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 establishthe 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 theChamaeleon I North cloud, achieving a limiting luminosity of log L_t ~ 27 erg/s(0.5-8 keV band) in a 0.8x0.8 pc region. Of the 107 X-ray sources, 37 areassociated with Galactic stars of which 27 are previously recognized cloudmembers. These include three PMS brown dwarfs; the protostellar brown dwarf ISO192 has a particularly high level of magnetic activity. Followup opticalphotometry and spectroscopy establishes that 9-10 of the Chandra sources areprobably magnetically active background stars. Several previously proposedcloud members are also inferred to be interlopers due to the absence of X-rayemission at the level expected from the L_t - K correlation. No new X-raydiscovered stars were confidently found despite the high sensitivity. From these findings, we argue that the sample of 27 PMS cloud members in theChandra field is uncontaminated and complete down to K = 12 or M ~ 0.1 Mo. Theinitial mass function (IMF) derived from our sample is deficient in 0.1-0.3 Mostars compared to the IMF of the rich Orion Nebula Cluster and other Galacticpopulations. We can not discriminate whether this is due to different starformation processes, mass segregation, or dynamical ejection of lower massstars.