37 research outputs found
The complex kinematics of rotating star clusters in a tidal field
We broaden the investigation of the dynamical properties of tidally
perturbed, rotating star clusters by relaxing the traditional assumptions of
coplanarity, alignment, and synchronicity between the internal and orbital
angular velocity vector of their initial conditions. We show that the interplay
between the internal evolution of these systems and their interaction with the
external tidal field naturally leads to the development of a number of
evolutionary features in their three-dimensional velocity space, including a
precession and nutation of the global rotation axis and a variation of its
orientation with the distance from the cluster centre. In some cases, such a
radial variation may manifest itself as a counter-rotation of the outermost
regions relative to the inner ones. The projected morphology of these systems
is characterized by a non-monotonic ellipticity profile and, depending on the
initial inclination of the rotation axis, it may also show a twisting of the
projected isodensity contours. These results provide guidance in the
identification of non-trivial features which may emerge in upcoming
investigations of star cluster kinematics and a dynamical framework to
understand some of the complexities already hinted by recent observational
studies.Comment: 5 pages, 4 figures, accepted for publication in MNRA
The inefficiency of satellite accretion in forming extended star clusters
The distinction between globular clusters and dwarf galaxies has been
progressively blurred by the recent discoveries of several extended star
clusters, with size (20-30 pc) and luminosity (-6 < Mv < -2) comparable to the
one of faint dwarf spheroidals. In order to explain their sparse structure, it
has been suggested that they formed as star clusters in dwarf galaxy satellites
that later accreted onto the Milky Way. If these clusters form in the centre of
dwarf galaxies, they evolve in a tidally-compressive environment where the
contribution of the tides to the virial balance can become significant, and
lead to a super-virial state and subsequent expansion of the cluster, once
removed. Using N-body simulations, we show that a cluster formed in such an
extreme environment undergoes a sizable expansion, during the drastic variation
of the external tidal field due to the accretion process. However, we show that
the expansion due to the removal of the compressive tides is not enough to
explain the observed extended structure, since the stellar systems resulting
from this process are always more compact than the corresponding clusters that
expand in isolation due to two-body relaxation. We conclude that an accreted
origin of extended globular clusters is unlikely to explain their large spatial
extent, and rather favor the hypothesis that such clusters are already extended
at the stage of their formation.Comment: 5 pages, 4 figures, 1 table. Accepted for publication in MNRAS
Letter
Kinematical evolution of multiple stellar populations in star clusters
We present the results of a suite of \Nbody simulations aimed at
understanding the fundamental aspects of the long-term evolution of the
internal kinematics of multiple stellar populations in globular clusters. Our
models enable us to study the cooperative effects of internal,
relaxation-driven processes and external, tidally-induced perturbations on the
structural and kinematic properties of multiple-population globular clusters.
To analyse the dynamical behaviour of the multiple stellar populations in a
variety of spin-orbit coupling conditions, we have considered three reference
cases in which the tidally perturbed star cluster rotates along an axis
oriented in different directions with respect to the orbital angular momentum
vector. We focus specifically on the characterisation of the evolution of the
degree of differential rotation and anisotropy in the velocity space, and we
quantify the process of spatial and kinematic mixing of the two populations. In
light of recent and forthcoming explorations of the internal kinematics of this
class of stellar systems by means of line-of sight and astrometric
measurements, we also investigate the implications of projection effects and
spatial distribution of the stars adopted as tracers. The kinematic and
structural richness emerging from our models further emphasises the need and
the importance of observational studies aimed at building a complete
kinematical picture of the multiple population phenomenon.Comment: 14 pages, 12 figures, Accepted by MNRA
Evolution of star clusters on eccentric orbits
We study the evolution of star clusters on circular and eccentric orbits
using direct -body simulations. We model clusters with initially and single stars of the same mass, orbiting around a
point-mass galaxy. For each orbital eccentricity that we consider, we find the
apogalactic radius at which the cluster has the same lifetime as the cluster
with the same on a circular orbit. We show that then, the evolution of
bound particle number and half-mass radius is approximately independent of
eccentricity. Secondly, when we scale our results to orbits with the same
semi-major axis, we find that the lifetimes are, to first order, independent of
eccentricity. When the results of Baumgardt and Makino for a singular
isothermal halo are scaled in the same way, the lifetime is again independent
of eccentricity to first order, suggesting that this result is independent of
the Galactic mass profile. From both sets of simulations we empirically derive
the higher order dependence of the lifetime on eccentricity. Our results serve
as benchmark for theoretical studies of the escape rate from clusters on
eccentric orbits. Finally, our results can be useful for generative models for
cold streams and cluster evolution models that are confined to spherical
symmetry and/or time-independent tides, such as Fokker-Planck models, Monte
Carlo models, and (fast) semi-analytic models.Comment: 7 pages, 8 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
Central Dynamics of Multi-mass Rotating Star Clusters
We investigate the evolutionary nexus between the morphology and internal
kinematics of the central regions of collisional, rotating, multi-mass stellar
systems, with special attention to the spatial characterisation of the process
of mass segregation. We report results from idealized, purely -body
simulations that show multi-mass, rotating, and spherical systems rapidly form
an oblate, spheroidal massive core, unlike single-mass rotating or multi-mass
non-rotating configurations with otherwise identical initial properties,
indicating that this evolution is a result of the interplay between the
presence of a mass spectrum and angular momentum. This feature appears to be
long-lasting, preserving itself for several relaxation times. The degree of
flattening experienced by the systems is directly proportional to the initial
degree of internal rotation. In addition, this morphological effect has a clear
characterisation in terms of orbital architecture, as it lowers the inclination
of the orbits of massive stars. We offer an idealised dynamical interpretation
that could explain the mechanism underpinning this effect and we highlight
possible useful implications, from kinematic hysteresis to spatial distribution
of dark remnants in dense stellar systems.Comment: 13 pages, 12 figures, Accepted for publication in MNRA
Early dynamical evolution of rotating star clusters in a tidal field
In order to explore how the early internal rotational properties of star
clusters are affected by the external potential of their host galaxies, we have
run a suite of -body simulations following the early dynamical evolution and
violent relaxation of rotating star clusters embedded in a tidal field. Our
study focuses on models for which the cluster's rotation axis has a generic
orientation relative to the torque of the tidal field. The interaction between
the violent relaxation process, angular momentum of the cluster, and the
external torque creates a complex kinematic structure within the cluster, most
prominently a radial variation in the position of the rotation axis along both
the polar and azimuthal directions. We also examine the cluster's velocity
dispersion anisotropy and show that the projected anisotropy may be affected by
the variation of the rotation axis directions within the cluster; the
combination of projection effects and the complex kinematical features may
result in the measurement of tangential anisotropy in the cluster's inner
regions. We also characterize the structural properties of our clusters as a
function of their initial rotation and virial ratio and find that clusters may
develop a triaxial morphology and a radial variation of the minor axis not
necessarily aligned with the rotation axis. Finally, we examine the long-term
evolution of these complex kinematic features.Comment: 15 pages, 12 figures, accepted by MNRA