We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star formation rates are discussed, and updated prescriptions for calculating star formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds. Subject headings: star formation, galaxies, Milky Way 1. OVERVIEW 1.1. Introduction Star formation encompasses the origins of stars and planetary systems, but it is also a principal agent of galaxy formation and evolution, and hence a subject at the roots of astrophysics on its largest scales.The past decade has witnessed an unprecedented stream of new observational information on star formation on all scales, thanks in no small part to new facilities such as the Galaxy Evolution Explorer (GALEX), the Spitzer Space Telescope, the Herschel Space Observatory, the introduction of powerful new instruments on the Hubble Space Telescope (HST), and a host of groundbased optical, infrared, submillimeter, and radio telescopes. These new observations are providing a detailed reconstruction of the key evolutionary phases and physical processes that lead to the formation of individual stars in interstellar clouds, while at the same time extending the reach of integrated measurements of star formation rates (SFRs) to the most distant galaxies known. The new data have also stimulated a parallel renaissance in theoretical investigation and numerical modelling of the star formation process, on scales ranging from individual protostellar and protoplanetary systems to the scales of molecular clouds and star clusters, entire galaxies and ensembles of galaxies, even to the first objects, which are thought to have reionized the Universe and seeded today's stellar populations and Hubble sequence of galaxies.This immense expansion of the subject, both in terms of the volume of results and the range of physical scales explored, may help to explain one of its idiosyncracies, namely the relative isolation between the community studying individual star-forming regions and stars in the Milky Way (usually abbreviated hereafter as MW, but sometimes referred to as "the Galaxy"), and the largely extragalactic community that attempts to characterise the star formation process on galactic and cosmologiElectronic address: robk@ast.cam.as.uk Electronic address: nje@astro.as.utexas.edu cal scales. Some aspects of this separation have been understandable. The key physical processes that determine how molecular clouds contract and fragment into clumps and cores and finally clusters and individual stars can be probed up close only in the Galaxy, and much of the progress in this subject has come from in-depth case studies of individual star-forming regions. Such detailed observations have been impossible to obtain for even relatively nearby galaxies. Instead the extragalactic branch of the subject has focused on the co...

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.

We investigate the relation between star formation rate (SFR) and gas surface densities in Galactic star forming regions using a sample of young stellar objects (YSOs) and massive dense clumps. Our YSO sample consists of objects located in 20 large molecular clouds from the Spitzer cores to disks (c2d) and Gould's Belt (GB) surveys. These data allow us to probe the regime of low-mass star formation essentially invisible to tracers of high-mass star formation used to establish extragalactic SFR-gas relations. We estimate the gas surface density (Σ gas ) from extinction (A V ) maps and YSO SFR surface densities (Σ SFR ) from the number of YSOs, assuming a mean mass and lifetime. We also divide the clouds into evenly spaced contour levels of A V , counting only Class I and Flat SED YSOs, which have not yet migrated from their birthplace. For a sample of massive star forming clumps, we derive SFRs from the total infrared luminosity and use HCN gas maps to estimate gas surface densities. We find that c2d and GB clouds lie above the extragalactic SFR-gas relations (e.g., Kennicutt-Schmidt Law) by factors up to 17. Cloud regions with high Σ gas lie above extragalactic relations up to a factor of 54 and overlap with high-mass star forming regions. We use 12 CO and 13 CO gas maps of the Perseus and Ophiuchus clouds from the COMPLETE survey to estimate gas surface densities and compare to measurements from A V maps. We find that 13 CO, with the standard conversions to total gas, underestimates the A Vbased mass by factors of ∼4-5. 12 CO may underestimate the total gas mass at Σ gas 200 M ⊙ pc −2 by 30%; however, this small difference in mass estimates does not explain the large discrepancy between Galactic and extragalactic relations. We find evidence for a threshold of star formation (Σ th ) at 129±14 M ⊙ pc −2 . At Σ gas > Σ th , the Galactic SFR-gas relation is linear. A possible reason for the difference between Galactic and extragalactic relations is that much of Σ gas is below Σ th in extragalactic studies, which detect all the CO-emitting gas. If the Kennicutt-Schmidt relation (Σ SFR ∝ Σ 1.4 gas ) and a linear relation between dense gas and star formation is assumed, the fraction of dense star forming gas (f dense ) increases as ∼ Σ 0.4 gas . When Σ gas reaches ∼300Σ th , the fraction of dense gas is ∼1, creating a maximal starburst.

Aims. To study the structure of nearby (<500 pc) dense starless and star-forming cores with the particular goal to identify and understand evolutionary trends in core properties, and to explore the nature of Very Low Luminosity Objects (≤0.1 L ; VeLLOs). Methods. Using the MAMBO bolometer array, we create maps unusually sensitive to faint (few mJy per beam) extended (≈5 ) thermal dust continuum emission at 1.2 mm wavelength. Complementary information on embedded stars is obtained from Spitzer, IRAS, and 2MASS. Results. Our maps are very rich in structure, and we characterize extended emission features ("subcores") and compact intensity peaks in our data separately to pay attention to this complexity. We derive, e.g., sizes, masses, and aspect ratios for the subcores, as well as column densities and related properties for the peaks. Combination with archival infrared data then enables the derivation of bolometric luminosities and temperatures, as well as envelope masses, for the young embedded stars. Conclusions. Starless and star-forming cores occupy the same parameter space in many core properties; a picture of dense core evolution in which any dense core begins to actively form stars once it exceeds some fixed limit in, e.g., mass, density, or both, is inconsistent with our data. A concept of necessary conditions for star formation appears to provide a better description: dense cores fulfilling certain conditions can form stars, but they do not need to, respectively have not done so yet. Comparison of various evolutionary indicators for young stellar objects in our sample (e.g., bolometric temperatures) reveals inconsistencies between some of them, possibly suggesting a revision of some of these indicators. Finally, we challenge the notion that VeLLOs form in cores not expected to actively form stars, and we present a first systematic study revealing evidence for structural differences between starless and candidate VeLLO cores.

We present an unbiased census of deeply embedded protostars in Perseus, Serpens, and Ophiuchus, assembled by combining large-scale 1.1 mm Bolocam continuum and Spitzer Legacy surveys. We identify protostellar candidates based on their mid-infrared properties, correlate their positions with 1.1 mm core positions from Enoch et al. (2006), and Enoch et al. (2007, and construct well-sampled SEDs using our extensive wavelength coverage (λ = 1.25 − 1100 µm). Source classification based on the bolometric temperature yields a total of 39 Class 0 and 89 Class I sources in the three cloud sample. We compare to protostellar evolutionary models using the bolometric temperature-luminosity diagram, finding a population of low luminosity Class I sources that are inconsistent with constant or monotonically decreasing mass accretion rates. This result argues strongly for episodic accretion during the Class I phase, with more than 50% of sources in a "sub-Shu" (dM/dt < 10 −6 M ⊙ yr −1 ) accretion state. Average spectra are compared to protostellar radiative transfer models, which match the observed spectra fairly well in Stage 0, but predict too much near-IR and too little mid-IR flux in Stage I. Finally, the relative number of Class 0 and Class I sources are used to estimate the lifetime of the Class 0 phase; the three cloud average yields a Class 0 lifetime of 1.7 ± 0.3 × 10 5 yr, ruling out an extremely rapid early accretion phase. Correcting photometry for extinction results in a somewhat shorter lifetime (1.1 × 10 5 yr). In Ophiuchus, however, we find very few Class 0 sources (N Class 0 /N Class I ∼ 0.1 − 0.2), similar to previous studies of that cloud. The observations suggest a consistent picture of nearly constant average accretion rate through the entire embedded phase, with accretion becoming episodic by at least the Class I stage, and possibly earlier.