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Published April 2009 | Published
Journal Article Open

The Spitzer c2d Legacy results: star-formation rates and efficiencies; evolution and lifetimes


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 spectral energy distribution (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 15-30%. Star-formation rates and rates per unit area vary from cloud to cloud; taken together, the five clouds are producing about 260 M 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 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 , 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.

Additional Information

© 2009 American Astronomical Society. Print publication: Issue 2 (2009 April); received 2008 August 2; accepted for publication 2008 November 3; published 2009 March 11. We thank the referee, Charles Lada, for a very thoughtprovoking report, which helped us to improve the clarity of the paper. We thank the Lorentz Center in Leiden for hosting several meetings that contributed to this paper. Part of the work was done while in residence at theKavli Institute for Theoretical Physics in Santa Barbara, California. We thank M. Krumholz for enlightening discussions and derivations of the speed of star formation in dense gas and F. Comer´on for drawing our attention to biases caused by low-mass objects dropping out of the sample as they age. We are also grateful to D. Ward- Thompson for helpful suggestions. Support for this work, part of the Spitzer Legacy Science Program, was provided by NASA through contracts 1224608, 1230779, 1288658, 1288664, and 1230782 issued by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. Additional support came from NASA Origins grants NNG04GG24G and NNX07AJ72G to N.J.E. While several coauthors were at the Kavli Institute, the research was supported in part by the NSF under Grant No. NSF PHY05-51164. M.L.E. acknowledges support of an NSF Graduate Research Fellowship and a Spitzer Space Telescope Postdoctoral Fellowship. Astrochemistry in Leiden is supported by a NWO Spinoza grant and a NOVA grant. Facilities: Spitzer, CSO

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