarfs to be ≥2.8±1.6x10^–3pc^–3 from a survey that covered 2798.4 arcmin² and was complete up to a distance of 119pc.