5,508 research outputs found
Models of rotating coronae
Fitting equilibrium dynamical models to observational data is an essential
step in understanding the structure of the gaseous hot haloes that surround our
own and other galaxies. However, the two main categories of models that are
used in the literature are poorly suited for this task: (i) simple barotropic
models are analytic and can therefore be adjusted to match the observations,
but are clearly unrealistic because the rotational velocity does
not depend on the distance from the galactic plane, while (ii) models
obtained as a result of cosmological galaxy formation simulations are more
realistic, but are impractical to fit to observations due to high computational
cost. Here we bridge this gap by presenting a general method to construct
axisymmetric baroclinic equilibrium models of rotating galactic coronae in
arbitrary external potentials. We consider in particular a family of models
whose equipressure surfaces in the plane are ellipses of varying axis
ratio. These models are defined by two one-dimensional functions, the axial
ratio of pressure and the value of the pressure along the galaxy's symmetry axis. These models can have a rotation
speed that realistically decreases as one moves away from the
galactic plane, and can reproduce the angular momentum distribution found in
cosmological simulations. The models are computationally cheap to construct and
can thus be used in fitting algorithms. We provide a python code that given
, and returns ,
, , . We show a few examples of these models using
the Milky Way as a case study.Comment: Accepted for publication in MNRA
Dynamics in the Fitness-Income plane: Brazilian states vs World countries
In this paper we introduce a novel algorithm, called Exogenous Fitness, to calculate the Fitness of subnational entities and we apply it to the states of Brazil. In the last decade, several indices were introduced to measure the competitiveness of countries by looking at the complexity of their export basket. Tacchella et al (2012) developed a non-monetary metric called Fitness. In this paper, after an overview about Brazil as a whole and the comparison with the other BRIC countries, we introduce a new methodology based on the Fitness algorithm, called Exogenous Fitness. Combining the results with the Gross Domestic Product per capita (GDPp), we look at the dynamics of the Brazilian states in the Fitness-Income plane. Two regimes are distinguishable: one with high predictability and the other with low predictability, showing a deep analogy with the heterogeneous dynamics of the World countries. Furthermore, we compare the ranking of the Brazilian states according to the Exogenous Fitness with the ranking obtained through two other techniques, namely Endogenous Fitness and Economic Complexity Index
Periodicity makes galactic shocks unstable - I. Linear analysis
We study the dynamical stability of stationary galactic spiral shocks. The
steady-state equilibrium flow contains a shock of the type derived by Roberts
in the tightly wound approximation. We find that boundary conditions are
critical in determining whether the solutions are stable or not. Shocks are
unstable if periodic boundary conditions are imposed. For intermediate
strengths of the spiral potential, the instability disappears if boundary
conditions are imposed such that the upstream flow is left unperturbed as in
the classic analysis of D'yakov and Kontorovich. This reconciles apparently
contradictory findings of previous authors regarding the stability of spiral
shocks. This also shows that the instability is distinct from the
Kelvin-Helmholtz instability, confirming the findings of Kim et al. We suggest
that instability is a general characteristics of periodic shocks, regardless of
the presence of shear, and provide a physical picture as to why this is the
case. For strong spiral potentials, high post-shock shear makes the system
unstable also to parasitic Kelvin-Helmholtz instability regardless of the
boundary conditions. Our analysis is performed in the context of a simplified
problem that, while preserving all the important characteristics of the
original problem, strips it from unnecessary complications, and assumes that
the gas is isothermal, non self-gravitating, non-magnetised.Comment: Accepted for publication in MNRA
Black holes in the low mass gap: Implications for gravitational wave observations
Binary neutron-star mergers will predominantly produce black-hole remnants of
mass , thus populating the putative \emph{low mass gap}
between neutron stars and stellar-mass black holes. If these low-mass black
holes are in dense astrophysical environments, mass segregation could lead to
"second-generation" compact binaries merging within a Hubble time. In this
paper, we investigate possible signatures of such low-mass compact binary
mergers in gravitational-wave observations. We show that this unique population
of objects, if present, will be uncovered by the third-generation
gravitational-wave detectors, such as Cosmic Explorer and Einstein Telescope.
Future joint measurements of chirp mass and effective spin
could clarify the formation scenario of compact objects in the
low mass gap. As a case study, we show that the recent detection of GW190425
(along with GW170817) favors a double Gaussian mass model for neutron stars,
under the assumption that the primary in GW190425 is a black hole formed from a
previous binary neutron star merger.Comment: 8 pages, 4 figures, 1 table. v4: matches the version accepted for
publication in Phys. Rev.
Distinguishing double neutron star from neutron star-black hole binary populations with gravitational wave observations
Gravitational waves from the merger of two neutron stars cannot be easily
distinguished from those produced by a comparable-mass mixed binary in which
one of the companions is a black hole. Low-mass black holes are interesting
because they could form in the aftermath of the coalescence of two neutron
stars, from the collapse of massive stars, from matter overdensities in the
primordial Universe, or as the outcome of the interaction between neutron stars
and dark matter. Gravitational waves carry the imprint of the internal
composition of neutron stars via the so-called tidal deformability parameter,
which depends on the stellar equation of state and is equal to zero for black
holes. We present a new data analysis strategy powered by Bayesian inference
and machine learning to identify mixed binaries, hence low-mass black holes,
using the distribution of the tidal deformability parameter inferred from
gravitational-wave observations.Comment: 13 pages, 6 figures - v2: matches the published version in Phys. Rev.
D 102, 02302
Iginio Tansini revisited
The origin of the muscolocutaneous latissimus dorsi flap dates back to 1906 when Igino Tansini, an Italian surgeon, described a procedure to reconstruct the mastectomy defect. After a detailed study of Tansini's original description and drawings, new insights about the pedicle of its compound flap have been found, showing that it has the same pedicle of the scapular flap. In the end, Tansini's flap should be more correctly considered as a compound musculocutaneous scapular flap
Syntactic characterization in Lisp of the polynomial complexity classes and hierarchy
The definition of a class C of functions is syntactic if membership to C can be decided from the construction of its elements. Syntactic
characterizations of PTIMEF, of PSPACEF, of the polynomial hierarchy PH, and of its subclasses Delta^p_n are presented. They are obtained by
progressive restrictions of recursion in Lisp, and may be regarded as predicative according to a foundational point raised by Leivant
- …