The Dynamical Ejections of Massive Stars from Young Star Clusters

Abstract

Massive stars form in star clusters or OB associations which may be expanded clusters. Such stars can be expelled with a high velocity from their birth cluster through a strong close encounter with other cluster members. In particular, massive stars can be efficiently shot out from their birth cluster via dynamical ejection processes since they are mostly found in the centre of the cluster where the stellar density is the highest. In this thesis, we perform a large set of N-body calculations using the direct N-body code NBODY6. To study the ejects of initial configurations of star clusters, we vary several initial parameters, such as the initial size and mass of the clusters, initial mass segregation, and initial binary populations. We investigate several aspects of dynamical ejections of massive stars from young star clusters using the N-body calculations. This study constitutes the hitherto largest theoretical investigation of dynamical OB-star ejections. <br /> Firstly, we study the eject of dynamical processes, such as dynamical ejections and stellar collisions, on the relation between the maximum stellar mass and cluster mass. We show that the initially most massive star in a cluster can be dynamically ejected from dense, massive clusters. Secondly, we study the efficiency of O-star ejections as a function of cluster mass. We discover that the ejection fraction of O-star systems peaks at a cluster mass of ~3000 solar masses. We estimate that, for our most realistic models, about 15% of O-star systems that form in a Milky-Way-type galaxy are dynamically ejected from their birth cluster to the field. Our results show that the observed fractions of field and runaway O stars, and the binary fractions among them, can be well understood theoretically if all O stars form in embedded clusters. Thirdly, we investigate how the dynamical ejections of massive stars vary with the initial conditions of star clusters. We present several properties of ejected massive systems that are dependent on the initial conditions. The ejections of (massive) stars can change the properties of stars inside clusters. Because the ejection efficiency of stars increases with stellar mass, particularly for models that are efficient in ejecting massive stars, the mass functions become top-heavy for ejected stars and bottom-heavy for stars that remain in the cluster. Lastly, we show that a very massive (>300 solar masses) binary, such as R144 in the 30 Dor region in the Large Magellanic Cloud, can be dynamically ejected from a young massive star cluster like R136 through a binary–binary encounter. In addition, the R136-type cluster can populate several very massive (>100 solar masses) stars outside of the cluster through dynamical ejections. This implies that the isolated formation scenario is unnecessary for very massive stars/binaries in relative isolation. <br /> Throughout this thesis, we show that the massive stars can be efficiently ejected from their birth cluster through energetic close encounters and that outcomes of the dynamical ejections depend on the initial conditions of star clusters and their massive star population. The latter suggests that studying massive stars outside of star clusters can help to understand how massive stars form in a star cluster, by assuming that they all form in star clusters. When a large kinematic survey of massive stars in the Galaxy becomes available, for example through the astrometric space mission Gaia, our models can be used to constrain the initial configurations of massive stars and their birth clusters, which are end-products of the star formation process, leading towards a better understanding of massive star formation.