3,336 research outputs found
The Real Meaning of Complex Minkowski-Space World-Lines
In connection with the study of shear-free null geodesics in Minkowski space,
we investigate the real geometric effects in real Minkowski space that are
induced by and associated with complex world-lines in complex Minkowski space.
It was already known, in a formal manner, that complex analytic curves in
complex Minkowski space induce shear-free null geodesic congruences. Here we
look at the direct geometric connections of the complex line and the real
structures. Among other items, we show, in particular, how a complex world-line
projects into the real Minkowski space in the form of a real shear-free null
geodesic congruence.Comment: 16 page
Spin and lattice excitations of a BiFeO3 thin film and ceramics
We present a comprehensive study of polar and magnetic excitations in BiFeO3
ceramics and a thin film epitaxially grown on an orthorhombic (110) TbScO3
substrate. Infrared reflectivity spectroscopy was performed at temperatures
from 5 to 900 K for the ceramics and below room temperature for the thin film.
All 13 polar phonons allowed by the factor-group analysis were observed in
theceramic samples. The thin-film spectra revealed 12 phonon modes only and an
additional weak excitation, probably of spin origin. On heating towards the
ferroelectric phase transition near 1100 K, some phonons soften, leading to an
increase in the static permittivity. In the ceramics, terahertz transmission
spectra show five low-energy magnetic excitations including two which were not
previously known to be infrared active; at 5 K, their frequencies are 53 and 56
cm-1. Heating induces softening of all magnetic modes. At a temperature of 5 K,
applying an external magnetic field of up to 7 T irreversibly alters the
intensities of some of these modes. The frequencies of the observed spin
excitations provide support for the recently developed complex model of
magnetic interactions in BiFeO3 (R.S. Fishman, Phys. Rev. B 87, 224419 (2013)).
The simultaneous infrared and Raman activity of the spin excitations is
consistent with their assignment to electromagnons
Optical Properties of (SrMnO3)n/(LaMnO3)2n superlattices: an insulator-to-metal transition observed in the absence of disorder
We measure the optical conductivity of (SrMnO3)n/(LaMnO3)2n superlattices
(SL) for n=1,3,5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator
to metal transition (IMT) for n \leq 3, driven by the softening of a polaronic
mid-infrared band. At n = 5 that softening is incomplete, while at the
largest-period n=8 compound the MIR band is independent of T and the SL remains
insulating. One can thus first observe the IMT in a manganite system in the
absence of the disorder due to chemical doping. Unsuccessful reconstruction of
the SL optical properties from those of the original bulk materials suggests
that (SrMnO3)n/(LaMnO3)2n heterostructures give rise to a novel electronic
state.Comment: Published Online in Nano Letters, November 8, 2010;
http://pubs.acs.org/doi/abs/10.1021/nl1022628; 5 pages, 3 figure
Revealing a Ring-like Cluster Complex in a Tidal Tail of the Starburst Galaxy NGC 2146
We report the discovery of a ring-like cluster complex in the starburst
galaxy NGC 2146. The Ruby Ring, so named due to its appearance, shows a clear
ring-like distribution of star clusters around a central object. It is located
in one of the tidal streams which surround the galaxy. NGC 2146 is part of the
Snapshot Hubble U-band Cluster Survey (SHUCS). The WFC3/F336W data has added
critical information to the available archival Hubble Space Telescope imaging
set of NGC 2146, allowing us to determine ages, masses, and extinctions of the
clusters in the Ruby Ring. These properties have then been used to investigate
the formation of this extraordinary system. We find evidence of a spatial and
temporal correlation between the central cluster and the clusters in the ring.
The latter are about 4 Myr younger than the central cluster, which has an age
of 7 Myr. This result is supported by the H alpha emission which is strongly
coincident with the ring, and weaker at the position of the central cluster.
From the derived total H alpha luminosity of the system we constrain the star
formation rate density to be quite high, e.g. ~ 0.47 Msun/yr/kpc^2. The Ruby
Ring is the product of an intense and localised burst of star formation,
similar to the extended cluster complexes observed in M51 and the Antennae, but
more impressive because is quite isolated. The central cluster contains only 5
% of the total stellar mass in the clusters that are determined within the
complex. The ring-like morphology, the age spread, and the mass ratio support a
triggering formation scenario for this complex. We discuss the formation of the
Ruby Ring in a "collect & collapse" framework. The predictions made by this
model agree quite well with the estimated bubble radius and expansion velocity
produced by the feedback from the central cluster, making the Ruby Ring an
interesting case of triggered star formation.Comment: 11 pages, 7 figures, 1 table; Accepted for publication in MNRA
Towards elimination of measles and rubella in Italy. Progress and challenges
Introduction In the WHO European Region, endemic transmission of measles and rubella had been interrupted by 37 and 42 of the 53 member states (MSs), respectively, by 2018. Sixteen MSs are still endemic for measles, 11 for rubella and nine for both diseases, the latter including Italy. Elimination is documented by each country’s National Verification Committee (NVC) through an annual status update (ASU). Objective By analysing data used to produce the ASUs, we aimed to describe the advances made by Italy towards elimination of measles and rubella. Moreover, we propose a set of major interventions that could facilitate the elimination process. Methods A total of 28 indicators were identified within the six core sections of the ASU form and these were evaluated for the period 2013–2018. These indicators relate to the incidence of measles/rubella; epidemiological investigation of cases; investigation of outbreaks; performance of the surveillance system; population immunity levels; and implementation of supplemental immunization activities (SIAs). Results From 2013 to 2018, epidemiological and laboratory analyses of measles cases in Italy improved substantially, allowing timely investigation in 2017 and 2018 of most outbreak and sporadic cases and identification of the majority of genotypic variants. Moreover, since 2017, vaccination coverage has increased significantly. Despite these improvements, several areas of concern emerged, prompting the following recommendations: i) improve outbreak monitoring; ii) strengthen the MoRoNet network; iii) increase the number of SIAs; iv) reinforce vaccination services; v) maintain regional monitoring; vi) design effective communication strategies; vii) foster the role of general practitioners and family paediatricians. Conclusions The review of national ASUs is a crucial step to provide the NVC with useful insights into the elimination process and to guide the development of targeted interventions. Against this background, the seven recommendations proposed by the NVC have been shared with the Italian Ministry of Health and the Technical Advisory Group on measles and rubella elimination and have been incorporated into the new Italian Elimination Plan 2019–2023 as a technical aid to facilitate the achievement of disease elimination goals
Spectroscopic Constraints on the Form of the Stellar Cluster Mass Function
This contribution addresses the question of whether the initial cluster mass
function (ICMF) has a fundamental limit (or truncation) at high masses. The
shape of the ICMF at high masses can be studied using the most massive young
(<10 Myr) clusters, however this has proven difficult due to low-number
statistics. In this contribution we use an alternative method based on the
luminosities of the brightest clusters, combined with their ages. If a
truncation is present, a generic prediction (nearly independent of the cluster
disruption law adopted) is that the median age of bright clusters should be
younger than that of fainter clusters. In the case of an non-truncated ICMF,
the median age should be independent of cluster luminosity. Here, we present
optical spectroscopy of twelve young stellar clusters in the face-on spiral
galaxy NGC 2997. The spectra are used to estimate the age of each cluster, and
the brightness of the clusters is taken from the literature. The observations
are compared with the model expectations of Larsen (2009) for various ICMF
forms and both mass dependent and mass independent cluster disruption. While
there exists some degeneracy between the truncation mass and the amount of mass
independent disruption, the observations favour a truncated ICMF. For low or
modest amounts of mass independent disruption, a truncation mass of 5-6*10^5
Msun is estimated, consistent with previous determinations. Additionally, we
investigate possible truncations in the ICMF in the spiral galaxy M83, the
interacting Antennae galaxies, and the collection of spiral and dwarf galaxies
present in Larsen (2009) based on photometric catalogues taken from the
literature, and find that all catalogues are consistent with having a
(environmentally dependent) truncation in the cluster mass functions.Comment: 6 pages, 5 figures, in press, A&A Research Note
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