An X-Ray Census of Young Stars in the Chamaeleon I North Cloud

Sensitive X-ray imaging surveys provide a new and effective tool to establish the census of pre-main-sequence (PMS) stars in nearby young stellar clusters. We report here a deep Chandra X-Ray Observatory observation of PMS stars in the Chamaeleon I North cloud, achieving a limiting luminosity of log L_t ≃ 27 ergs s^-1 (0.5-8 keV band) in a 0.8 × 0.8 pc² region. Of the 107 X-ray sources, 37 are associated with Galactic stars of which 27 are previously recognized cloud members. These include three PMS brown dwarfs; the protostellar brown dwarf ISO 192 has a particularly high level of magnetic activity. Follow-up optical photometry and spectroscopy establish that 9-10 of the Chandra sources are probably magnetically active background stars. Several previously proposed cloud members are also inferred to be interlopers because of the absence of X-ray emission at the level expected from the log L_t-K correlation. No new X-ray-discovered stars were confidently found despite the high sensitivity. From these findings, we argue that the sample of 27 PMS cloud members in the Chandra field is uncontaminated and complete down to K = 12 or M ≃ 0.1 M_☉. The initial mass function (IMF) derived from our sample is deficient in 0.1-0.3 M_☉ stars compared with the IMF of the rich Orion Nebula cluster and other Galactic populations. We cannot discriminate whether this is due to different star formation processes, mass segregation, or dynamical ejection of lower mass stars.