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Astron. Astrophys. 343, 477-495 (1999)

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Faint members of the Chamaeleon I cloud *

F. Comerón 1, G.H. Rieke 2 and R. Neuhäuser 3

1 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany (e-mail: fcomeron@eso.org)
2 Steward Observatory, University of Arizona, Tucson, AZ 85721, USA (e-mail: grieke@as.arizona.edu)
3 MPI Extraterrestrische Physik, Giessenbachstrasse 1, D-85740 Garching bei München, Germany (e-mail: rne@mpe.mpg.de)

Received 23 July 1998 / Accepted 23 November 1998


We present a survey of the central [FORMULA] arcmin2 of the Chamaeleon I star forming cloud, including objective prism spectroscopy in the H[FORMULA] 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 [FORMULA] 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 [FORMULA], [FORMULA] 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 [FORMULA] 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 (Neuhäuser & Comerón 1998, Science , 282, 83). The near-infrared properties of the H[FORMULA] emission objects suggest that, unlike at higher masses, strong H[FORMULA] 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 H[FORMULA]-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.

Key words: stars: low-mass, brown dwarfs – stars: luminosity function, mass function – stars: pre-main sequence – infrared: stars

* Based on observations collected at the European Southern Observatory, Chile, program 58.E-0429.

Send offprint requests to: F. Comerón

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999