THE SPITZER c2d SURVEY OF LARGE, NEARBY, INTERSTELLAR CLOUDS. X. THE CHAMAELEON II PRE-MAIN-SEOUENCE POPULATION AS OBSERVED WITH IRAC AND MIPS

摘要

We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and NIR data of the Chamaeleon Ⅱ (Cha Ⅱ) dark cloud. We perform a census of the young population in an area of ～1.75 deg~2 and study the spatial distribution and properties of the cloud members and candidate pre-main-sequence (PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime (M ≈0.03 solar mass). From the analysis of the volume density of the PMS objects and candidates we find two groups of objects with volume densities higher than 25 solar mass pc~(-3) and 5-10 members each. A multiplicity fraction of about 13% ± 3% is observed for objects with separations 0.8″ < θ < 6.0″ (142-1065 AU). No evidence for variability between the two epochs of the c2d IRAC data set, Δt ～ 6 hr, is detected. We estimate a star formation efficiency of l%-4%, consistent with the estimates for Taurus and Lupus, but lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that Cha II is turning some 6-7 solar mass into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%-80% that we estimate in Cha II is much higher than in other star-forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed; a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50, has been discovered.