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

By N. Evans II, M. Dunham, J. Jørgensen, M. Enoch, B. Mer'in, E. V. van Dishoeck, J. Alcalá, P. Myers, K. Stapelfeldt, T. Huard and 8 more. 2008

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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 Tbol, the values for Lbol 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.

Subfields: Astronomy and Astrophysics, Spectroscopy

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Citations in the corpus

The CO-to-H2 Conversion Factor (2013, neutral): “21 may yield a 40% overestimate (Evans et al. 2009).”
TOWARD A COMPLETE ACCOUNTING OF ENERGY AND MOMENTUM FROM STELLAR FEEDBACK IN GALAXY FORMATION SIMULATIONS (2012, methodology): “In the regime of ∆x∼ 50 − 100pc, adopted for the galactic disks in § 4.3, where cell sizes matches observed sizes of massive GMCs, this is not an issue.”
STAR FORMATION RATES IN MOLECULAR CLOUDS AND THE NATURE OF THE EXTRAGALACTIC SCALING RELATIONS (2011, background): “These classical relations are often super linear and moreover, as Heiderman et al. (2010) point out, they under predict the ΣSFR in local regions by factors of 17 - 50 (see also Evans et al. 2009).”
Stellar Feedback in Galaxies and the Origin of Galaxy-scale Winds (2011, background): “…2012 The Authors Monthly Notices of the Royal Astronomical Society C© 2012 RAS at H arvard U niversity on A pril 18, 2013 http://m nras.oxfordjournals.org/ D ow nloaded from lifetime is only a few to several per cent (Zuckerman & Evans 1974; Williams & McKee 1997; Evans 1999; Evans et al. 2009).”
WIDE-FIELD INFRARED SURVEY EXPLORER OBSERVATIONS OF THE EVOLUTION OF MASSIVE STAR-FORMING REGIONS (2012, methodology): “The c2d (Evans et al. 2009) Spitzer survey of five molecular clouds within 300 pc of the Sun and GLIMPSE Spitzer survey of the inner Galaxy (|l| < 65◦ and |b| 1◦; Benjamin et al. 2003, 2008) have shown the power of extended mid- to far-infrared surveys to uncover the extent of star formation beyond…”
Globular clusters as the relics of regular star formation in ‘normal’ high-redshift galaxies (2015, methodology): “Finally, we adopt a SFE ofǫ = 0.05 (e.g.Lada & Lada 2003; Evans et al. 2009; Kruijssen 2014), and use the CFE model ofKruijssen (2012) to obtainΓ(Σ,Ω, Q).”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “In the former category, the recent Spitzer Cores to Disks legacy survey of low-mass star-forming clouds near the Sun gives ff ∼ 0.01 − 0.1 for clouds with mean densities nH2 ∼ 103 cm−3 (Evans et al. 2009).”
A Keplerian disk around a Class 0 source: ALMA observations of VLA1623A (2013, methodology): “…age obtained through this method is inconsistent with the expected age from Tbol (Ladd et al. 1998) and the estimated Class 0 lifetime obtained by Evans et al. (2009), which is on the order of 105 yr, although they conceded that for ρ Ophiuchus the lifetime is 4 × 104 yr, an order of magnitude…”
On the diversity and statistical properties of protostellar discs (2018, background): “Class 0 objects are thought to have typical lifetimes of ≈ 105 yrs (Evans et al. 2009).”
Microarcsecond Radio Astrometry (2013, background): “Most of our knowledge about the formation of stars like the Sun comes from in-depth studies of these regions, for example, from the Spitzer c2d survey (Evans et al. 2009), the XMM Newton Extended Survey of Taurus (Güdel et al. 2008) and the Herschel Gould Belt Survey (André et al. 2010).”
LOW VIRIAL PARAMETERS IN MOLECULAR CLOUDS: IMPLICATIONS FOR HIGH-MASS STAR FORMATION AND MAGNETIC FIELDS (2013, background): “The absence of such cores is significant, given that many starless cores exist in non–HMSF regions (see, e.g., Evans et al. 2009 for statistics).”
26Al‐26Mg isotope systematics of the first solids in the early solar system (2013, neutral): “Evans et al. (2009) estimated the lifetimes of individual classes of YSOs, based on Spitzer surveys in multiple large molecular clouds, as shown in Table 1.”
THE STAR FORMATION RATE OF TURBULENT MAGNETIZED CLOUDS: COMPARING THEORY, SIMULATIONS, AND OBSERVATIONS (2012, methodology): “The observational data are from Galactic observations of clouds and YSOs identified in the Spitzer Cores-to-Disks (C2D) and GB surveys (Evans et al. 2009) of massive dense clumps (Wu et al. 2010), and of the Taurus molecular cloud (Pineda et al. 2010; Rebull et al. 2010).”
Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “8, indicating the times before the final burst that destroys the cloud, together with observational data for various types of clouds, from low-mass star forming clouds (Evans et al. 2009; Lada et al. 2010, 2013) to high-mass regions like the OMC1 cloud and other mini-bursts (Louvet et al. 2014).”
THE FORMATION OF LOW-MASS BINARY STAR SYSTEMS VIA TURBULENT FRAGMENTATION (2010, background): “This is consistent with observations that the mean accretion rate falls as protostars transistion from the Class 0 to Class II stages (Andre & Montmerle 1994; Evans et al. 2009).”
HYDROGEN GREENHOUSE PLANETS BEYOND THE HABITABLE ZONE (2011, background): “At 3 AU, the predicted timescales for these two phases of accretion are ∼ 106 and 107 yr, respectively, and bracket the lifetime of gaseous disks (Haisch Jr. et al. 2001; Evans et al. 2009).”
RADIATION-HYDRODYNAMIC SIMULATIONS OF THE FORMATION OF ORION-LIKE STAR CLUSTERS. II. THE INITIAL MASS FUNCTION FROM WINDS, TURBULENCE, AND RADIATION (2012, background): “…by observations even in the absence of the problems rapid star formation creates in the IMF, because observed star formation rates per free-fall time are always a few percent across a very wide range of star-forming environments (Krumholz & Tan 2007; Evans et al. 2009; Krumholz et al. 2012).”