Optical spectroscopy of isolated planetary mass objects in the Orionis cluster ^*

¹ Departamento Física Teórica, C-XI-506, Universidad Autónoma de Madrid, 28049 Madrid, Spain
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Division of Geological & Planetary Sciences, California Institute of Technology, MS 150-21, Pasadena, CA 91125, USA
⁴ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁵ Consejo Superior de Investigaciones Científicas, CSIC, Spain
⁶ Institute of Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

Corresponding author: D. Barrado y Navascués, barrado@pollux.ft.uam.es

Received: 29 May 2001
Accepted: 15 August 2001

Abstract

We have obtained low resolution optical spectra of 15 isolated planetary mass objects (IPMOs) in the σ Orionis cluster, and derived spectral types by comparison with nearby M and L dwarfs. The spectral types are in the range late M -mid L, in agreement with our expectations based on colors and magnitudes for bona fide members. Therefore, most of these objects have masses below the deuterium burning limit. About 2/3 show Hα in emission at our spectral resolution. From our spectroscopic and photometric data, we infer that three IPMOs in this sample may be binaries with components of similar masses. These results confirm that the substellar mass function of the σ Orionis cluster, in the form dN/dM, keeps rising in the planetary domain.