47 research outputs found
Forming circumnuclear disks and rings in galactic nuclei: a competition between supermassive black hole and nuclear star cluster
We investigate the formation of circumnuclear gas structures from the tidal
disruption of molecular clouds in galactic nuclei, by means of smoothed
particle hydrodynamics simulations. We model galactic nuclei as composed of a
supermassive black hole (SMBH) and a nuclear star cluster (NSC) and consider
different mass ratios between the two components. We find that the relative
masses of the SMBH and the NSC have a deep impact on the morphology of the
circumnuclear gas. Extended disks form only inside the sphere of influence of
the SMBH. In contrast, compact rings naturally form outside the SMBH's sphere
of influence, where the gravity is dominated by the NSC. This result is in
agreement with the properties of the Milky Way's circumnuclear ring, which
orbits outside the SMBH sphere of influence. Our results indicate that compact
circumnuclear rings can naturally form outside the SMBH sphere of influence.Comment: Accepted for publication in ApJ. 12 pages, 6 figures, 3 tables.
Comments welcom
Constraining the binarity of black hole candidates: a proof-of-concept study of Gaia BH1 and Gaia BH2
Nearly a hundred of binary black holes (BBHs) have been discovered with
gravitational-wave signals emitted at their merging events. Thus, it is quite
natural to expect that significantly more abundant BBHs with wider separations
remain undetected in the universe, or even in our Galaxy. We consider a
possibility that star-BH binary candidates may indeed host an inner BBH,
instead of a single BH. We present a detailed feasibility study of constraining
the binarity of the currently available two targets, Gaia BH1 and Gaia BH2.
Specifically, we examine three types of radial velocity (RV) modulations of a
tertiary star in star-BBH triple systems; short-term RV modulations induced by
the inner BBH, long-term RV modulations induced by the nodal precession, and
long-term RV modulations induced by the von Zeipel-Kozai-Lidov oscillations.
Direct three-body simulations combined with approximate analytic models reveal
that Gaia BH1 system may exhibit observable signatures of the hidden inner BBH
if it exists at all. The methodology that we examine here is quite generic, and
is expected to be readily applicable to future star-BH binary candidates in a
straightforward manner.Comment: 19 pages, 10 figures, 1 table, submitted to Ap
Extreme eccentricities of triple systems: Analytic results
Triple stars and compact objects are ubiquitously observed in nature. Their
long-term evolution is complex; in particular, the von-Zeipel-Lidov-Kozai (ZLK)
mechanism can potentially lead to highly eccentric encounters of the inner
binary. Such encounters can lead to a plethora of interacting binary phenomena,
as well as stellar and compact-object mergers. Here we find explicit analytical
formulae for the maximal eccentricity, , of the inner binary
undergoing ZLK oscillations, where both the test particle limit (parametrised
by the inner-to-outer angular momentum ratio ) and the double-averaging
approximation (parametrised by the period ratio, ) are
relaxed, for circular outer orbits. We recover known results in both limiting
cases (either or ) and verify the validity of
our model using numerical simulations. We test our results with two accurate
numerical N-body codes, for Newtonian dynamics and
for general-relativistic (GR) dynamics, and find excellent
correspondence. We discuss the implications of our results for stellar triples
and both stellar and supermassive triple black hole mergers
Merging black hole binaries with the SEVN code
Studying the formation and evolution of black hole binaries (BHBs) is essential for the interpretation of current and forthcoming gravitational wave (GW) detections. We investigate the statistics of BHBs that form from isolated binaries, by means of a new version of the
SEVN population-synthesis code. SEVN integrates stellar evolution by interpolation over a grid of stellar evolution tracks. We upgraded SEVN to include binary stellar evolution processes and we used it to evolve a sample of 1.5 x 10(8) binary systems, with metallicity in the range [10(-4); 4 x 10(-2)]. From our simulations, we find that the mass distribution of black holes (BHs) in double compact-object binaries is remarkably similar to the one obtained considering only single stellar evolution. The maximum BH mass we obtain is similar to 30, 45, and 55 M-circle dot at metallicity Z = 2 x 10(-2), 6 x 10(-3), and 10(-4), respectively. A few massive single BHs may also form (less than or similar to 0.1 per cent of the total number of BHs), with mass up to similar to 65, 90, and 145 M-circle dot at Z = 2 x 10(-2), 6 x 10(-3),
and 10(-4), respectively. These BHs fall in the mass gap predicted from pair-instability supernovae. We also show that the most massive BHBs are unlikely to merge within a Hubble time. In our simulations, merging BHs like GW151226 and GW170608, form at all metallicities, the high-mass systems (like GW150914, GW170814, and GW170104) originate from metal-poor (Z less than or similar to 6 x 10(-3)) progenitors, whereas GW170729-like systems are hard to form, even at Z = 10(-4). The BHB merger rate in the local Universe obtained from our simulations is similar to 90Gpc(-3)yr(-1), consistent with the rate inferred from LIGO-Virgo data
Compact object mergers: exploring uncertainties from stellar and binary evolution with SEVN
Population-synthesis codes are an unique tool to explore the parameter space
of massive binary star evolution and binary compact object (BCO) formation.
Most population-synthesis codes are based on the same stellar evolution model,
limiting our ability to explore the main uncertainties. Our code SEVN overcomes
this issue by interpolating the main stellar properties from a set of
pre-computed evolutionary tracks. With SEVN, we evolved
binaries in the metallicity range , exploring a number
of models for electron-capture, core-collapse and pair-instability supernovae,
different assumptions for common envelope, stability of mass transfer,
quasi-homogeneous evolution and stellar tides. We find that stellar evolution
has a dramatic impact on the formation of single and binary compact objects.
Just by slightly changing the overshooting parameter () and the pair-instability model, the maximum mass of a black hole
can vary from to . Furthermore,
the formation channels of BCOs and the merger efficiency we obtain with SEVN
show significant differences with respect to the results of other
population-synthesis codes, even when the same binary-evolution parameters are
used. For example, the main traditional formation channel of BCOs is strongly
suppressed in our models: at high metallicity () only % of
the merging binary black holes and binary neutron stars form via this channel,
while other authors found fractions %. The local BCO merger rate density
of our fiducial models is consistent with the most recent estimates by the
LIGO--Virgo--KAGRA collaboration.Comment: Submitted to MNRAS, comments welcome! The SEVN code is available at
https://gitlab.com/sevncodes/sevn.git. All the data underlying this article
are available in Zenodo at the link https://doi.org/10.5281/zenodo.7260771.
All the Jupyter notebooks used to produce the plots in the paper are
available in the gitlab repository https://gitlab.com/iogiul/iorio22_plot.gi