Internal Structure of Stellar Clusters: Geometry of Star Formation

Abstract

Abstract. The study of the internal structure of star clusters provides important clues concerning their formation mechanism and dynamical evolution. There are both observational and numerical evidences indicating that open clusters evolve from an initial clumpy structure, presumably a direct consequence of the formation in a fractal medium, toward a centrally condensed state. This simple picture has, however, several drawbacks. There can be very young clusters exhibiting radial patterns maybe reflecting the early effect of gravity on primordial gas. There can be also very evolved clusters showing fractal patterns that either have survived through time or have been generated subsequently by some (unknown) mechanism. Additionally, the fractal structure of some open clusters is much clumpier than the average structure of the interstellar medium in the Milky Way, although in principle a very similar structure should be expected. Here we summarize and discuss observational and numerical results concerning this subject.

The complex distribution of recently formed stars. Bimodal stellar clustering in the star-forming region NGC 346

“…On the other hand, recent numerical simulations by Girichidis et al (2012) and Dale et al (2013) find a higher degree of hierarchy for stars formed from turbulent molecular clouds with smaller values of D3 ∼ 1.6. These studies, which apply the Qparameter (Cartwright & Whitworth 2004), find similar fractal dimensions with that inferred from the same method for the Taurus molecular complex (D3 ∼ 1.5), in line with the fractal dimensions of the ISM in this region (see, e.g., Alfaro & Sánchez 2011). While the differences in the derived fractal dimensions may reflect discrepancies in the methods used, the may as well demonstrate the fact that stars have a different spatial distribution to the gas from which they formed, or that the gas is not similarly structured everywhere.…”

“…On the other hand, recent numerical simulations by Girichidis et al (2012) and Dale et al (2013) find a higher degree of hierarchy for stars formed from turbulent molecular clouds with smaller values of D3 ∼ 1.6. These studies, which apply the Qparameter (Cartwright & Whitworth 2004), find similar fractal dimensions with that inferred from the same method for the Taurus molecular complex (D3 ∼ 1.5), in line with the fractal dimensions of the ISM in this region (see, e.g., Alfaro & Sánchez 2011).…”