Linear thermal instability and formation of clumpy gas clouds including ambipolar diffusion

Abstract

Thermal instability is one of the most important processes in the formation of clumpy substructure in magnetic molecular clouds. On the other hand, ambipolar diffusion, or ion–neutral friction, has long been thought to be an important energy dissipation mechanism in these clouds. Thus, it would interesting to investigate the effect of ambipolar diffusion on the thermal instability and formation of clumps in the magnetic molecular clouds. For this purpose, as a first step, we turn our attention to the linear perturbation stage. In this way, we obtain a non‐dimensional characteristic equation that reduces to the prior characteristic equation in the absence of the magnetic field and ambipolar diffusion. With numerical manipulation of this characteristic equation, we conclude that there are solutions where the thermal instability allows compression along the magnetic field but not perpendicular to it. We infer that this aspect might be in evidence in the formation of observed disc‐like (oblate) clumps in magnetic molecular clouds.

Thermal instability in molecular clouds, including dust particles, Hall effect and ambipolar diffusion

“…The fifth‐order polynomial describes unstable modes, and two stable wave modes are described by setting the second‐order polynomial equal to zero. A study of the fifth‐order polynomial leads to the results of Nejad‐Asghar & Ghanbari (2003). We can conclude that in the ambipolar regime, we have the isentropic mode at θ= 0 (or Θ= 1).…”

“…In Fig. 4, it is apparent that at θ= 0° and θ= 90°, the Hall effect does not affect the results of Nejad‐Asghar & Ghanbari (2003), but at θ= 45° the increase of the non‐dimensional Hall coefficient from zero to one does not significantly affect the diagram. By increasing to higher values, the diagram tends to isentropic mode.…”

“…We can write the characteristic equation in the following form: The coefficients of Y are defined as follows: Here, we have If , , and α vanish, then the characteristic reduces to the characteristic equation of Field (1965). If we neglect the effect of Ohmic and Hall diffusion, the characteristic reduces to the characteristic equation of Nejad‐Asghar & Ghanbari (2003).…”

“…For example, Birk (2000) studied the effect of ambipolar diffusion or ion‐neutral friction on condensation modes in weakly ionized plasmas. Later, Nejad‐Asghar & Ghanbari (2003) investigated the effect that this diffuse mechanism had on thermal instability, and studied the domains of stability. They found that thermal instability causes the cloud to contract along the magnetic field.…”

Investigating thermal evolution of the self-gravitating one dimensional molecular cloud by smoothed particle hydrodynamics

“…Birk (2000) has used the two-fluid technique to find the thermal condensation modes in weakly ionized hydrogen plasma. Nejad-Asghar & Ghanbari (2003) studied the effect of linear thermal instability in a weakly ionized magnetic molecular cloud within the one-fluid description. Nejad-Asghar (2007) has recently made the assumption that the molecular cloud is initially an uniform ensemble which then fragments due to thermal instability.…”