In this paper we investigate the level of star formation activity within nearby molecular clouds. We employ a uniform set of infrared extinction maps to provide accurate assessments of cloud mass and structure and compare these with inventories of young stellar objects within the clouds. We present evidence indicating that both the yield and rate of star formation can vary considerably in local clouds, independent of their mass and size. We find that the surface density structure of such clouds appears to be important in controlling both these factors. In particular, we find that the star formation rate (SFR) in molecular clouds is linearly proportional to the cloud mass (M 0.8 ) above an extinction threshold of A K ≈ 0.8 magnitudes, corresponding to a gas surface density threshold of Σ gas ≈ 116 M ⊙ pc −2 . We argue that this surface density threshold corresponds to a gas volume density threshold which we estimate to be n(H 2 ) ≈ 10 4 cm −3 . Specifically we find SFR (M ⊙ yr −1 ) = 4.6 ± 2.6 × 10 −8 M 0.8 (M ⊙ ) for the clouds

Context. Stars form in the cold dense cores of interstellar molecular clouds and the detailed knowledge of the spectrum of masses of such cores is clearly a key for the understanding of the origin of the IMF. To date, observations have presented somewhat contradictory evidence relating to this issue. Aims. In this paper we propose to derive the mass function of a complete sample of dense molecular cores in a single cloud employing a robust method that uses uses extinction of background starlight to measure core masses and enables the reliable extension of such measurements to lower masses than previously possible. Methods. We use a map of near-infrared extinction in the nearby Pipe dark cloud to identify the population of dense cores in the cloud and measure their masses. Results. We identify 159 dense cores and construct the mass function for this population. We present the first robust evidence for a departure from a single power-law form in the mass function of a population of cores and find that this mass function is surprisingly similar in shape to the stellar IMF but scaled to a higher mass by a factor of about 3. This suggests that the distribution of stellar birth masses (IMF) is the direct product of the dense core mass function and a uniform star formation efficiency of 30%±10%, and that the stellar IMF may already be fixed during or before the earliest stages of core evolution. These results are consistent with previous dust continuum studies which suggested that the IMF directly originates from the core mass function. The typical density of ∼10 4 cm −3 measured for the dense cores in this cloud suggests that the mass scale that characterizes the dense core mass function may be the result of a simple process of thermal (Jeans) fragmentation.

In this paper, we investigate scaling relations between star formation rates and molecular gas masses for both local Galactic clouds and a sample of external galaxies. We specifically consider relations between the star formation rates and measurements of dense, as well as total, molecular gas masses. We argue that there is a fundamental empirical scaling relation that directly connects the local star formation process with that operating globally within galaxies. Specifically, the total star formation rate in a molecular cloud or galaxy is linearly proportional to the mass of dense gas within the cloud or galaxy. This simple relation, first documented in previous studies, holds over a span of mass covering nearly nine orders of magnitude and indicates that the rate of star formation is directly controlled by the amount of dense molecular gas that can be assembled within a star formation complex. We further show that the star formation rates and total molecular masses, characterizing both local clouds and galaxies, are correlated over similarly large scales of mass and can be described by a family of linear star formation scaling laws, parameterized by f DG , the fraction of dense gas contained within the clouds or galaxies. That is, the underlying star formation scaling law is always linear for clouds and galaxies with the same dense gas fraction. These considerations provide a single unified framework for understanding the relation between the standard (nonlinear) extragalactic Schmidt-Kennicutt scaling law, that is typically derived from CO observations of the gas, and the linear star formation scaling law derived from HCN observations of the dense gas.

We present high-resolution, high dynamic range column-density and color-temperature maps of the Orion complex using a combination of Planck dust-emission maps, Herschel dust-emission maps, and 2MASS NIR dust-extinction maps. The column-density maps combine the robustness of the 2MASS NIR extinction maps with the resolution and coverage of the Herschel and Planck dustemission maps and constitute the highest dynamic range column-density maps ever constructed for the entire Orion complex, covering 0.01 mag < A K < 30 mag, or 2 × 10 20 cm −2 < N < 5 × 10 23 cm −2 . We determined the ratio of the 2.2 µm extinction coefficient to the 850 µm opacity and found that the values obtained for both Orion A and B are significantly lower than the predictions of standard dust models, but agree with newer models that incorporate icy silicate-graphite conglomerates for the grain population. We show that the cloud projected probability distribution function, over a large range of column densities, can be well fitted by a simple power law. Moreover, we considered the local Schmidt-law for star formation, and confirm earlier results, showing that the protostar surface density Σ * follows a simple law Σ * ∝ Σ β gas , with β ∼ 2.

Aims. We present a 8• × 6• , high resolution extinction map of the Pipe nebula using 4.5 million stars from the Two Micron All Sky Survey (2MASS) point source catalog. Methods. The use of N , A&A, 377, 1023, a robust and optimal technique to map the dust column density, allows us to detect a A V = 0.5 mag extinction at a 3-σ level with a 1 arcmin resolution. Results. (i) We find for the Pipe nebula a normal reddening law,We measure the cloud distance using Hipparchos and Tycho parallaxes, and obtain 130 +24 −58 pc. This, together with the total estimated mass, 10 4 M , makes the Pipe the closest massive cloud complex to Earth. (iii) We compare the N extinction map to the NANTEN 12 CO observations and derive with unprecedented accuracy the relationship between the near-infrared extinction and the 12 CO column density and hence (indirectly) the 12 CO X-factor, that we estimate to be 2.91 × 10 20 cmWe identify approximately 1500 OH/IR stars located within the Galactic bulge in the direction of the Pipe field. This represents a significant increase of the known numbers of such stars in the Galaxy. Conclusions. Our analysis confirms the power and simplicity of the color excess technique to study molecular clouds. The comparison with the NANTEN 12 CO data corroborates the insensitivity of CO observations to low column densities (up to approximately 2 mag in A V ), and shows also an irreducible uncertainty in the dust-CO correlation of about 1 mag of visual extinction.

We present an overview of data available for the Ophiuchus and Perseus molecular clouds from ``Phase I'' of the COMPLETE Survey of Star-Forming Regions. This survey provides a range of data complementary to the Spitzer Legacy Program ``From Molecular Cores to Planet Forming Disks.'' Phase I includes: Extinction maps derived from 2MASS near-infrared data using the NICER algorithm; extinction and temperature maps derived from IRAS 60 and 100um emission; HI maps of atomic gas; 12CO and 13CO maps of molecular gas; and submillimetre continuum images of emission from dust in dense cores. Not unexpectedly, the morphology of the regions appears quite different depending on the column-density tracer which is used, with IRAS tracing mainly warmer dust and CO being biased by chemical, excitation and optical depth effects. Histograms of column-density distribution are presented, showing that extinction as derived from 2MASS/NICER gives the closest match to a log-normal distribution as is predicted by numerical simulations. All the data presented in this paper, and links to more detailed publications on their implications are publically available at the COMPLETE website.Comment: Accepted by AJ. Full resolution version available from: http://www.cfa.harvard.edu/COMPLETE/papers/complete_phase1.pd

Abstract.We generalize the technique of Lada et al. (1994) to map dust column density through a molecular cloud (Nice) to an optimized multi-band technique (Nicer) that can be applied to any multi-band survey of molecular clouds. We present a first application to a ∼625 deg 2 subset of the Two Micron All Sky Survey (2MASS) data and show that when compared to Nice, the optimized Nicer technique (i) achieves the same extinction peak values, (ii) improves the noise variance of the map by a factor of 2 and (iii) is able to reach 3 σ dust extinction measurements as low as AV 0.5 magnitudes, better than or equivalent to classical optical star count techniques and below the threshold column density for the formation of CO, the brightest H2 tracer in radio-spectroscopy techniques. The application of the Nicer techniques to near-infrared data obtained with a 8 meter-class telescope with a state-of-the-art NIR camera, such as the VLT-ISAAC combination, will be able to achieve dynamic ranges from below 10 21 protons cm −2 to over 10 23 protons cm −2 (AV in the range [0.3, 60]) and spatial resolutions <10 , making use of a single and straightforward column density tracer, extinction by interstellar dust.

In this paper we present the results of a systematic investigation of an entire population of predominately starless dust cores within a single molecular cloud, the Pipe Nebula. Analysis of extinction data shows the cores to be dense objects characterized by a narrow range of density with a median value of n(H 2 ) ¼ 7 ; 10 3 . The nonthermal velocity dispersions measured in molecular emission lines are found to be subsonic for the large majority of the cores and show no correlation with core mass (or size). Thermal pressure is found to be the dominate source of internal gas pressure and support for most of the core population. The total internal pressures of the cores are found to be roughly independent of core mass over the entire (0.2Y20 M ) range of the core mass function (CMF) indicating that the cores are in pressure equilibrium with an external source of pressure. This external pressure is most likely provided by the weight of the surrounding molecular cloud. Most of the cores appear to be pressure confined, gravitationally unbound entities whose fundamental physical properties are determined by only a few factors, which include self-gravity, gas temperature, and the simple requirement of pressure equilibrium with the surrounding environment. The entire core population is found to be characterized by a single critical Bonnor-Ebert mass of approximately 2 M . This mass coincides with the characteristic mass of the Pipe CMF suggesting that the CMF (and ultimately the stellar IMF) has its origin in the physical process of thermal fragmentation in a pressurized medium.