2013MNRAS.430.1208P

We have performed a 970 arcmin2 (1.75 pc2) survey of the young cluster IC 2391 using the High Acuity Wide field K-band Imager on the Very Large Telescope. Observations were made in both the CH4 and H filters, targeting the methane absorption found in T dwarfs. From our survey of the cluster, five candidate T dwarfs were found. This is substantially smaller than the number that would be found by an equivalent near-infrared broad-band imaging survey, and allowed rapid spectroscopy of all five candidates. Follow-up spectroscopy was carried out using the Folded-port Infrared Echellette on Magellan. These spectra confirm that none of these objects are T dwarfs. This negative result emphasizes the critical importance of follow-up spectroscopy for any photometrically selected candidates, even those from methane imaging. Simulations of the photometric data set, combined with data for known IC 2391 members, imply that the power-law mass function in IC 2391 between 0.003 and 0.13 M☉ has a slope of α < 1.7.