SCYON Abstract

Received on November 14 2012

Local-Density Driven Clustered Star Formation

AuthorsGenevieve Parmentier (1) and Susanne Pfalzner (2)
Affiliation(1) Zentrum für Astronomie, Heidelberg Universitäat, Germany
(2) Max-Planck Institut für Radioastronomie, Bonn, Germany
Accepted byAstronomy & Astrophysics


A positive power-law trend between the local surface densities of molecular gas, Σgas, and young stellar objects, Σstar, in molecular clouds of the Solar Neighbourhood has recently been identified by Gutermuth et al. How it relates to the properties of embedded clusters, in particular to the recently established radius-density relation, has so far not been investigated. In this paper, we model the development of the stellar component of molecular clumps as a function of time and initial local volume density so as to provide a coherent framework able to explain both the molecular-cloud and embedded-cluster relations quoted above. To do so, we associate the observed volume density gradient of molecular clumps to a density-dependent free-fall time. The molecular clump star formation history is obtained by applying a constant SFE per free-fall time, εff. For volume density profiles typical of observed molecular clumps (i.e. power-law slope ≈ -1.7), our model gives a star-gas surface-density relation Σstar α Σgas2, in very good agreement with the Gutermuth et al relation. Taking the case of a molecular clump of mass M0 ≈ 104 M(sun) and radius R ≈ 6 pc experiencing star formation during 2 Myr, we derive what SFE per free-fall time matches best the normalizations of the observed and predicted (Σstar, Σgas) relations. We find εff ≈ 0.1. We show that the observed growth of embedded clusters, embodied by their radius-density relation, corresponds to a surface density threshold being applied to developing star-forming regions. The consequences of our model in terms of cluster survivability after residual star-forming gas expulsion are that due to the locally high SFE in the inner part of star-forming regions, global SFE as low as 10% can lead to the formation of bound gas-free star clusters.