EPoS Contribution

Analytical model of hierarchical fragmentation within molecular clouds: towards a new perspective? Benjamin Thomasson IPAG, Grenoble, FR
In star forming regions, most of the newborn stars are found in binary/multiple systems and/or are clustered within small groups. These stellar systems are hosted inside much larger molecular clouds that provide the mass reservoir for the star formation events. The fragmentation of this large-scale gaseous environment results in a coupled origin for stellar multiplicity and stellar masses. In this presentation, I will introduce an analytical model of fragmentation that both describes the multi-scale nature of the cloud and establishes its sub-structuration as the outcome of a hierarchical fragmentation stochastic process resulting from a gravo-turbulent cascade. In this paradigm, turbulence generates local density fluctuations that are the seeds for local collapse to occur and eventually set the stage for future gravitational collapse at smaller scales. This hierarchical mode of fragmentation cascades along increasingly smaller gaseous sub-structures until stellar systems are formed. Such a process may be decisive to set both the multiplicity of stellar groups and their stellar masses. In particular, I will show that hierarchical fragmentation is a self-regulating mechanism that tends to form identical systems on average. Due to the existence of two distinct fragmentation drivers, hierarchical fragmentation is unlikely a scale-free process. In addition, the fragment rate which tracks the productivity of fragments within a cloud, decreases as the gaseous fragments gets denser such that fragmentation stops when the collapse is no longer isothermal. Finally, I will use this model of fragmentation to derive the conditions required to recover a Salpeter IMF from a top-heavy Core Mass Function. This work suggests that hierarchical fragmentation may seed the spatial properties of young stellar groups as well as the IMF of individual young stars.

Caption: Variations of the fragmentation rate (equivalent to a net fragment production) as a function of the fragment size for different mass transfer rates ξ (equivalent to a net fragment formation efficiency) using an adiabatic equation of state of polytropic index p. The black dotted line is associated with an isothermal equation of state with a constant p = 1. The transition between turbulent and thermal fragmentation is delimited by a grey line. Fragmentation stops for scales at which the fragmentation rate cancels out.
Collaborators: I. Joncour, IPAG, FR E. Moraux, IPAG, FR F. Motte, IPAG, FR	Relevant topic(s): Fragmentation Initial MF
Relevant Big Question: What mechanism controls the fragmentation of molecular clouds and the formation of multiple stellar systems?