165 research outputs found
Treatment of realistic tidal field in Monte Carlo simulations of star clusters
We present a new implementation of the Monte Carlo method to simulate the
evolution of star clusters. The major improvement with respect to the
previously developed codes is the treatment of the external tidal field taking
into account for both the loss of stars from the cluster boundary and the
disk/bulge shocks. We provide recipes to handle with eccentric orbits in
complex galactic potentials. The first calculations for stellar systems
containing 21000 and 42000 equal-mass particles show good agreement with direct
N-body simulations in terms of the evolution of both the enclosed mass and the
Lagrangian radii provided that the mass-loss rate does not exceed a critical
value.Comment: 17 pages, 13 figures, accepted for publication by MNRA
Probing the formation history of the nuclear star cluster at the Galactic Centre with millisecond pulsars
The origin of the Nuclear Star Cluster in the centre of our Galaxy is still
unknown. One possibility is that it formed after the disruption of stellar
clusters that spiralled into the Galactic Centre due to dynamical friction. We
trace the formation of the Nuclear Star Cluster around the central black hole,
using state-of-the-art N-body simulations, and follow the dynamics of the
neutron stars born in the clusters. We then estimate the number of Millisecond
Pulsars (MSPs) that are released in the Nuclear Star Cluster, during its
formation. The assembly and tidal dismemberment of globular clusters lead to a
population of MSPs distributed over a radius of about 20 pc, with a peak near 3
pc. No clustering is found on the sub-parsec scale. We simulate the
detectability of this population with future radio telescopes like the MeerKAT
radio telescope and SKA1, and find that about of order ten MSPs can be observed
over this large volume, with a paucity of MSPs within the central parsec. This
helps discriminating this scenario from the in-situ formation model for the
Nuclear Star Cluster that would predict an over abundance of MSPs closer to the
black hole. We then discuss the potential contribution of our MSP population to
the gamma-ray excess at the Galactic Centre.Comment: 11 pages, 8 figures, accepted for publication in MNRA
High performance astrophysics computing
The application of high end computing to astrophysical problems, mainly in
the galactic environment, is under development since many years at the Dep. of
Physics of Sapienza Univ. of Roma. The main scientific topic is the physics of
self gravitating systems, whose specific subtopics are: i) celestial mechanics
and interplanetary probe transfers in the solar system; ii) dynamics of
globular clusters and of globular cluster systems in their parent galaxies;
iii) nuclear clusters formation and evolution; iv) massive black hole formation
and evolution; v) young star cluster early evolution. In this poster we
describe the software and hardware computational resources available in our
group and how we are developing both software and hardware to reach the
scientific aims above itemized.Comment: 2 pages paper presented at the Conference "Advances in Computational
Astrophysics: methods, tools and outcomes", to be published in the ASP
Conference Series, 2012, vol. 453, R. Capuzzo-Dolcetta, M. Limongi and A.
Tornambe' ed
A Monte Carlo analysis of the velocity dispersion of the globular cluster Palomar 14
We present the results of a detailed analysis of the projected velocity
dispersion of the globular cluster Palomar 14 performed using recent
high-resolution spectroscopic data and extensive Monte Carlo simulations. The
comparison between the data and a set of dynamical models (differing in
fraction of binaries, degree of anisotropy, mass-to-light ratio M/L, cluster
orbit and theory of gravity) shows that the observed velocity dispersion of
this stellar system is well reproduced by Newtonian models with a fraction of
binaries f_b<30% and a M/L compatible with the predictions of stellar evolution
models. Instead, models computed with a large fraction of binaries
systematically overestimate the cluster velocity dispersion. We also show that,
across the parameter space sampled by our simulations, models based on the
Modified Newtonian Dynamics theory can be reconciled with observations only
assuming values of M/L lower than those predicted by stellar evolution models
under standard assumptions.Comment: 13 pages, 13 figures, accepted for publication by Ap
Jeans modelling of the Milky Way's nuclear stellar disc
The nuclear stellar disc (NSD) is a flattened stellar structure that dominates the gravitational potential of the Milky Way at Galactocentric radii 30âČRâČ300pcâ . In this paper, we construct axisymmetric Jeans dynamical models of the NSD based on previous photometric studies and we fit them to line-of-sight kinematic data of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and silicon monoxide (SiO) maser stars. We find that (i) the NSD mass is lower but consistent with the mass independently determined from photometry by Launhardt et al. Our fiducial model has a mass contained within spherical radius r=100pc of M(r 1. Observations and theoretical models of the star-forming molecular gas in the central molecular zone suggest that large vertical oscillations may be already imprinted at stellar birth. However, the finding Ïz/ÏR > 1 depends on a drop in the velocity dispersion in the innermost few tens of parsecs, on our assumption that the NSD is axisymmetric, and that the available (extinction corrected) stellar samples broadly trace the underlying light and mass distributions, all of which need to be established by future observations and/or modelling. (iii) We provide the most accurate rotation curve to date for the innermost 500pc of our Galaxy
NBSymple, a double parallel, symplectic N-body code running on Graphic Processing Units
We present and discuss the characteristics and performances, both in term of
computational speed and precision, of a numerical code which numerically
integrates the equation of motions of N 'particles' interacting via Newtonian
gravitation and move in an external galactic smooth field. The force evaluation
on every particle is done by mean of direct summation of the contribution of
all the other system's particle, avoiding truncation error. The time
integration is done with second-order and sixth-order symplectic schemes. The
code, NBSymple, has been parallelized twice, by mean of the Computer Unified
Device Architecture to make the all-pair force evaluation as fast as possible
on high-performance Graphic Processing Units NVIDIA TESLA C 1060, while the
O(N) computations are distributed on various CPUs by mean of OpenMP Application
Program. The code works both in single precision floating point arithmetics or
in double precision. The use of single precision allows the use at best of the
GPU performances but, of course, limits the precision of simulation in some
critical situations. We find a good compromise in using a software
reconstruction of double precision for those variables that are most critical
for the overall precision of the code. The code is available on the web site
astrowww.phys.uniroma1.it/dolcetta/nbsymple.htmlComment: Paper composed by 29 pages, including 9 figures. Submitted to New
Astronomy
Smooth kinematic and metallicity gradients between the Milky Way's nuclear star cluster and nuclear stellar disc. Different components of the same structure?
The innermost regions of most galaxies are characterised by the presence of
extremely dense nuclear star clusters. Nevertheless, these clusters are not the
only stellar component present in galactic nuclei, where larger stellar
structures known as nuclear stellar discs, have also been found. Understanding
the relation between nuclear star clusters and nuclear stellar discs is
challenging due to the large distance towards other galaxies which limits their
analysis to integrated light. The Milky Way's centre, at only 8 kpc, hosts a
nuclear star cluster and a nuclear stellar disc, constituting a unique template
to understand their relation and formation scenario. We aim to study the
kinematics and stellar metallicity of stars from the Milky Way's nuclear star
cluster and disc to shed light on the relation between these two Galactic
centre components. We used publicly available photometric, proper motions, and
spectroscopic catalogues to analyse a region of centred on
the Milky Way's nuclear star cluster. We built colour magnitude diagrams, and
applied colour cuts to analyse the kinematic and metallicity distributions of
Milky Way's nuclear star cluster and disc stars with different extinction along
the line of sight. We detect kinematics and metallicity gradients for the
analysed stars along the line of sight towards the Milky Way's nuclear star
cluster, suggesting a smooth transition between the nuclear stellar disc and
cluster. We also find a bi-modal metallicity distribution for all the analysed
colour bins, which is compatible with previous work on the bulk population of
the nuclear stellar disc and cluster. Our results suggest that these two
Galactic centre components might be part of the same structure with the Milky
Way's nuclear stellar disc being the grown edge of the nuclear star cluster.Comment: Submitted to A&A. 13 pages, 9 figure
Gaia and Hubble unveil the kinematics of stellar populations in the Type II globular clusters {\omega} Centauri and M 22
The origin of multiple stellar populations in Globular Clusters (GCs) is one
of the greatest mysteries of modern stellar astrophysics. N-body simulations
suggest that the present-day dynamics of GC stars can constrain the events that
occurred at high redshift and led to the formation of multiple populations.
Here, we combine multi-band photometry from the Hubble Space Telescope (HST)
and ground-based facilities with HST and Gaia Data Release 2 proper motions to
investigate the spatial distributions and the motions in the plane of the sky
of multiple populations in the type II GCs NGC 5139 (Centauri) and
NGC 6656 (M 22). We first analyzed stellar populations with different
metallicities. Fe-poor and Fe-rich stars in M 22 share similar spatial
distributions and rotation patterns and exhibit similar isotropic motions.
Similarly, the two main populations with different iron abundance in
Centauri share similar ellipticities and rotation patterns. When
analyzing different radial regions, we find that the rotation amplitude
decreases from the center towards the external regions. Fe-poor and Fe-rich
stars of Centauri are radially anisotropic in the central region and
show similar degrees of anisotropy. We also investigate the stellar populations
with different light-element abundances and find that their N-rich stars
exhibit higher ellipticity than N-poor stars. In Centauri Centauri
both stellar groups are radially anisotropic. Interestingly, N-rich, Fe-rich
stars exhibit different rotation patterns than N-poor stars with similar
metallicities. The stellar populations with different nitrogen of M 22 exhibit
similar rotation patterns and isotropic motions. We discuss these findings in
the context of the formation of multiple populations.Comment: 24 pages, 13 figures, accepted for publication in Ap
A Deep View into the Nucleus of the Sagittarius Dwarf Spheroidal Galaxy with MUSE. II. Kinematic Characterization of the Stellar Populations
The Sagittarius dwarf spheroidal galaxy is in an advanced stage of disruption but still hosts its nuclear star cluster (NSC), M54, at its center. In this paper, we present a detailed kinematic characterization of the three stellar populations present in M54: young metal-rich (YMR); intermediate-age metal-rich (IMR); and old metal-poor (OMP), based on the spectra of ~6500 individual M54 member stars extracted from a large Multi-Unit Spectroscopic Explorer (MUSE)/Very Large Telescope data set. We find that the OMP population is slightly flattened with a low amount of rotation (~0.8 km sâ1) and with a velocity dispersion that follows a Plummer profile. The YMR population displays a high amount of rotation (~5 km sâ1) and a high degree of flattening, with a lower and flat velocity dispersion profile. The IMR population shows a high but flat velocity dispersion profile, with some degree of rotation (~2 km sâ1). We complement our MUSE data with information from Gaia DR2 and confirm that the stars from the OMP and YMR populations are comoving in 3D space, suggesting that they are dynamically bound. While dynamical evolutionary effects (e.g., energy equipartition) are able to explain the differences in velocity dispersion between the stellar populations, the strong differences in rotation indicate different formation paths for the populations, as supported by an N-body simulation tailored to emulate the YMRâOMP system. This study provides additional evidence for the M54 formation scenario proposed in our previous work, where this NSC formed via GC accretion (OMP) and in situ formation from gas accretion in a rotationally supported disk (YMR)
The Unexpected Kinematics of Multiple Populations in NGC 6362: Do Binaries Play a Role?
We present a detailed analysis of the kinematic properties of the multiple populations (MPs) in the low-mass Galactic globular cluster (GC) NGC 6362 based on a sample of about 500 member stars for which radial velocities (RVs), and Fe and Na abundances have been homogeneously derived. At distances from the cluster center larger than about 0.5r h , we find that first-generation (FGâNa-poor) and second-generation (SGâNa-rich) stars show hints of different line-of-sight velocity dispersion profiles, with FG stars being dynamically hotter. This is the first time that differences in the velocity dispersion of MPs are detected using only RVs. While kinematic differences between MPs in GCs are usually described in terms of anisotropy differences driven by the different radial distributions, this explanation hardly seems viable for NGC 6362, where SG and FG stars are spatially mixed. We demonstrate that the observed difference in the velocity dispersion profiles can be accounted for by the effect of binary stars. In fact, thanks to our multi-epoch RV measurements, we find that the binary fraction is significantly larger in the FG sample (f ~ 14%) than in the SG population (f < 1%), and we show that such a difference can inflate the velocity dispersion of FG with respect to SG by the observed amount in the relevant radial range. Our results nicely match the predictions of state-of-the art N-body simulations of the co-evolution of MPs in GCs that include the effects of binaries
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