Populations of Young Stellar Objects in Nearby Molecular Clouds

We develop a new method to identify YSOs from star-forming regions using the photometry data from Spitzer’s c2d Legacy Project. The aim is to obtain YSO lists as complete as possible for studying the statistical properties, such as Star Formation Rate (SFR) and lifetimes of YSOs in different evolutionary stages. The largest obstacle for identifying YSOs comes from background galaxies with similar SEDs to YSOs. Traditionally, selected color-color and color-magnitude criteria are used to separate YSOs and galaxies. However, since there is no obvious boundary between YSOs and galaxies in Color-Color Diagrams (CCDs) and Color-Magnitude Diagrams (CMDs), those criteria may exclude faint YSOs near the boundary. In this paper, we separate the YSOs and galaxies in multi-dimensional (Multi-D) magnitude space, which is equivalent to using all variations of CMDs simultaneously. Comparing sources from molecular clouds to Spitzer’s SWIRE data, which have negligible amount of YSOs, we can naturally identify YSO candidates locating outside of the galaxy populated regions in the Multi-D space. In the five c2d-surveyed clouds, we select 322 new YSO candidates (YSOc), miss/exclude 33 YSOc compared to Evans et al. (2009) and result in 1313 YSOc in total. As a result, SFR increases 28% correspondingly, but the lifetimes of YSOs in different evolutionary stages remain unchanged. Comparing to theories (Krumholz & McKee 2005), our derived SFR suggests that star formation in large scale is dominated by supersonic turbulence rather than magnetic fields. Furthermore, we identify 7 new Very Low Luminosity Objects. Subject headings: ISM: clouds – Stars: luminosity function, mass function – Stars: pre-main sequence – Stars: protostars – brown dwarfs – Stars: formation