50 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
Weighing the IMBH candidate CO-0.40-0.22* in the Galactic Centre
The high velocity gradient observed in the compact cloud CO-0.40-0.22, at a
projected distance of 60 pc from the centre of the Milky Way, has led its
discoverers to identify the closeby mm continuum emitter, CO-0.40-0.22*, with
an intermediate mass black hole (IMBH) candidate. We describe the interaction
between CO-0.40-0.22 and the IMBH, by means of a simple analytical model and of
hydrodynamical simulations. Through such calculation, we obtain a lower limit
to the mass of CO-0.40-0.22* of few . This result
tends to exclude the formation of such massive black hole in the proximity of
the Galactic Centre. On the other hand, CO-0.40-0.22* might have been brought
to such distances in cosmological timescales, if it was born in a dark matter
halo or globular cluster around the Milky Way.Comment: 9 pages, 4 figures. To be published on MNRA
Finding Black Holes with Black Boxes -- Using Machine Learning to Identify Globular Clusters with Black Hole Subsystems
Machine learning is a powerful technique, becoming increasingly popular in
astrophysics. In this paper, we apply machine learning to more than a thousand
globular cluster (GC) models simulated as part of the 'MOCCA-Survey Database I'
project in order to correlate present-day observable properties with the
presence of a subsystem of stellar mass black holes (BHs). The machine learning
model is then applied to available observed parameters for Galactic GCs to
identify which of them that are most likely to be hosting a sizeable number of
BHs and reveal insights into what properties lead to the formation of BH
subsystems. With our machine learning model, we were able to shortlist 21
Galactic GCs that are most likely to contain a BH subsystem. We show that the
clusters shortlisted by the machine learning classifier include those in which
BH candidates have been observed (M22, M10 and NGC 3201) and that our results
line up well with independent simulations and previous studies that manually
compared simulated GC models with observed properties of Galactic GCs. These
results can be useful for observers searching for elusive stellar mass BH
candidates in GCs and further our understanding of the role BHs play in GC
evolution. In addition, we have released an online tool that allows one to get
predictions from our model after they input observable properties.Comment: 20 pages, 9 figures, 7 tables. Accepted for publication in MNRAS.
Source code available at
https://github.com/ammaraskar/black-holes-black-boxe