Network of Star Formation: Fragmentation Controlled by Scale-dependent Turbulent Pressure and Accretion onto the Massive Cores Revealed in the Cygnus-X GMC Complex

Abstract

Molecular clouds have complex density structures produced by processes including turbulence and gravity. We propose a triangulation-based method to dissect the density structure of a molecular cloud and study the interactions between dense cores and their environments. In our approach, a Delaunay triangulation is constructed, which consists of edges connecting these cores. Starting from this construction, we study the physical connections between neighboring dense cores and the ambient environment in a systematic fashion. We apply our method to the Cygnus-X massive GMC complex and find that the core separation is related to the mean surface density by ${{\rm{\Sigma }}}_{\mathrm{edge}}\propto {l}_{\mathrm{core}}^{-0.28}$ , which can be explained by fragmentation controlled by a scale-dependent turbulent pressure (where the pressure is a function of scale, e.g., p ~ l^2/3). We also find that the masses of low-mass cores ( ${M}_{\mathrm{core}}\lt 10\,{M}_{\odot }$ ) are determined by fragmentation, whereas massive cores ( ${M}_{\mathrm{core}}\gt 10\,{M}_{\odot }$ ) grow mostly through accretion. The transition from fragmentation to accretion coincides with the transition from a log-normal core mass function (CMF) to a power-law CMF. By constructing surface density profiles measured along edges that connect neighboring cores, we find evidence that the massive cores have accreted a significant fraction of gas from their surroundings and thus depleted the gas reservoir. Our analysis reveals a picture where cores form through fragmentation controlled by scale-dependent turbulent pressure support, followed by accretion onto the massive cores, and the method can be applied to different regions to achieve deeper understandings in the future.