We present a survey of the central ∼100 arcmin2
of the Chamaeleon I star forming cloud, including objective prism spectroscopy in the Hα region and deep imaging in the near-infrared. We estimate the expected number of very low mass objects within the survey, taking as a reference the higher mass members identified in previous studies, and assuming different ages and slopes of the initial mass function of the Chamaeleon I population. A new approach is introduced to estimate the contribution of background objects to the counts of low luminosity sources. This method takes advantage of the fact that the contribution of Chamaeleon I members should be negligible at the faintest magnitudes covered by our survey for any reasonable shape of the initial mass function. K-band source counts indicate the absence of a significant population of very low mass stars, implying that the initial mass function at very low masses, approximated by a power law, has a form Φ(M) d M ∝M–1
dM or flatter. This conclusion is in qualitative agreement with the discovery of six new emission line objects in the objective prism survey, and with the fact that only 2-3 faint objects are detected in the region of the (J-H), (H-K) diagram diagnostic of near infrared excesses of circumstellar origin. The masses of the new emission line objects, derived from recent pre-main sequence evolutionary tracks, are found to be near, and possibly below, the hydrogen burning limit, and their ages to be younger than 3x106
years. One of them is found to be a bona-fide brown dwarf, and its detection in a deep ROSAT exposure makes it the first, and so far the only, brown dwarf known to emit X-rays (Neuhaeuser & Comeron, 1998Sci...282...83N
). The near-infrared properties of the Hα emission objects suggest that, unlike at higher masses, strong Hα emission near the hydrogen-burning limit is not accompanied by infrared excess detectable in the K band. Comparing the numbers of very low mass objects expected from K band counts with the number of new Hα-emitting members, for which we derive individual masses and ages, we find that the spectroscopic survey samples the initial mass function completely, or nearly completely, down to the hydrogen-burning limit.