Structure and Fragmentation of Molecular Clouds

The molecular gas out of which stars form is mostly found in what are known as “Molecular Clouds” (MCs), which occupy a small fraction of the volume of the interstellar medium (ISM). Despite their overall small volume, MCs are fundamental in the evolution and appearance of galaxies because that is the place where star formation (SF) occurs.

MCs have typical masses in the range ~ 103 - 105 M⊙ and show a hierarchical structure that extends from the scale of the cloud down to more compact and denser regions which are termed clumps. Clumps are the regions within MCs where eventually stellar clusters can form. The even more compact cores, which are gravitationally bound, represent the regions out of which individual stars (or multiple stars) finally form.

During the process of SF the relevant physical parameters (such as density and temperature) undergo a transformation by many orders of magnitude. However, the exact physical process(es) that regulate the evolution from the MC to the clump phase and then to the core is not yet fully understood. Specifically, the interplay between magnetic fields, turbulence and gas dynamics within MCs is very difficult to study both theoretically and observationally.

Figure 1. Plots of the position in the Galactic plane of clusters of clumps selected from the Hi-GAL survey. Spiral arms, are plotted with different colours, with the arc-colour correspondence reported in the upper-left corner of the plot. (Olmi et al. 2018, MNRAS 480,1831).

Figure 2. Integrated intensity of C18O(2-1) as obtained with the ALMA interferometer towards a compact Hi-GAL clump. The image shows several cores in which the clump has fragmented. Each box in the coordinate grid (in Galactic coordinates) has a linear size of about 0.03pc (Olmi et al. in prep.).

Our knowledge of these processes is being improved by studies of the structure and kinematics of the gas in MCs at several scale-lengths and densities, and by measuring the distribution and strength of magnetic fields, also at different scales. These observations must then be related to the physical properties of the more compact clumps and cores. Recent works show that matter in MCs, and actually in most of the ISM, is distributed in structures with filamentary shapes, often with signs of internal fragmentation, which are also characterized by the orderly presence of magnetic fields.

The Arcetri Stars and Star Formation Group has contributed to these observations both at large (e.g., with the Herschel telescope, see Fig. 1) and small scales (e.g., with the ALMA array, see Fig. 2). All these and previous literature results suggest a connection between the processes acting at the MC scale with the formation of stars, passing through the shaping of local (sub)structures. This potential link can be explored through a systematic study of the statistical properties of large samples of clumps (as shown in Fig. 1) and the detailed study of individual clumps and cores at the smallest scales (as shown in Fig. 2).