Space spaghetti in magnetic sauce

Molecular clouds are to stars what the maternal placenta is to babies: a reservoir of nutrients to ensure the development of the embryo, to be disposed of after birth. However, the analogy has now a major flaw: while "placenta" is the greek word for "pancake", the structure of molecular clouds is much more "filamentary" than "slab-like".

It was already known that the diffuse, atomic gas (the so-called cold neutral medium), mostly observed through HI 21-cm line absorption, had a complex spatial distribution, taking the shape sometimes of corrugated sheets, sometimes of ribbons with overlapping knots (Heiles & Troland 2003, ApJ, 586, 1067). Thanks to the spectacular capabilities of the ESA submillimeter Space Observatory Herschel (launched in 2009), we now know that such a complexity of shapes also extends to the dense molecular clouds from wich stars form. In star forming regions where we previously pictured fluffy clouds, Herschel's infrared eyes have revealed intricate networks of intertwined filaments, tufts of hair, plates of spaghetti: in short, a structure replicating on a small scale the web-like cosmic fabric of the Universe (Molinari et al. 2010, A&A, 518, L100).

What are the causes of such a complexity? No one knows for sure. Turbulent shear motions and magnetic stretching are likely involved, but both processes are poorly understood. According to the current picture, the filaments are expected to fragment into equally spaced "cores", and the latter to collapse into a necklace of stars, like beads on a string. It is hoped that detailed observational studies of the physical properties of the filaments will shed some light on the origin and the evolution of these structures. However, the results obtained so far are puzzling: first, the sample filamentary clouds studied by Herschel seem to be characterized by a universal width, of the order of 0.03 pc, despite a variation of the peak density and other physical characteristics by over two orders of magnitude (Arzoumanian et al. 2011, A&A, 529, L6); second, the filaments appear to be made of many thread-like subfilaments with similar kinematics and chemical composition, woven into tight "bundles" (Hacar & Tafalla 2011, A&A, 533, A34). Both properties are mysterious.

A first step toward an understanding of the physical properties of filamentary clouds was undertaken by a student of the University of Florence, Claudia Toci, who summarized the results of her master thesis in two papers recently published in MNRAS in collaboration with Daniele Galli (INAF-Observatory of Arcetri). The first study "Polytropic models of filamentary interstellar clouds - I. Structure and stability" (Toci & Galli 2015, MNRAS, 446, 2110) proved the inadequacy of traditional models for self-gravitating filaments based on a simple isothermal equation of state, that fail to reproduce their observed radial density profiles. Instead, the data seem to require a "softer" equation of state, as expected for a structure supported by a superposition of Alfven waves, rather than thermal pressure. These polytropic models were further developed in the second paper "Polytropic models of filamentary interstellar clouds - II. Helical magnetic fields" (Toci & Galli 2015, MNRAS, 446, 2118), where the effects of a large-scale helical magnetic field were included. The hoop stresses associated to the latter may result in significant squeezing of the filament, and promote the onset of fierce varicose instabilities that break the filament into subpieces.

Despite their idealization, these studies indicate in the Galactic magnetic field a crucial and powerful agent able to forge and shape the dense gas of molecular clouds. Certainly not the only one, but for sure one of the most formidably difficult to model and to measure. But brave theoreticians remain undaunted.

Fig.1: Radial density profiles (normalised to the central density) of self-gravitating cylinders with polytropic exponent 2, 3/2, 4/3 (short-dashed lines, from left to right) and 1/3, 1/2, 2/3 and 3/4 (long-dashed lines, from right to left). The thick solid lines show the density profiles of a model with an isothermal and a logatropic equation of state. Dotted lines are singular solutions. The hatched area corresponds to the observed mean density profile of filaments in the star forming region IC5146.

Edited by Daniele Galli and Anna Gallazzi, 11/12/2014