The Dynamical Ejections of Massive Stars from Young Star Clusters

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. 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. 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

Mass Distribution in the Central Few Parsecs of Our Galaxy

We estimate the enclosed mass profile in the central 10 pc of the Milky Way by analyzing the infrared photometry and the velocity observations of dynamically relaxed stellar population in the Galactic center. HST/NICMOS and Gemini Adaptive Optics images in the archive are used to obtain the number density profile, and proper motion and radial velocity data were compiled from the literature to find the velocity dispersion profile assuming a spherical symmetry and velocity isotropy. From these data, we calculate the enclosed mass and density profiles in the central 10 pc of the Galaxy using the Jeans equation. Our improved estimates can better describe the exact evolution of the molecular clouds and star clusters falling down to the Galactic center, and constrain the star formation history of the inner part of the Galaxy.Comment: To appear in the Journal of The Korean Astronomical Society, vol. 42, p. 17 (2009

Runaway massive stars from R136: VFTS 682 is very likely a "slow runaway"

We conduct a theoretical study on the ejection of runaway massive stars from R136 --- the central massive, star-burst cluster in the 30 Doradus complex of the Large Magellanic Cloud. Specifically, we investigate the possibility of the very massive star (VMS) VFTS 682 being a runaway member of R136. Recent observations of the above VMS, by virtue of its isolated location and its moderate peculiar motion, have raised the fundamental question whether isolated massive star formation is indeed possible. We perform the first realistic N-body computations of fully mass-segregated R136-type star clusters in which all the massive stars are in primordial binary systems. These calculations confirm that the dynamical ejection of a VMS from a R136-like cluster, with kinematic properties similar to those of VFTS 682, is common. Hence the conjecture of isolated massive star formation is unnecessary to account for this VMS. Our results are also quite consistent with the ejection of 30 Dor 016, another suspected runaway VMS from R136. We further note that during the clusters' evolution, mergers of massive binaries produce a few single stars per cluster with masses significantly exceeding the canonical upper-limit of 150 solar mass. The observations of such single super-canonical stars in R136, therefore, do not imply an IMF with an upper limit greatly exceeding the accepted canonical 150 solar mass limit, as has been suggested recently, and they are consistent with the canonical upper limit.Comment: 21 pages (AASTeX preprint format), 4 figures, 4 tables, 2 online tables. Accepted for publication in The Astrophysical Journa

The influence of stellar-dynamical ejections and collisions on the relation between the maximum-star and star-cluster-mass

We perform the largest currently available set of direct N-body calculations of young star cluster models to study the dynamical influence, especially through the ejections of the most massive star in the cluster, on the current relation between the maximum-stellar-mass and the star-cluster-mass. We vary several initial parameters such as the initial half-mass radius of the cluster, the initial binary fraction, and the degree of initial mass segregation. Two different pairing methods are used to construct massive binaries for more realistic initial conditions of massive binaries. We find that lower mass clusters (<= 10^2.5 Msun) do not shoot out their heaviest star. In the case of massive clusters (>= 1000 Msun), no most-massive star escapes the cluster within 3 Myr regardless of the initial conditions if clusters have initial half-mass radii, r_0.5, >= 0.8 pc. However, a few of the initially smaller sized clusters (r_0.5 = 0.3 pc), which have a higher density, eject their most massive star within 3 Myr. If clusters form with a compact size and their massive stars are born in a binary system with a mass-ratio biased towards unity, the probability that the mass of the most massive star in the cluster changes due to the ejection of the initially most massive star can be as large as 20 per cent. Stellar collisions increase the maximum-stellar-mass in a large number of clusters when clusters are relatively dense (M_ecl >= 10^3 Msun and r_0.5 = 0.3 pc) and binary-rich. Overall, we conclude that dynamical effects hardly influence the observational maximum-stellar-mass -- cluster mass relation.Comment: 16 pages, 8 figures, 5 tables, accepted for publication in MNRA

The emergence of super-canonical stars in R136-type star-burst clusters

[abridged] Among the most remarkable features of the stellar population of R136, the central, young, massive star cluster in the 30 Doradus complex of the Large Magellanic Cloud, are the single stars whose masses substantially exceed the canonical stellar upper mass limit of 150 M_sun. A recent study by us, viz., that of Banerjee, Kroupa & Oh (2012; Paper I), which involves realistic N-body computations of star clusters mimicking R136, indicates that such "super-canonical" (SC) stars can be formed out of a dense stellar population with a canonical initial mass function (IMF) through dynamically induced mergers of the most massive binaries. Here we study the formation of SC stars in the R136 models of Paper I in detail. To avoid forming extraneous SC stars from initially highly eccentric primordial binaries as in Paper I, we compute additional models with only initially circular primordial binaries. We also take into account the mass-evolution of the SC stars using detailed stellar evolutionary models that incorporate updated treatments of stellar winds. We find that SC stars begin to form via dynamical mergers of massive binaries from approx. 1 Myr cluster age. We obtain SC stars with initial masses up to approx. 250 M_sun from these computations. Multiple SC stars are found to remain bound to the cluster simultaneously within a SC-lifetime. These properties of the dynamically formed SC stars are consistent with those observed in R136. In fact, the stellar evolutionary models of SC stars imply that had they formed primordially along with the rest of the R136 cluster, i.e., violating the canonical upper limit, they would have evolved below the canonical 150 M_sun limit by approx. 3 Myr, the likely age of R136, and would not have been observable as SC stars at the present time in R136. This strongly supports the dynamical formation scenario of the observed SC stars in R136.Comment: 12 pages, 4 figures, Accepted for publication in MNRA

An analytical description of the evolution of binary orbital-parameter distributions in N-body computations of star clusters

A new method is presented to describe the evolution of the orbital-parameter distributions for an initially universal binary population in star clusters by means of the currently largest existing library of N-body models. It is demonstrated that a stellar-dynamical operator exists, which uniquely transforms an initial orbital parameter distribution function for binaries into a new distribution depending on the initial cluster mass and half-mass radius, after some time of dynamical evolution. For the initial distribution the distribution functions derived by Kroupa (1995a,b) are used, which are consistent with constraints for pre-main sequence and Class I binary populations. Binaries with a lower energy and a higher reduced-mass are dissolved preferentially. The stellar-dynamical operator can be used to efficiently calculate and predict binary properties in clusters and whole galaxies without the need for further N-body computations. For the present set of N-body models it is found that the binary populations change their properties on a crossing time-scale such that the stellar dynamical operator can be well parametrized as a function of the initial cluster density. Furthermore it is shown that the binary-fraction in clusters with similar initial velocity dispersions follows the same evolutionary tracks as a function of the passed number of relaxation-times. Present-day observed binary populations in star clusters put constraints on their initial stellar densities which are found to be in the range 10^2 - 2x10^5 M_sun pc^-3 for open clusters and a few x 10^3 - 10^8 M_sun pc^-3 for globular clusters, respectively.Comment: accepted for publication in MNRAS, 20 pages, 10 figures, 2 table

MASS DISTRIBUTION IN THE CENTRAL FEW PARSECS OF OUR GALAXY

ABSTRACT We estimate the enclosed mass profile in the central 10 pc of the Milky Way by analyzing the infrared photometry and the velocity observations of dynamically relaxed stellar population in the Galactic center. HST/NICMOS and Gemini Adaptive Optics images in the archive are used to obtain the number density profile, and proper motion and radial velocity data were compiled from the literature to find the velocity dispersion profile assuming a spherical symmetry and velocity isotropy. From these data, we calculate the the enclosed mass and density profiles in the central 10 pc of the Galaxy using the Jeans equation. Our improved estimates can better describe the exact evolution of the molecular clouds and star clusters falling down to the Galactic center, and constrain the star formation history of the inner part of the Galaxy