Comparison of Star Formation in Five Nearby Molecular Clouds

Abstract

We have calculated bolometric temperature (Tbol) and bolometric luminosity (Lbol) for 383 young stellar objects (YSOs) in five molecular clouds within 200 pc in Corona Australis (CrA), Ophiuchus (Oph), Taurus (Tau), Chamaeleon (Cha), and Lupus (Lup). We used Tbol, Lbol, and bolometric luminosity-temperature (BLT) diagrams to characterize and compare the overall-star-formation activity of the clouds on a self-consistent basis. The main results are the following: (1) the YSO populations in the clouds can be differentiated by the fraction of their low-Tbol sources, which increases systematically from Lup and Cha to Tau and to Oph and CrA. This trend is interpreted as increasing current star-forming activity in the same order; (2) the clouds with higher cold source fractions also seem to have higher bright source fractions; (3) In the BLT diagram, the CrA and Oph sources are more uniformly distributed while the Cha and Lup sources are aggregated near the zero-age mainsequence (ZAMS). Tau sources appear to be an intermediate case. Taurus also seems to contain more cold (Tbol < 1000 K) and low-luminosity (Lbol < 1 L⊙) sources than the other complexes; (4) the YSOs show a characteristic distribution in the median BLT diagram. This distribution is qualitatively consistent with the early YSO evolution from a protostar to a pre-main-sequence star and provides a unique observational test to star-formation models; (5) for Lup pre-main-sequence stars, the ratio of their Tbol to Teff increases during their approach to the ZAMS. This increase can be explained by the disk and envelope dissipation during the pre-main-sequence evolution; (6) the most active star-forming clouds (Oph and CrA) also have denser molecular cores as measured by C18O J = 1-0 line emission, suggesting that the star formation occurs in the densest parts of the molecular clouds; and (7) we find an anti-correlation between Tbol and C18O emission for the class 0 and I Tau sources (Tbol < 650 K). This shows that Tbol measures the intrinsic redness of YSOs, rather than their disk-envelope orientation. The disk orientation may have a more important effect on Tbol of the pre-main-sequence stars.