Magnetic field effects in the initial collapse of polytropic clouds

Abstract

In this paper, we investigate the inuence of the magnetic field and the temperature gradients on the stability of a spherical cloud. Observational data confirm a power-law relation between the magnetic field and the gas density. Here, we study the stability of a magnetized cloud with a toroidal magnetic field and the polytropic equation of state. We find that density and mass profiles of the modified clouds are departed from the non-magnetized isothermal case. As the result, the critical mass, radius and density contrast at the onset of the gravitational instability differ from the critical Bonnor-Ebert mass and its critical radius and density contrast. Thus, both the magnetic field and the temperature gradients play important roles in the structure of the cloud. Furthermore, the cloud critical mass is increased for the higher values of the polytropic exponent, irrespective of the values of the magnetic to thermal pressure ratios or the field gradients. Different values of the magnetic field gradient and strength change the values of the critical density contrast and radius of the cloud; however, both of them fade their importance in the values of the critical mass.