The physical and the geometrical properties of simulated cold H i structures

Abstract

The objective of this paper is to help shedding some light on the nature and the properties of the cold structures formed via thermal instability in the magnetized atomic interstellar medium. To this end, we searched for clumps formed in forced (magneto)hydrodynamic simulations with an initial magnetic field ranging from 0 to 8.3 μG. We statistically analysed, through the use of Kernel density estimations, the physical and the morphological properties of a sample containing ∼1500 clumps, as well as the relative alignments between the main direction of clumps and the internal velocity and magnetic field. The density (n ∼50-200 cm-3), the thermal pressure (Pth/k ∼4.9 × 103-104 K cm-3), the mean magnetic field (∼3-11 μG), and the sonic Mach number of the selected clumps have values comparable to those reported in observations. We find, however, that the cloud sample cannot be described by a single regime concerning their pressure balance and their Alfvénic Mach number. We measured the morphological properties of clumps mainly through the asphericity and the prolatness, which appear to be more sensitive than the aspect ratios. From this analysis, we find that the presence of magnetic field, even if it is weak, does qualitatively affect the morphology of the clumps by increasing the probability of having highly aspherical and highly plolate clumps by a factor of two, that is by producing more filamentary clumps. Finally, we find that the angle between the main direction of the clumps and the local magnetic field lies between ∼π/4 and π/2 and shifts to more perpendicular alignments as the intensity of this field increases, while the relative direction between the local density structure and the local magnetic field transits from parallel to perpendicular.