Gravity or turbulence? VII. The Schmidt-Kennicutt law, the star formation efficiency, and the mass density of clusters from gravitational collapse rather than turbulent support

Abstract

We explore the Schmidt-Kennicutt (SK) relations and the star formation efficiency per free-fall time (${\epsilon }_{\mathrm{ff}}$), mirroring observational studies, in numerical simulations of filamentary molecular clouds undergoing gravitational contraction. We find that (a)~collapsing clouds accurately replicate the observed SK relations for galactic clouds and (b)~${\epsilon }_{\mathrm{ff}}$ is small and constant in space and in time, with values similar to those found in local clouds. We propose that this constancy arises from the similar radial scaling of the free-fall time (${\tau }_{\mathrm{ff}}$) and the internal mass in density structures with spherically-averaged density profiles near $r^{-2}$. We additionally show that (c)~the star formation rate (SFR) increases rapidly in time; (d)~the low values of ${\epsilon }_{\mathrm{ff}}$ result from evaluating ${\tau }_{\mathrm{ff}}$ and the characteristic star-formation time scale over different time intervals, combined with the increasing SFR, and (e)~the fact that star clusters are significantly denser than the gas clumps from which they form is a natural consequence of the rapidly increasing SFR, the continuous replenishment of the star-forming gas by the accretion flow, and the near $r^{-2}$ density profile induced by the collapse. Finally, we argue that interpreting ${\epsilon }_{\mathrm{ff}}$ as an efficiency is problematic since it is not bounded by unity, and because the gas mass in clouds evolves. Instead, we propose that viewing ${\epsilon }_{\mathrm{ff}}$ as the ratio of the actual SFR to the gas free-fall rate. In summary, our results show that the SK relation, the low values of ${\epsilon }_{\mathrm{ff}}$, and the mass density of stellar clusters arise naturally from gravitational contraction.

Accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS)

References in corpus (31)

• Theory of Star Formation
• Slow Star Formation in Dense Gas: Evidence and Implications
• Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions
• ATLASGAL --- properties of a complete sample of Galactic clumps
• Herschel-Planck dust optical-depth and column-density maps: I. Method description and results for Orion
• A robust numerical scheme for highly compressible magnetohydrodynamics: Nonlinear stability, implementation and tests
• Rapid Molecular Cloud and Star Formation: Mechanisms and Movies
• Observational Evidence of Dynamic Star Formation Rate in Milky Way Giant Molecular Clouds
• Star Formation Efficiency and Dispersal of Giant Molecular Clouds with UV Radiation Feedback: Dependence on Gravitational Boundedness and Magnetic Fields
• Star Formation Relations in the Milky Way
• Star Formation Laws and Efficiencies across 80 Nearby Galaxies
• Disruption of giant molecular clouds and formation of bound star clusters under the influence of momentum stellar feedback
• Star Formation in Self-Gravitating Turbulent Fluids
• Structure and mass segregation in Galactic stellar clusters
• Tree-based solvers for adaptive mesh refinement code FLASH - I: gravity and optical depths
• Effects of the environment and feedback physics on the initial mass function of stars in the STARFORGE simulations
• The Formation of Stellar Clusters in Magnetized, Filamentary Infrared Dark Clouds
• The Single-Cloud Star Formation Relation
• Are fibres in molecular cloud filaments real objects?
• Slow Star Formation in the Milky Way: Theory Meets Observations
• HP2 survey: III The California Molecular Cloud--A Sleeping Giant Revisited
• Density profile evolution during prestellar core collapse: Collapse starts at the large scale
• The effect of photoionising feedback on the shaping of hierarchically-forming stellar clusters
• Striations in the Taurus molecular cloud: Kelvin-Helmholtz instability or MHD waves?
• Magnetized converging flows towards the hot core in the intermediate/high-mass star-forming region NGC 6334 V
• Gravity Versus Magnetic Fields in Forming Molecular Clouds
• Molecular Cloud Turbulence and Star Formation
• Non-adiabatic turbulence driving during gravitational collapse
• Simultaneous evolution of the virial parameter and star formation rate in molecular clumps undergoing global hierarchical collapse
• Gravity or turbulence V: Star forming regions undergoing violent relaxation
• The kinetic and magnetic energy budget of hub-filament systems during the gravitational fragmentation of molecular clouds

Cited by in corpus (1)

• The Star Formation Factory revisited I. The impact of metallicity on collapsing star-forming clouds