Search and characterization of T-type planetary mass candidates in the σ Orionis cluster

¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife Spain
e-mail: karla@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
³ Centro de Astrobiología (CSIC-INTA), 28850 Torrejón de Ardoz, Madrid, Spain
⁴ Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain
⁵ Leibniz Institute for Astrophysics Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 2 March 2011
Accepted: 16 May 2011

Abstract

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, are likely to have masses between 3 and 7 M_Jup if their age is 3 Myr. It awaits confirmation whether S Ori 73 has a methane atmosphere.

Aims. We aim to: i) confirm the presence of methane absorption in S Ori 73 by performing methane imaging; ii) study S Ori 70 and 73 cluster membership via photometric colors and accurate proper motion analysis; and 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 arcmin² 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 comparing 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.