46 research outputs found
Taking apart the dynamical clock. Fat-tailed dynamical kicks shape the blue-straggler star bimodality
In globular clusters, blue straggler stars are heavier than the average star,
so dynamical friction strongly affects them. The radial distribution of BSS,
normalized to a reference population, appears bimodal in a fraction of Galactic
GCs, with a density peak in the core, a prominent zone of avoidance at
intermediate radii, and again higher density in the outskirts. The zone of
avoidance appears to be located at larger radii the more relaxed the host
cluster, acting as a sort of dynamical clock. We use a new method to compute
the evolution of the BSS radial distribution under dynamical friction and
diffusion. We evolve our BSS in the mean cluster potential under dynamical
friction plus a random fluctuating force, solving the Langevin equation with
the Mannella quasi symplectic scheme. This amounts to a new simulation method
which is much faster and simpler than direct N-body codes but retains their
main feature: diffusion powered by strong, if infrequent, kicks. We compute the
radial distribution of initially unsegregated BSS normalized to a reference
population as a function of time. We trace the evolution of its minimum,
corresponding to the zone of avoidance. We compare the evolution under kicks
extracted from a Gaussian distribution to that obtained using a Holtsmark
distribution. The latter is a fat tailed distribution which correctly models
the effects of close gravitational encounters. We find that the zone of
avoidance moves outwards over time, as expected based on observations, only
when using the Holtsmark distribution. Thus the correct representation of near
encounters is crucial to reproduce the dynamics of the system. We confirm and
extend earlier results that showed how the dynamical clock indicator depends
both on dynamical friction and effective diffusion powered by dynamical
encounters.Comment: 8 pages, 6 figures. Version accepted in Astronomy & Astrophysic
-body chaos and the continuum limit in numerical simulations of self-gravitating systems, revisited
We revisit the r\^{o}le of discreteness and chaos in the dynamics of
self-gravitating systems by means of -body simulations with active and
frozen potentials, starting from spherically symmetric stationary states and
considering the orbits of single particles in a frozen -body potential as
well as the orbits of the system in the full -dimensional phase space. We
also consider the intermediate case where a test particle moves in the field
generated by non-interacting particles, which in turn move in a static
smooth potential. We investigate the dependence on and on the softening
length of the largest Lyapunov exponent both of single particle orbits and of
the full -body system. For single orbits we also study the dependence on the
angular momentum and on the energy. Our results confirm the expectation that
orbital properties of single orbits in finite- systems approach those of
orbits in smooth potentials in the continuum limit and that the
largest Lyapunov exponent of the full -body system does decrease with ,
for sufficiently large systems with finite softening length. However, single
orbits in frozen models and active self-consistent models have different
largest Lyapunov exponents and the -dependence of the values in non-trivial,
so that the use of frozen -body potentials to gain information on large-
systems or on the continuum limit may be misleading in certain cases.Comment: 13 pages 16 figures. Version accepted in MNRA
Dissipationless collapse and the dynamical mass-ellipticity relation of elliptical galaxies in Newtonian gravity and MOND
Context. Deur (2014) and Winters et al. (2023) proposed an empirical relation
between the dark to total mass ratio and ellipticity in elliptical galaxies
from their observed total dynamical mass-to-light ratio data M/L = (14.1 +/-
5.4){\epsilon}. In other words, the larger is the content of dark matter in the
galaxy, the more the stellar component would be flattened. Such observational
claim, if true, appears to be in stark contrast with the common intuition of
the formation of galaxies inside dark halos with reasonably spherical symmetry.
Aims. Comparing the processes of dissipationless galaxy formation in different
theories of gravity, and emergence of the galaxy scaling relations therein is
an important frame where, in principle one could discriminate them. Methods. By
means of collisionless N-body simulations in modified Newtonian dynamics (MOND)
and Newtonian gravity with and without active dark matter halos, with both
spherical and clumpy initial structure, I study the trends of intrinsic and
projected ellipticities, S\'ersic index and anisotropy with the total dynamical
to stellar mass ratio. Results. It is shown that, the end products of both cold
spherical collapses and mergers of smaller clumps depart more and more from the
spherical symmetry for increasing values of the total dynamical mass to stellar
mass, at least in a range of halo masses. The equivalent Newtonian systems of
the end products of MOND collapses show a similar behaviour. The M/L relation
obtained from the numerical experiments in both gravities is however rather
different from that reported by Deur and coauthors.Comment: 10 pages, 8 figures, submitted, comments welcom
Dynamics of heterogeneous clusters under intense laser fields
We study the dynamics of atomic and molecular nano-clusters exposed to short
and intense X-fel pulsesComment: PhD Thesis, 133 pages, 60 (low quality) figure
Radially anisotropic systems with forces. II: radial-orbit instability
We continue to investigate the dynamics of collisionless systems of particles
interacting via additive interparticle forces. Here we focus on
the dependence of the radial-orbit instability on the force exponent .
By means of direct -body simulations we study the stability of equilibrium
radially anisotropic Osipkov-Merritt spherical models with Hernquist density
profile and with . We determine, as a function of , the
minimum value for stability of the anisotropy radius and of the
maximum value of the associated stability indicator . We find that, for
decreasing , decreases and increases, i.e.
longer-range forces are more robust against radial-orbit instability. The
isotropic systems are found to be stable for all the explored values of
. The end products of unstable systems are all markedly triaxial with
minor-to-major axial ratio , so they are never flatter than an E7 system.Comment: 12 pages, 6 figure
Proton ejection from molecular hydride clusters exposed to strong X-ray pulses
Clusters consisting of small molecules containing hydrogen do eject fast
protons when illuminated by short X-ray pulses. A suitable overall charging of
the cluster controlled by the X-ray intensity induces electron migration from
the surface to the bulk leading to efficient segregation of the protons and to
a globally hindered explosion of the heavy atoms even outside the screened
volume. We investigate this peculiar effect systematically along the
iso-electronic sequence of methane over ammonia and water to the atomic limit
of neon as a reference. In contrast to core-shell systems where the outer shell
is sacrificed to reduce radiation damage, the intricate proton dynamics of
hydride clusters allows one to keep the entire backbone of heavy atoms intact.Comment: 5 pages, 5 figure