Turbulent, Molecular Clouds Regulated by Radiation Feedback

Abstract

Radiation feedback from stellar clusters is expected to play a key role in setting the rate and efficiency of star formation in giant molecular clouds (GMCs) and across whole galaxies. In particular, stellar radiation may quench star formation by driving outflows and unbinding stellar clusters. To investigate how radiation forces influence realistic clouds, we have conducted a series of simulations employing the {\it Hyperion} radiation hydrodynamics solver, considering the regime that is optically thick to ultraviolet and optically thin to infrared radiation. Our model clouds cover initial surface densities between $\Sigma_{\rm cl,0} \sim 10-300~M_{\odot}~{\rm pc^{-2}}$, with varying initial turbulence and magnetic field strength ($B_{z,0}$). We follow them through turbulent, self-gravitating collapse, formation of star clusters, and cloud dispersal by stellar radiation. All our models display a lognormal distribution of gas surface density $\Sigma$ as seen by both the observer and the central cluster. For an initial virial parameter $\alpha_{\rm vir,0} = 2$, the lognormal standard deviation is $\sigma_{\rm ln \Sigma} = 1-1.5$ and the star formation rate (SFR) coefficient $\varepsilon_{\rm ff,\bar\rho} = 0.3-0.5$, both of which are sensitive to turbulence, and magnetic fields, but not radiation feedback. Embedded stars are more centrally concentrated than the gas so that above $\Sigma_{\rm cl,0} \sim 60~M_{\odot}~{\rm pc^{-2}}$, the star cluster remains intact even when surrounding gas is dispersed. The net star formation efficiency depends primarily on the distribution of Eddington ratios in the cloud and therefore increases with $\Sigma_{\rm cl,0}$ and decreases with both $\alpha_{\rm vir,0}$ and $B_{z,0}$. This also has implications for outflows, since low surface density regions may be driven outwards to nearly $10$ times their initial escape speed ($v_{\rm esc}$). However, the overall efficiency of momentum injection to the gas is reduced because much of the radiation escapes and irrespective of $\Sigma_{\rm cl,0}$, the mean outflow velocity is approximately twice $v_{\rm esc}$. Unless GMCs are highly magnetized and turbulent, the lognormal structure of gas modulates the effect of radiative feedback in disrupting clouds, so that it cannot alone explain the low observed galactic SFR.