A survey for low-mass spectroscopic binary stars in the young clusters around σ Orionis and λ Orionis

We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs (BDs) in the young clusters around sigma Ori and lambda Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their Na I 8200 lines and the spectral type from their TiO band strength. We have identified 11 new binary stars among the cluster members based on their variable radial velocity and an additional binary from the variation in its linewidth and shape. Of these, six are double-lined spectroscopic binaries (SB2) where the components of the binary are of comparable brightness. The others are single-lined binaries (SB1) in which the companion is faint or the spectra of the stars are blended. There are three narrow-lined SB1 binaries in our sample for which the companion is more than 2.5 mag fainter than the primary. This suggests that the mass ratio distribution for the spectroscopic binaries in our sample is broad but that there may be a peak in the distribution near q = 1. The sample covers the magnitude range I-C = 14-18.9 (mass approximate to 0.55-0.03 M-circle dot), but all of the binary stars are brighter than I-C = 16.6 (mass approximate to 0.12 M-circle dot) and 10 are brighter than I-C = 15.5 (mass approximate to 0.23 M-circle dot). There is a significant lack of spectroscopic binaries in our sample at faint magnitudes even when we account for the decrease in sensitivity with increasing magnitude. We can reject the hypothesis that the fraction of spectroscopic binaries is a uniform function of I-C magnitude with more than 99 per cent confidence. The spectroscopic binary fraction for stars more massive than about 0.1 M-circle dot (I-C < 16.9) is f(bright) = 0.095(-0.028)(+0.012). The 90 per cent confidence upper limit to the spectroscopic binary fraction for very low-mass (VLM) stars (mass < 0.1 M-circle dot) and BDs is f(faint) < 7.5 per cent. The hypothesis that f(bright) and f(faint) are equal can be rejected with 90 per cent confidence. The average detection probability for our survey is 50 per cent or more for binaries with separations up to 0.28 au for stars with I-C < 16.9 and 0.033 au for the fainter stars in our sample. We conclude that we have found strong evidence for a change in the fraction of spectroscopic binaries among young VLM stars and BDs when compared to more massive stars in the same star-forming region. This implies a difference in the total binary fraction between VLM stars and BDs compared to more massive stars or a difference in the distribution of semimajor axes, or both.