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Register a new userHigh-precision star formation efficiency measurements in nearby clouds
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On average molecular clouds convert only a small fraction epsilon_ff of their mass into stars per free-fall time, but differing star formation theories make contrasting claims for how this low mean efficiency is achieved. To test these theories, we need precise measurements of both the mean value and the scatter of epsilon_ff, but high-precision measurements have been difficult because they require determining cloud volume densities, from which we can calculate free-fall times. Until recently, most density estimates assume clouds as uniform spheres, while their real structures are often filamentary and highly non-uniform, yielding systematic errors in epsilon_ff estimates and smearing real cloud-to-cloud variations. We recently developed a theoretical model to reduce this error by using column density distributions in clouds to produce more accurate volume density estimates. In this letter, we apply this model to recent observations of 12 nearby molecular clouds. Compared to earlier analyses, our method reduces the typical dispersion of epsilon_ff within individual clouds from 0.35 dex to 0.31 dex, and decreases the median value of epsilon_ff over all clouds from ~ 0.02 to ~ 0.01. However, we find no significant change in the ~ 0.2 dex cloud-to-cloud dispersion of epsilon_ff, suggesting the measured dispersions reflect real structural differences between clouds.
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We test some ideas for star formation relations against data on local molecular clouds. On a cloud by cloud basis, the relation between the surface density of star formation rate and surface density of gas divided by a free-fall time, calculated from the mean cloud density, shows no significant correlation. If a crossing time is substituted for the free-fall time, there is even less correlation. Within a cloud, the star formation rate volume and surface densities increase rapidly with the corresponding gas densities, faster than predicted by models using the free-fall time defined from the local density. A model in which the star formation rate depends linearly on the mass of gas above a visual extinction of 8 mag describes the data on these clouds, with very low dispersion. The data on regions of very massive star formation, with improved star formation rates based on free-free emission from ionized gas, also agree with this linear relation.
We estimate the star formation efficiency per gravitational free fall time, $epsilon_{rm ff}$, from observations of nearby galaxies with resolution matched to the typical size of a Giant Molecular Cloud. This quantity, $epsilon_{rm ff}$, is theoretically important but so far has only been measured for Milky Way clouds or inferred indirectly in a few other galaxies. Using new, high resolution CO imaging from the PHANGS-ALMA survey, we estimate the gravitational free-fall time at 60 to 120 pc resolution, and contrast this with the local molecular gas depletion time to estimate $epsilon_{rm ff}$. Assuming a constant thickness of the molecular gas layer ($H = 100$ pc) across the whole sample, the median value of $epsilon_{rm ff}$ in our sample is $0.7%$. We find a mild scale-dependence, with higher $epsilon_{rm ff}$ measured at coarser resolution. Individual galaxies show different values of $epsilon_{rm ff}$, with the median $epsilon_{rm ff}$ ranging from $0.3%$ to $2.6%$. We find the highest $epsilon_{rm ff}$ in our lowest mass targets, reflecting both long free-fall times and short depletion times, though we caution that both measurements are subject to biases in low mass galaxies. We estimate the key systematic uncertainties, and show the dominant uncertainty to be the estimated line-of-sight depth through the molecular gas layer and the choice of star formation tracers.
We investigate the origin of observed local star formation relations using radiative magnetohydrodynamic simulations with self-consistent star formation and ionising radiation. We compare these clouds to the density distributions of local star-forming clouds and find that the most diffuse simulated clouds match the observed clouds relatively well. We then compute both observationally-motivated and theoretically-motivated star formation efficiencies (SFEs) for these simulated clouds. By including ionising radiation, we can reproduce the observed SFEs in the clouds most similar to nearby Milky Way clouds. For denser clouds, the SFE can approach unity. These observed SFEs are typically 3 to 10 times larger than the total SFEs, i.e. the fraction of the initial cloud mass converted to stars. Converting observed to total SFEs is non-trivial. We suggest some techniques for doing so, though estimate up to a factor of ten error in the conversion.
The SFiNCs (Star Formation in Nearby Clouds) project is an X-ray/infrared study of the young stellar populations in 22 star forming regions with distances <=1 kpc designed to extend our earlier MYStIX survey of more distant clusters. Our central goal is to give empirical constraints on cluster formation mechanisms. Using parametric mixture models applied homogeneously to the catalog of SFiNCs young stars, we identify 52 SFiNCs clusters and 19 unclustered stellar structures. The procedure gives cluster properties including location, population, morphology, association to molecular clouds, absorption, age (AgeJX), and infrared spectral energy distribution (SED) slope. Absorption, SED slope, and AgeJX are age indicators. SFiNCs clusters are examined individually, and collectively with MYStIX clusters, to give the following results. (1) SFiNCs is dominated by smaller, younger, and more heavily obscured clusters than MYStIX. (2) SFiNCs cloud-associated clusters have the high ellipticities aligned with their host molecular filaments indicating morphology inherited from their parental clouds. (3) The effect of cluster expansion is evident from the radius-age, radius-absorption, and radius-SED correlations. Core radii increase dramatically from ~0.08 to ~0.9 pc over the age range 1--3.5 Myr. Inferred gas removal timescales are longer than 1 Myr. (4) Rich, spatially distributed stellar populations are present in SFiNCs clouds representing early generations of star formation. An Appendix compares the performance of the mixture models and nonparametric Minimum Spanning Tree to identify clusters. This work is a foundation for future SFiNCs/MYStIX studies including disk longevity, age gradients, and dynamical modeling.
Ever since their discovery, Infrared dark clouds (IRDCs) are generally considered to be the sites just at the onset of high-mass (HM) star formation. In recent years, it has been realized that not all IRDCs harbour HM Young Stellar Objects (YSOs). Only those IRDCs satisfying a certain mass-size criterion, or equivalently above a certain threshold density, are found to contain HMYSOs. In all cases, IRDCs provide ideal conditions for the formation of stellar clusters. In this paper, we study the massive stellar content of IRDCs to re-address the relation between IRDCs and HM star formation. For this purpose, we have identified all IRDCs associated to a sample of 12 Galactic molecular clouds (MCs). The selected MCs have been the target of a systematic search for YSOs in an earlier study. The catalogued positions of YSOs have been used to search all YSOs embedded in each identified IRDC. In total, we have found 834 YSOs in 128 IRDCs. The sample of IRDCs have mean surface densities of 319 Mo/pc2, mean mass of 1062 Mo, and a mass function power-law slope -1.8, which are similar to the corresponding properties for the full sample of IRDCs and resulting physical properties in previous studies. We find that all those IRDCs containing at least one intermediate to high-mass young star satisfy the often-used mass-size criterion for forming HM stars. However, not all IRDCs satisfying the mass-size criterion contain HM stars. We find that the often used mass-size criterion corresponds to 35% probability of an IRDC forming a massive star. Twenty five (20%) of the IRDCs are potential sites of stellar clusters of mass more than 100 Mo.
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