Faint members of the Chamaeleon I cloud

Details Faint members of the Chamaeleon I cloud Faint members of the Chamaeleon I cloud

Mar 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999a%26a...343..477c&link_type=abstract

Astronomy and Astrophysics, v.343, p.477-495 (1999)

Astronomy and Astrophysics

Astrophysics

57

Stars: Low-Mass, Brown Dwarfs, Stars: Luminosity Function, Mass Function, Stars: Pre-Main Sequence, Infrared: Stars

Scientific paper

We present a survey of the central ~ 100 arcmin(2) 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 Phi ({cal M}) d {cal M} ~ {cal M}(-1) d{cal M} 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 3 x 10(6) 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 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. Based on observations collected at the European Southern Observatory, Chile, program 58.E-0429.

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