THE SPITZER c2d LEGACY RESULTS: STAR-FORMATION RATES AND EFFICIENCIES; EVOLUTION AND LIFETIMES

Abstract

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 ⊙ 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.

Infrared study of transitional disks in Ophiuchus withHerschel

“…Our sample is different from that in Evans et al (2009), since they had photometrically selected objects, including many objects with other classes, while we have only spectroscopically confirmed Class II YSOs. All of these YSOs fell within the coverage of the maps used, and 49 were detected in at least one Herschel band, 49 in PACS, and 19 in SPIRE.…”

“…Another study took into account the sources detected by Spitzer and catalogued by c2d (Evans et al 2009) in each of the apertures defined per band and per detected source to estimate possible contamination due to Herschel resolution. The flux in Notes.…”

Disk Masses for Embedded Class I Protostars in the Taurus Molecular Cloud

“…As such, the Class I disks, which are younger (e.g., Evans et al 2009;Dunham et al 2015) and have had less time for dust processing and planet formation to occur, should better represent the initial mass budget of disks for forming planets. And although the Class I disk sample appears to be more massive, on average, than the Class II sample, it remains unclear from our results whether Class I disks are massive enough to form giant planets.…”

“…If this is the case, then studying the disks around the younger (∼0.5 Myr; Evans et al 2009) Class I disks, which have had less significant dust processing, may give a better picture of the initial mass budget for forming planets. The masses of these disks are more difficult to determine because they are still embedded in their natal envelope, and any millimeter flux measurement will include a contribution from both the disk and envelope.…”

Primeval Jets from Young Stars

“…In the Class II phase the photosphere of the newborn star is directly visible for the first time: it is now a classical T Tauri star or a Herbig Ae/Be star if it has a higher mass. The Class 0, I and II phases are thought to last about 10 5 , 5.10 5 and 2.10 6 years respectively (Evans et al 2009).…”