On the evolution of the mass density profile of dense molecular clouds

Abstract

We aim to obtain the equations which govern the evolution of the mass density profile of a dense irrotational molecular cloud (MC). Our study is based on the notion of "ensemble of MCs", introduced in our previous work. The MCs are modeled by use of the "ensemble abstract representative member": a spherically symmetric, isotropic and isothermal MC which accretes radially matter from its surroundings. Applying the equations of hydrodynamics to a self-gravitating isothermal spherical gas cloud, we obtain a system of 2 first-order non-linear partial differential equations, which govern the evolution of two unknown fields: the exponent of density profile and the accretion velocity. Assuming a steady-state flow, we get approximate solutions using the method of leading-order terms. Far from the cloud centre the obtained density profile is $\varrho=\ell^{-2}$ and the accretion velocity is constant, while near to the centre $\varrho=\ell^{-3/2}$ and $v_{\rm a}\propto\ell^{-1/2}$. Through our dynamical equations, the obtained solutions coincide completely with those found using the equation of energy conservation of a fluid element (in our previous work). Also, combining the equations of energy balance for a fluid element, we arrive at the conclusion that the cloud layers far from the centre are in a stable dynamical state if the accretion velocity flow is sub- or transsonic; otherwise they are marginally stable (moderately supersonic flow) or unstable (supersonic flow). Both solutions are consistent only if the accretion flow is subsonic and hence the outer layers are stable. Finally, under the assumption that both the accretion velocity and density scale with $\ell$ and their power-law exponents are position-independent, we show that the density scaling exponent far away from the centre is $p=2$ and this value is an attractor. Hence this value should be observable in dense MCs.