S Ori J053825.4-024241: a classical T Tauri-like object at the substellar boundary

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain e-mail: zvezda@ll.iac.es
² University of Central Florida, Dept. of Physics, PO Box 162385, Orlando, FL 32816-2385, USA
³ LAEFF-INTA, PO Box 50727, 28080, Madrid, Spain
⁴ Proyecto Gran Telescopio Canarias, Instituto de Astrofísica de Canarias
⁵ Consejo Superior de Investigaciones Científicas, Spain
⁶ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK
⁷ Main Astronomical Observatory of Academy of Sciences of Ukraine, Golosiiv Woods, Kyiv-127, 03680, Ukraine
⁸ Institute for Astronomy, University of Hawai'i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

Received: 18 May 2005
Accepted: 24 August 2005

Abstract

We present a spectrophotometric analysis of S Ori J053825.4-024241, a candidate member close to the substellar boundary of the young (1–8 Myr), nearby (~360 pc) σ Orionis star cluster. Our optical and near-infrared photometry and low-resolution spectroscopy indicate that S Ori J053825.4-024241 is a likely cluster member with a mass estimated from evolutionary models at 0.06 , which makes the object a probable brown dwarf. The radial velocity of S Ori J053825.4-024241 is similar to the cluster systemic velocity. This target, which we have classified as an M 6..0 low-gravity object, shows excess emission in the near-infrared and anomalously strong photometric variability for its type (from the blue to the J band), suggesting the presence of a surrounding disc. The optical spectroscopic observations show a continuum excess at short wavelengths and a persistent and resolved Hα emission (pseudo-equivalent width of ~-250 Å) in addition to the presence of other forbidden and permitted emission lines, which we interpret as indicating accretion from the disc and possibly mass loss. We conclude that despite the low mass of S Ori J053825.4-024241, this object exhibits some of the properties typical of active classical T Tauri stars.