16 research outputs found
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 binaryâbinary 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