Seungkyung Oh

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, M ecl,rh dyn (t), exists, which uniquely transforms an initial (t = 0) orbital parameter distribution function for binaries, Din, into a new distribution, D M ecl,rh (t), depending on the initial cluster mass, Mecl, and half-mass radius, rh, after some time t of dynamical evolution. For Din 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 -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 M ecl,rh dyn (t) can be well parametrized as a function of the cluster density, ρecl. 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, ρecl, which are found to be in the range 10 2 . ρecl(6 rh)/M⊙ pc −3 . 2 × 10 5 for open clusters and a few×10 3 . ρecl(6 rh)/M⊙ pc −3 . 10 8 for globular clusters, respectively.

Search for OB stars running away from young star clusters - II. The NGC 6357 star-forming region

Dynamical few-body encounters in the dense cores of young massive star clusters are responsible for the loss of a significant fraction of their massive stellar content. Some of the escaping (runaway) stars move through the ambient medium supersonically and can be revealed via detection of their bow shocks (visible in the infrared, optical or radio). In this paper, which is the second of a series of papers devoted to the search for OB stars running away from young (several Myr) Galactic clusters and OB associations, we present the results of the search for bow shocks around the star-forming region NGC 6357. Using the archival data of the Midcourse Space Experiment (MSX) satellite and the Spitzer Space Telescope, and the preliminary data release of the Wide-Field Infrared Survey Explorer (WISE), we discovered seven bow shocks, whose geometry is consistent with the possibility that they are generated by stars expelled from the young star clusters, Pismis 24 and AH03 J1725-34.4, associated with NGC 6357. Two of the seven bow shocks are driven by the already known O stars. Follow-up spectroscopy of three other bow shock-producing stars showed that they are O-type stars as well, while the 2MASS photometry of the remaining two stars suggests that they could be B0 V stars, provided that both are located at the same distance as NGC 6357. Detection of numerous massive stars ejected from the very young clusters is consistent with the theoretical expectation that star clusters can effectively lose massive stars at the very beginning of their dynamical evolution and lends strong support to the idea that probably all field OB stars have been dynamically ejected from their birth clusters. A by-product of our search for bow shocks around NGC 6357 is the detection of three circular shells typical of luminous blue variable and late WN-type Wolf-Rayet stars.

DEPENDENCY OF DYNAMICAL EJECTIONS OF O STARS ON THE MASSES OF VERY YOUNG STAR CLUSTERS

Massive stars can be efficiently ejected from their birth star clusters through encounters with other massive stars. We study how the dynamical ejection fraction of O star systems varies with the masses of very young star clusters, , by means of direct N-body calculations. We include diverse initial conditions by varying the half-mass radius, initial mass segregation, initial binary fraction, and orbital parameters of the massive binaries. The results show robustly that the ejection fraction of O star systems exhibits a maximum at a cluster mass of for all models, even though the number of ejected systems increases with cluster mass. We show that lower mass clusters () are the dominant sources for populating the Galactic field with O stars by dynamical ejections, considering the mass function of embedded clusters. About 15% (up to ?38%, depending on the cluster models) of O stars of which a significant fraction are binaries, and which would have formed in a ?10 Myr epoch of star formation in a distribution of embedded clusters, will be dynamically ejected to the field. Individual clusters may eject 100% of their original O star content. A large fraction of such O stars have velocities up to only 10 km s?1. Synthesising a young star cluster mass function, it follows, given the stellar-dynamical results presented here, 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.

Theoretical Isochrones with Extinction in the K Band

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Dynamical ejections of massive stars from young star clusters under diverse initial conditions

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Revisiting the universality of (multiple) star formation in present-day star formation regions

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R144: a very massive binary likely ejected from R136 through a binary–binary encounter

We study the effects of initial conditions of star clusters and their massive star population on dynamical ejections of massive stars from star clusters up to an age of 3 Myr. We use a large set of direct N-body calculations for moderately massive star clusters (Mecl=

MASS DISTRIBUTION IN THE CENTRAL FEW PARSECS OF OUR GALAXY[Author`s correction]

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RUNAWAY MASSIVE STARS FROM R136: VFTS 682 IS VERY LIKELY A “SLOW RUNAWAY”

Msun). We vary the initial conditions of the calculations such as the initial half-mass radius of the clusters, initial binary populations for massive stars and initial mass segregation. We find that the initial density is the most influential parameter for the ejection fraction of the massive systems. The clusters with an initial half-mass radius of 0.1 (0.3) pc can eject up to 50% (30%) of their O-star systems on average. Most of the models show that the average ejection fraction decreases with decreasing stellar mass. For clusters efficient at ejecting O stars, the mass function of the ejected stars is top-heavy compared to the given initial mass function (IMF), while the mass function of stars that remain in the cluster becomes slightly steeper (top-light) than the IMF. The top-light mass functions of stars in 3 Myr old clusters in our N-body models agree well with the mean mass function of young intermediate-mass clusters in M31, as reported previously. We show that the multiplicity fraction of the ejected massive stars can be as high as 60%, that massive high-order multiple systems can be dynamically ejected, and that high-order multiples become common especially in the cluster. We also discuss binary populations of the ejected massive systems. When a large kinematic survey of massive field stars becomes available, for instance through Gaia, our results may be used to constrain the birth configuration of massive stars in star clusters. (Abridged)

The emergence of super-canonical stars in R136-type starburst clusters

Populations of multiple stars inside clustered regions are known to change through dynamical interactions. The efficiency of binary disruption is thought to be determined by stellar density. King and collaborators recently investigated the multiplicity properties in young star forming regions and in the Galactic field. They concluded that stellar density-dependent modification of a universal initial binary population (the standard or null hypothesis model) cannot explain the observations. We re-visit their results, analyzing the data within the framework of different model assumptions, namely non-universality without dynamical modification and universality with dynamics. We illustrate that the standard model does account for all known populations if regions were significantly denser in the past. Some of the effects of using present-day cluster properties as proxies for their past values are emphasized and that the degeneracy between age and density of a star forming region can not be omitted when interpreting multiplicity data. A new analysis of the Corona Australis region is performed within the standard model. It is found that this region is likely as unevolved as Taurus and an initial density of