1999A&A...343..477C

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.