THE <i>SPITZER</i> c2d LEGACY RESULTS: STAR-FORMATION RATES AND EFFICIENCIES; EVOLUTION AND LIFETIMES

Abstract

(Abridged) The c2d Spitzer Legacy project obtained images and photometry with\nboth IRAC and MIPS instruments for five large, nearby molecular clouds. This\npaper combines information drawn from studies of individual clouds into a\ncombined and updated statistical analysis of star formation rates and\nefficiencies, numbers and lifetimes for SED classes, and clustering properties.\nCurrent star formation efficiencies range from 3% to 6%. Taken together, the\nfive clouds are producing about 260 solar masses of stars per Myr. The star\nformation surface density is more than an order of magnitude larger than would\nbe predicted from the Kennicutt relation used in extragalactic studies.\nMeasured against the dense gas probed by the maps of dust continuum emission,\nthe efficiencies are much higher, and the current stock of dense cores would be\nexhausted in 1.8 Myr on average. The derived lifetime for the Class I phase is\n0.44 to 0.54 Myr, considerably longer than some estimates. Similarly, the\nlifetime for the Class 0 SED class, 0.10 to 0.16 Myr, is longer than early\nestimates. The great majority (90%) of young stars lie within loose clusters\nwith at least 35 members and a stellar density of 1 solar mass per cubic pc.\nAccretion at the sound speed from an isothermal sphere over the lifetime\nderived for the Class I phase could build a star of about 0.25 solar masses,\ngiven an efficiency of 0.3. Our data confirm and aggravate the "luminosity\nproblem" for protostars. Our results strongly suggest that accretion is time\nvariable, with prolonged periods of very low accretion. Based on a very simple\nmodel and this sample of sources, half the mass of a star would be accreted\nduring only 7% of the Class I lifetime, as represented by the eight most\nluminous objects.\n