Extending the census at the bottom of the stellar mass function in Chamaeleon I

We present the results of a deep, wide field objective prism survey of the entire Chameleon I cloud, followed by long slit spectroscopy of objects with detected Hα emission that were either previously unidentified, or suspected to be members only on the basis of their mid-infrared emission. We identify 9 new members and confirm 9 objects already suspected as members, with spectral types ranging from late K to M8.5. The latter limit corresponds to an object with an estimated mass of 0.03 M� , making it the latest-type brown dwarf spectroscopically confirmed so far in Chamaeleon I. A comparison with theoretical pre- main sequence tracks indicates an age of most of the members between 1 and 5 Myr, consistent with previous studies. However, we find that the objects with Hα equivalent widths exceeding 100 A tend to have apparent ages above 5 Myr, and in the two most extreme cases their positions in the temperature-luminosity diagram, if interpreted at face value, would place them below the main sequence. These two extreme objects display surprising differences in their emission line spectra in spite of the otherwise very similar broad-band spectral energy distributions and spectral types, being strongly dominated by accretion-tracing and outflow-tracing emission lines, respectively. We interpret the identification of an apparently underluminous object with strong accretion signatures and only weak outflow signatures as a further support, already discussed in previous works, for the apparent underluminosity of objects with very large Hα equivalent widths being a real, intrinsic feature rather than being due to partial blocking by an edge-on disk. Given that Hα emission is the most common feature among young stellar objects we consider that the present work is an important contribution towards a complete census of the Chamaeleon I star forming region down to the hydrogen-burning limit.