Thermal condensation modes of an expanding medium including self-gravity and diffusion mechanisms

Abstract

Thermal instability of a self-gravitating magnetic molecular cloud is investigated by means of a linear perturbation analysis, when the unperturbed background is expanding uniformly. The previous studies did not precisely consider the effect of Hall diffusion, since they supposed that the electrons and ions move together. Then, we revisit the effect of Hall diffusion on the growth of the thermal instability. Our results indicate that the non-ideal effects highly reduce the stabilizing role of the magnetic field in a thermally unstable medium. With increasing the diffusion parameters, the profile of the growth rate tends to the non-magnetized case. Thus, thermal instability in the presence of the diffusion mechanisms can explain the formation of low-mass condensations in molecular clouds. The wavenumber of the perturbations corresponding to the maximum growth rate also shifts to the smaller values (longer wavelengths) as the nondimensional diffusion parameters become large. We find that self-gravity shifts the growth rate of a system without self-gravity to the higher values, while the growth rate of such a medium is reduced because of the existence of expansion. Therefore the expanding motions within the molecular clouds cause a more stable system. But the diffusion mechanisms can enhance the growth rate and speed up the rate of structure formation in molecular clouds.