1,512 research outputs found
Spatial distribution of X-ray emitting ejecta in Tycho's SNR: indications of shocked Titanium
Young supernova remnants show a characteristic ejecta-dominated X-ray
emission that allows us to probe the products of the explosive nucleosynthesis
processes and to ascertain important information about the physics of the
supernova explosions. Hard X-ray observations have recently revealed the
radioactive decay lines of 44Ti at ~67.9 keV and ~78.4 keV in the Tycho's SNR.
We here analyze the set of XMM-Newton archive observations of the Tycho's SNR.
We produce equivalent width maps of the Fe K and Ca XIX emission lines and find
indications for a stratification of the abundances of these elements and
significant anisotropies. We then perform a spatially resolved spectral
analysis by identifying five different regions characterized by high/low values
of the Fe K equivalent width. We find that the spatial distribution of the Fe K
emission is correlated with that of the Cr XXII. We also detect the Ti K-line
complex in the spectra extracted from the two regions with the highest values
of the Fe and Cr equivalent widths. The Ti line emissions remains undetected in
regions where the Fe and Cr equivalent widths are low. Our results indicate
that the post-shock Ti is spatially co-located with other iron-peak nuclei in
Tycho's SNR, in agreement with the predictions of multi-D models of Type Ia
supernovae.Comment: Accepted for publication in Ap
String attractors and combinatorics on words
The notion of string attractor has recently been introduced in [Prezza, 2017] and studied in [Kempa and Prezza, 2018] to provide a unifying framework for known dictionary-based compressors. A string attractor for a word w = w[1]w[2] · · · w[n] is a subset Γ of the positions 1, . . ., n, such that all distinct factors of w have an occurrence crossing at least one of the elements of Γ. While finding the smallest string attractor for a word is a NP-complete problem, it has been proved in [Kempa and Prezza, 2018] that dictionary compressors can be interpreted as algorithms approximating the smallest string attractor for a given word. In this paper we explore the notion of string attractor from a combinatorial point of view, by focusing on several families of finite words. The results presented in the paper suggest that the notion of string attractor can be used to define new tools to investigate combinatorial properties of the words
Diffusion Anomaly in an Associating Lattice Gas Model
We investigate the relation between thermodynamic and dynamic properties of
an associating lattice gas (ALG) model. The ALG combines a two dimensional
lattice gas with particles interacting through a soft core potential and
orientational degrees of freedom. From the competition between the directional
attractive forces and the soft core potential results two liquid phases, double
criticality and density anomaly. We study the mobility of the molecules in this
model by calculating the diffusion constant at a constant temperature, . We
show that has a maximum at a density and a minimum at a
density . Between these densities the diffusivity
differs from the one expected for normal liquids. We also show that in the
pressure-temperature phase-diagram the line of extrema in diffusivity is close
to the liquid-liquid critical point and it is inside the temperature of maximum
density (TMD) line.Comment: 12 pages, 9 figure
Tuning effective interactions close to the critical point in colloidal suspensions
We report a numerical investigation of two colloids immersed in a critical
solvent, with the aim of quantifying the effective colloid-colloid interaction
potential. By turning on an attraction between the colloid and the solvent
particles we follow the evolution from the case in which the solvent density
close to the colloids changes from values smaller than the bulk to values
larger than the bulk. We thus effectively implement the so-called and
boundary conditions defined in field theoretical approaches focused on
the description of critical Casimir forces. We find that the effective
potential at large distances decays exponentially, with a characteristic decay
length compatible with the bulk critical correlation length, in full agreement
with theoretical predictions. We also investigate the case of boundary
condition, where the effective potential becomes repulsive. Our study provides
a guidance for a design of the interaction potential which can be exploited to
control the stability of colloidal systems
Theoretical description of a DNA-linked nanoparticle self-assembly
Nanoparticles tethered with DNA strands are promising building blocks for
bottom-up nanotechnology, and a theoretical understanding is important for
future development. Here we build on approaches developed in polymer physics to
provide theoretical descriptions for the equilibrium clustering and dynamics,
as well as the self-assembly kinetics of DNA-linked nanoparticles. Striking
agreement is observed between the theory and molecular modeling of DNA tethered
nanoparticles.Comment: Accepted for publication in Physical Review Letter
Circumstellar Disks in the Outer Galaxy: the Star-Forming Region NGC 1893
It is still debated whether star formation process depends on environment. In
particular it is yet unclear whether star formation in the outer Galaxy, where
the environmental conditions are, theoretically, less conducive, occurs in the
same way as in the inner Galaxy. We investigate the population of NGC1893, a
young cluster ~3-4 Myr in the outer part of the Galaxy (galactic radius >11
Kpc), to explore the effects of environmental conditions on star forming
regions. We present infrared observations acquired using the IRAC camera
onboard the Spitzer Space Telescope and analyze the color-color diagrams to
establish the membership of stars with excesses. We also merge this information
with that obtained from Chandra ACIS-I observations, to identify the Class III
population. We find that the cluster is very rich, with 242 PMS Classical
T-Tauri stars and 7 Class 0/I stars. We identify 110 Class III candidate
cluster members in the ACIS-I field of view. We estimate a disk fraction for
NGC1893 of about 67%, similar to fractions calculated for nearby star forming
regions of the same age. Although environmental conditions are unfavorable,
star formation can clearly be very successful in the outer Galaxy, allowing
creation of a very rich cluster like NGC1893.Comment: 10 pages,7 figures,4 table
Dynamics in a supercooled liquid of symmetric dumbbells: Reorientational hopping for small molecular elongations
We present extensive molecular dynamics simulations of a liquid of symmetric
dumbbells, for constant packing fraction, as a function of temperature and
molecular elongation. For large elongations, translational and rotational
degrees of freedom freeze at the same temperature. For small elongations only
the even rotational degrees of freedom remain coupled to translational motions
and arrest at a finite common temperature. The odd rotational degrees of
freedom remain ergodic at all investigated temperature and the temperature
dependence of the corresponding characteristic time is well described by an
Arrhenius law. Finally, we discuss the evidence in favor of the presence of a
type-A transition temperature for the odd rotational degrees of freedom,
distinct from the type-B transition associated with the arrest of the
translational and even rotational ones, as predicted by the mode-coupling
theory for the glass transition.Comment: 4 pages, 3 figure
General features of the energy landscape in Lennard-Jones like model liquids
Features of the energy landscape sampled by supercooled liquids are
numerically analyzed for several Lennard-Jones like model systems. The
properties of quasisaddles (minima of the square gradient of potential energy
W=|grad V|^2), are shown to have a direct relationship with the dynamical
behavior, confirming that the quasisaddle order extrapolates to zero at the
mode-coupling temperature T_MCT. The same result is obtained either analyzing
all the minima of W or the saddles (absolute minima of W), supporting the
conjectured similarity between quasisaddles and saddles, as far as the
temperature dependence of the properties influencing the slow dynamics is
concerned. We find evidence of universality in the shape of the landscape:
plots for different systems superimpose into master curves, once energies and
temperatures are scaled by T_MCT. This allows to establish a quantitative
relationship between T_MCT and potential energy barriers for LJ-like systems,
and suggests a possible generalization to different model liquids.Comment: 8 pages, 5 figure
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