Star Formation Efficiency and Dispersal of Giant Molecular Clouds with UV Radiation Feedback: Dependence on Gravitational Boundedness and Magnetic Fields

Molecular clouds are supported by turbulence and magnetic fields, but quantifying their influence on cloud life cycle and star formation efficiency (SFE) remains an open question. We perform radiation magnetohydrodynamic simulations of star-forming giant molecular clouds (GMCs) with UV radiation feedback, in which the propagation of UV radiation via ray tracing is coupled to hydrogen photochemistry. We consider 10 GMC models that vary in either initial virial parameter (1 <= alpha (vir,0) <= 5) or dimensionless mass-to-magnetic flux ratio (0.5 <= mu (phi,0) <= 8 and infinity ); the initial mass 10(5) M (circle dot) and radius 20 pc are fixed. Each model is run with five different initial turbulence realizations. In most models, the duration of star formation and the timescale for molecular gas removal (primarily by photoevaporation) are 4-8 Myr. Both the final SFE (epsilon (*)) and time-averaged SFE per freefall time (epsilon (ff)) are reduced by strong turbulence and magnetic fields. The median epsilon (*) ranges between 2.1% and 9.5%. The median epsilon (ff) ranges between 1.0% and 8.0%, and anticorrelates with alpha (vir,0), in qualitative agreement with previous analytic theory and simulations. However, the time-dependent alpha (vir)(t) and epsilon (ff,obs)(t) based on instantaneous gas properties and cluster luminosity are positively correlated due to rapid evolution, making observational validation of star formation theory difficult. Our median epsilon (ff,obs)(t) approximate to 2% is similar to observed values. We show that the traditional virial parameter estimates the true gravitational boundedness within a factor of 2 on average, but neglect of magnetic support and velocity anisotropy can sometimes produce large departures from traditional virial parameter estimates. Magnetically subcritical GMCs are unlikely to represent sites of massive star formation given their unrealistic columnar outflows, prolonged lifetime, and low escape fraction of radiation.

Automated summary

By simulating star-forming giant molecular clouds, the study shows that strong turbulence and magnetic fields reduce star formation efficiency. The difficulty of observational validation of star formation theory lies in their rapid evolution due to the influence of instantaneous gas properties and cluster luminosity. The traditional virial parameter estimates gravitational boundedness accurately on average, but neglecting magnetic support and velocity anisotropy can lead to significant departures from these estimates.