Search and characterization of T-type planetary mass candidates in the σ Orionis cluster (original) (raw)
2010, Proceedings of the International Astronomical Union
Context. The proper characterization of the least massive population of the young σ Orionis star cluster is required to understand the form of the cluster mass function and its impact on our comprehension of the substellar formation processes. S Ori 70 (T5.5 ± 1) and 73, two T-type cluster member candidates, would have likely masses between 3 and 7 M Jup if their age is 3 Myr. S Ori 73 awaits confirmation of its methane atmosphere. Aims. We aim to: i) confirm the presence of methane absorption in S Ori 73 through methane imaging; ii) study S Ori 70 and 73 cluster membership via photometric colors and accurate proper motion analysis; iii) perform a new search to identify additional T-type σ Orionis member candidates. Methods. We obtained HAWK-I (VLT) J, H, and CH 4off photometry of an area of 119.15 arcmin 2 in σ Orionis down to J comp = 21.7 and H comp = 21 mag. S Ori 70 and 73 are contained in the explored area. Near-infrared data were complemented with optical photometry using images acquired with OSIRIS (GTC) and VISTA as part of the VISTA Orion survey. Color-magnitude and color-color diagrams were constructed to characterize S Ori 70 and 73 photometrically, and to identify new objects with methane absorption and masses below 7 M Jup . We derived proper motions by comparison of the new HAWK-I and VISTA images with published near-infrared data taken 3.4 -7.9 yr ago. Results. S Ori 73 has a red H − CH 4off color indicating methane absorption in the H-band and a spectral type of T4 ± 1. S Ori 70 displays a redder methane color than S Ori 73 in agreement with its latter spectral classification. Our proper motion measurements (µ α cos δ = 26.7 ± 6.1, µ δ = 21.3 ± 6.1 mas yr −1 for S Ori 70, and µ α cos δ = 46.7 ± 4.9, µ δ = −6.3 ± 4.7 mas yr −1 for S Ori 73) are larger than the motion of σ Orionis, rendering S Ori 70 and 73 cluster membership uncertain. From our survey, we identified one new photometric candidate with J = 21.69 ± 0.12 mag and methane color consistent with spectral type ≥ T8. Conclusions. S Ori 73 has colors similar to those of T3-T5 field dwarfs, which in addition to its high proper motion suggests that it is probably a field dwarf located at 170-200 pc. The origin of S Ori 70 remains unclear: it can be a field, foreground mid-to late-T free-floating dwarf with peculiar colors, or an orphan planet ejected through strong dynamical interactions from σ Orionis or from a nearby star-forming region in Orion.
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