The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large , nearby molecular clouds .
Three of the clouds were also mapped in dust continuum emission at 1.1 mm , and optical spectroscopy has been obtained for some clouds .
This paper combines information drawn from studies of individual clouds into a combined and updated statistical analysis of star formation rates and efficiencies , numbers and lifetimes for SED classes , and clustering properties .
Current star formation efficiencies range from 3 % to 6 % ; if star formation continues at current rates for 10 Myr , efficiencies could reach 15 % to 30 % .
Star formation rates and rates per unit area vary from cloud to cloud ; taken together , the five clouds are producing about 260 M _ { \odot } of stars per Myr .
The star formation surface density is more than an order of magnitude larger than would be predicted from the Kennicutt relation used in extragalactic studies , reflecting the fact that those relations apply to larger scales , where more diffuse matter is included in the gas surface density .
Measured against the dense gas probed by the maps of dust continuum emission , the efficiencies are much higher , with stellar masses similar to masses of dense gas , and the current stock of dense cores would be exhausted in 1.8 Myr on average .
Nonetheless , star formation is still slow compared to that expected in a free fall time , even in the dense cores .
The derived lifetime for the Class I phase is 0.54 Myr , considerably longer than some estimates .
Similarly , the lifetime for the Class 0 SED class , 0.16 Myr , with the notable exception of the Ophiuchus cloud , is longer than early estimates .
If photometry is corrected for estimated extinction before calculating class indicators , the lifetimes drop to 0.44 Myr for Class I and to 0.10 for Class 0 .
These lifetimes assume a continuous flow through the Class II phase and should be considered median lifetimes or half-lives .
Star formation is highly concentrated to regions of high extinction , and the youngest objects are very strongly associated with dense cores .
The great majority ( 90 % ) of young stars lie within loose clusters with at least 35 members and a stellar density of 1 M _ { \odot } pc ^ { -3 } .
Accretion at the sound speed from an isothermal sphere over the lifetime derived for the Class I phase could build a star of about 0.25 M _ { \odot } , given an efficiency of 0.3 .
Building larger mass stars by using higher mass accretion rates could be problematic , as our data confirm and aggravate the “ luminosity problem ” for protostars .
At a given T _ { bol } , the values for L _ { bol } are mostly less than predicted by standard infall models and scatter over several orders of magnitude .
These results strongly suggest that accretion is time variable , with prolonged periods of very low accretion .
Based on a very simple model and this sample of sources , half the mass of a star would be accreted during only 7 % of the Class I lifetime , as represented by the eight most luminous objects .