Probing the brown dwarf population of the Chamaeleon I star forming region

We present observations of a sample of 13 very low mass stars and brown dwa rfs in the central region of the Chamaeleon I star forming cloud. The obser vations include slitless spectroscopy around H alpha to identify new member s, low resolution long-slit visible and near-infrared spectroscopy, deep RO SAT PSPC X-ray observations, and ISOCAM mid-infrared observations. Our samp le adds seven new objects to those discussed by Comeron, Rieke, and Neuhaus er (1999, A&A, 343, 477) and extends the range of spectral types up to Ms. We study different narrow-band indices as a tool for detecting and classify ing very late-type young stellar objects. As to K-band spectra, we find tha t the visible features are not appropriate to yield a spectral classificati on more accurate than a few subclasses at best beyond M6. None of our sources displays K-band excess emission, but four have excess a t 6.7 mu m suggesting that, although circumstellar disks are common around young very low mass stars, their inner regions are in general not hot enoug h to radiate significantly in the K band. Mid-infrared emission loosely cor relates with H alpha emission: sources without mid-IR excesses are always w eak H alpha emitters, while mid-IR excess sources have a broad range of H a lpha equivalent widths. X-ray emission is detected for 7 objects with spect ral type M6 or later, including one bona-fide brown dwarf and three objects near the border separating stars and brown dwarfs. X-ray to bolometric lum inosity ratios are typical of low mass, fully convective stars. The non-det ection of X-ray emission at comparable levels from more evolved brown dwarf s suggests that X-ray activity may be restricted to early stages of brown d warf evolution. We discuss in detail the temperatures and luminosities of our objects based on their magnitudes and spectra, and use the derived values to estimate ma sses and ages according to two different sets of pre-main sequence evolutio nary tracks. Both sets of models are in good agreement concerning the mass derived for our objects, showing that four of them are bona-fide brown dwar fs, six are transition objects, and three are low mass stars. Derived ages differ significantly depending on the adopted models, especially at the low est masses. This is mainly due to the objects lying on opposite sides of th e deuterium-burning main sequence depending on whether one or another set i s used. Using Baraffe et al. (1998, A&A, 337, 403) models for the dating of each object in the area of our survey with mass below 1 M-circle dot, we f ind that most have ages near 2.10(6) years, with a small spread around that value. However, a few objects appear to have ages near 2.10(7) years, sugg esting that most, but not all, star formation in that region of Chamaeleon I may have happened almost simultaneously in a recent burst. Comparing predictions on members of the star forming region based on K-band star counts with the number of members actually identified through H alpha emission suggests that sensitive H alpha surveys are very efficient in pro ducing a complete or nearly complete magnitude-limited census of young stel lar objects in Chamaeleon I. Under the assumption that our sample is comple te, we derive a mass function of Chamaeleon I between 0.03 and 1 M-circle d ot which can be approximated by a nearly flat powerlaw in logarithmic mass units, in agreement with results for other young aggregates.