Local‐density driven clustered star formation: Model and (some) implications

Parmentier, G.

doi:10.1002/asna.201412063

Cited by 8 publications

(13 citation statements)

References 18 publications

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“…This effect was already pointed out by Tan et al (2006) and is also discussed by Parmentier (2014). For Clump A, Equation (15) predicts a slightly lower global star formation efficiency, i.e., SFE(t = τ ff ) 0.11 (SFE = 0.12 in the numerical solution).…”

Section: Age Distributions Within Newly Born Star Clusterssupporting

confidence: 60%

Stellar Age Spreads in Clusters as Imprints of Cluster-Parent Clump Densities

Parmentier

Pfalzner

Grebel

2014

ApJ

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It has recently been suggested that high-density star clusters have stellar age distributions much narrower than that of the Orion Nebula Cluster, indicating a possible trend of narrower age distributions for denser clusters. We show this effect to likely arise from star formation being faster in gas with a higher density. We model the star formation history of molecular clumps in equilibrium by associating a star formation efficiency per free-fall time, ff , to their volume density profile. We focus on the case of isothermal spheres and we obtain the evolution with time of their star formation rate. Our model predicts a steady decline of the star formation rate, which we quantify with its half-life time, namely, the time needed for the star formation rate to drop to half its initial value. Given the uncertainties affecting the star formation efficiency per free-fall time, we consider two distinct values: ff = 0.1 and ff = 0.01. When ff = 0.1, the half-life time is of the order of the clump free-fall time, τ ff . As a result, the age distributions of stars formed in high-density clumps have smaller full-widths at half-maximum than those of stars formed in low-density clumps. When the star formation efficiency per free-fall time is 0.01, the half-life time is 10 times longer, i.e., 10 clump free-fall times. We explore what happens if the duration of star formation is shorter than 10τ ff , that is, if the half-life time of the star formation rate cannot be defined. There, we build on the invariance of the shape of the young cluster mass function to show that an anti-correlation between the clump density and the duration of star formation is expected. We therefore conclude that, regardless of whether the duration of star formation is longer than the star formation rate half-life time, denser molecular clumps yield narrower star age distributions in clusters. Published densities and stellar age spreads of young clusters and star-forming regions actually suggest that the timescale for star formation is of order 1-4τ ff . We also discuss how the age bin size and uncertainties in stellar ages affect our results. We conclude that there is no need to invoke the existence of multiple cluster formation mechanisms to explain the observed range of stellar age spreads in clusters.

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Section: Age Distributions Within Newly Born Star Clusterssupporting

confidence: 60%

Stellar Age Spreads in Clusters as Imprints of Cluster-Parent Clump Densities

Parmentier

Pfalzner

Grebel

2014

ApJ

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“…1). This is in line with the conclusions of analytical and semi-analytical calculations (Tan et al 2006;Elmegreen 2011;Parmentier 2014Parmentier , 2019, and of hydrodynamical simulations (Cho & Kim 2011;Girichidis et al 2011), which all demonstrate that a gas density gradient inside molecular clouds and clumps enhance their star formation rate. This immediately suggests that part of the scatter observed in the dense-gas relation has a physical origin, namely, the variations in the gas spatial distribution from one star-forming region to another.…”

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confidence: 88%

A new parameterization of the star formation rate-dense gas mass relation: embracing gas density gradients

Parmentier,

Pasquali

2020

Preprint

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It is well-established that a gas density gradient inside molecular clouds and clumps raises their star formation rate compared to what they would experience from a gas reservoir of uniform density. This effect should be observed in the relation between dense-gas mass M dg and star formation rate SF R of molecular clouds and clumps, with steeper gas density gradients yielding higher SF R/M dg ratios. The content of this paper is two-fold. Firstly, we build on the notion of magnification factor introduced by Parmentier (2019) to redefine the dense-gas relation (i.e. the relation between M dg and SF R). Not only does the SF R/M dg ratio depend on the mean freefall time of the gas and on its (intrinsic) star formation efficiency per free-fall time, it also depends on the logarithmic slope −p of the gas density profile and on the relative extent of the constant-density region at the clump center. Secondly, we show that nearby molecular clouds follow the newly-defined dense-gas relation, provided that their dense-gas mass is defined based on a volume density criterion. We also find the same trend for the dense molecular clouds of the Central Molecular Zone (CMZ) of the Galaxy, although this one is scaled down by a factor of 10 compared to nearby clouds. The respective locii of both nearby and CMZ clouds in the (p, SF R/M dg ) parameter space is discussed.

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“…7. CONCLUSIONS Molecular clumps have a higher star formation rate when they present a volume density gradient than when they are uniform in density (Tan et al 2006;Girichidis et al 2011;Cho & Kim 2011;Elmegreen 2011;Parmentier 2014). This effect arises from the clump inner regions being denser than the clump as a whole, yielding faster and more efficient star formation than would be expected based on the clump mean free-fall time (see Figs 3 and 4).…”

Section: Mapping the Magnification Factor ζmentioning

confidence: 90%

“…2 in Tan et al 2006). For a clump with a density profile of slope −2 combined to a small (≃ 10 −2 the clump radius) central core, Parmentier (2014) finds that the star formation efficiency achieved within one free-fall time amounts to 1.6ǫ ff , rather than ǫ ff , with ǫ ff the star formation efficiency per free-fall time. Additionally, Fig.…”

Section: Introductionmentioning

confidence: 96%

The Density Gradient Inside Molecular-gas Clumps as a Booster of Their Star Formation Activity

Parmentier

2019

ApJ

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Star-forming regions presenting a density gradient experience a higher star formation rate than if they were of uniform density. We refer to the ratio between the star formation rate of a spherical centrally-concentrated gas clump and the star formation rate that this clump would experience if it were of uniform density as the magnification factor ζ. We map ζ as a function of clump mass, radius, initial volume density profile and star formation time-span. For clumps with a steep density profile (i.e. power-law slope ranging from −3 to −4, as observed in some high-density regions of Galactic molecular clouds), we find the star formation rate to be at least an order of magnitude higher than its top-hat equivalent. This implies that such clumps experience faster and more efficient star formation than expected based on their mean free-fall time. This also implies that measurements of the star formation efficiency per free-fall time of clumps based on their global properties, namely, mass, mean volume density and star formation rate, present wide fluctuations. These reflect the diversity in the density profile of star-forming clumps, not necessarily variations in the physics of star formation. Steep density profiles inside star-cluster progenitors may be instrumental in the formation of multiple stellar populations, such as those routinely observed in old globular clusters.

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Local‐density driven clustered star formation: Model and (some) implications

Cited by 8 publications

References 18 publications

Stellar Age Spreads in Clusters as Imprints of Cluster-Parent Clump Densities

Stellar Age Spreads in Clusters as Imprints of Cluster-Parent Clump Densities

A new parameterization of the star formation rate-dense gas mass relation: embracing gas density gradients

The Density Gradient Inside Molecular-gas Clumps as a Booster of Their Star Formation Activity

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