Dependence of the Star Formation Efficiency on the Parameters of Molecular Cloud Formation Simulations

Abstract

We investigate the response of the star formation efficiency (SFE) to the main parameters of simulations of molecular cloud formation by the collision of warm diffuse medium (WNM) cylindrical streams, neglecting stellar feedback and magnetic fields. The parameters we vary are the Mach number of the inflow velocity of the streams, Msinf, the rms Mach number of the initial background turbulence in the WNM, and the total mass contained in the colliding gas streams, Minf. Because the SFE is a function of time, we define two estimators for it, the "absolute" SFE, measured at t = 25 Myr into the simulation's evolution (sfeabs), and the "relative" SFE, measured 5 Myr after the onset of star formation in each simulation (sferel). The latter is close to the "star formation rate per free-fall time" for gas at n = 100 cm^-3. We find that both estimators decrease with increasing Minf, although by no more than a factor of 2 as Msinf increases from 1.25 to 3.5. Increasing levels of background turbulence similarly reduce the SFE, because the turbulence disrupts the coherence of the colliding streams, fragmenting the cloud, and producing small-scale clumps scattered through the numerical box, which have low SFEs. Finally, the SFE is very sensitive to the mass of the inflows, with sferel decreasing from ~0.4 to ~0.04 as the the virial parameter in the colliding streams increases from ~0.15 to ~1.5. This trend is in partial agreement with the prediction by Krumholz & McKee (2005), since the latter lies within the same range as the observed efficiencies, but with a significantly shallower slope. We conclude that the observed variability of the SFE is a highly sensitive function of the parameters of the cloud formation process, and may be the cause of significant scatter in observational determinations.

19 pages, submitted to MNRAS

References in corpus (7)

• Theory of Star Formation
• Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions
• The Birth of Molecular Clouds: Formation of Atomic Precursors in Colliding Flows
• On the Structure of the Orion A Cloud and the Formation of the Orion Nebula Cluster
• High- and Low-Mass Star Forming Regions from Hierarchical Gravitational Fragmentation. High local Star Formation Rates with Low Global Efficiencies
• The Nature of the Velocity Field in Molecular Clouds. I. The Non-Magnetic Case
• Formation of an O-Star Cluster by Hierarchical Accretion in G20.08-0.14 N

Cited by in corpus (10)

• Regulation of Star Formation Rates in Multiphase Galactic Disks: a Thermal/Dynamical Equilibrium Model
• Maximally Star-Forming Galactic Disks I. Starburst Regulation Via Feedback-Driven Turbulence
• Gravity or turbulence? II. Evolving column density PDFs in molecular clouds
• Molecular Cloud Evolution V. Cloud Destruction by Stellar Feedback
• An Evolutionary Model for Collapsing Molecular Clouds and Their Star Formation Activity
• Star formation efficiency as a function of metallicity: from star clusters to galaxies
• The Free-Fall time of finite Sheets and Filaments
• On column density thresholds and the star formation rate
• Characterizing the structure of diffuse emission in Hi-GAL maps
• Interstellar Turbulence and Star Formation