815 research outputs found
Non-equilibrium ionization around clouds evaporating in the interstellar medium
It is of prime importance for global models of the interstellar medium to know whether dense clouds do or do not evaporate in the hot coronal gas. The rate of mass exchanges between phases depends very much on that. McKee and Ostriker's model, for instance, assumes that evaporation is important enough to control the expansion of supernova remnants, and that mass loss obeys the law derived by Cowie and McKee. In fact, the geometry of the magnetic field is nearly unknown, and it might totally inhibit evaporation, if the clouds are not regularly connected to the hot gas. Up to now, the only test of the theory is the U.V. observation (by the Copernicus and IUE satellites) of absorption lines of ions such as OVI or NV, that exist at temperatures of a few 100,000 K typical of transition layers around evaporating clouds. Other means of testing the theory are discussed
OVII and OVIII line emission in the diffuse soft X-ray background: heliospheric and galactic contributions
We study the 0.57 keV (O VII triplet) and 0.65 keV (O VIII) diffuse emission
generated by charge transfer collisions between solar wind (SW) oxygen ions and
interstellar H and He neutral atoms in the inner Heliosphere. These lines which
dominate the 0.3-1.0 keV energy interval are also produced by hot gas in the
galactic halo (GH) and possibly the Local Interstellar Bubble (LB). We
developed a time-dependent model of the SW Charge-Exchange (SWCX) X-ray
emission, based on the localization of the SW Parker spiral at each instant. We
include input SW conditions affecting three selected fields, as well as
shadowing targets observed with XMM-Newton, Chandra and Suzaku and calculate
X-ray emission fot O VII and O VIII lines. We determine SWCX contamination and
residual emission to attribute to the galactic soft X-ray background. We obtain
ground level intensities and/or simulated lightcurves for each target and
compare to X-ray data. The local 3/4 keV emission (O VII and O VIII) detected
in front of shadowing clouds is found to be entirely explained by the CX
heliospheric emission. No emission from the LB is needed at these energies.
Using the model predictions we subtract the heliospheric contribution to the
measured emission and derive the halo contribution. We also correct for an
error in the preliminary analysis of the Hubble Deep Field North (HDFN).Comment: 21 pages (3 on-line), 10 figures (4 on-line), accepted for
publication in Astronomy and Astrophysic
A new nearby pulsar wind nebula overlapping the RX J0852.0-4622 supernova remnant
Energetic pulsars can be embedded in a nebula of relativistic leptons which
is powered by the dissipation of the rotational energy of the pulsar. The
object PSR J0855-4644 is an energetic and fast-spinning pulsar (Edot =
1.1x10^36 erg/s, P=65 ms) discovered near the South-East rim of the supernova
remnant (SNR) RX J0852.0-4622 (aka Vela Jr) by the Parkes multibeam survey. The
position of the pulsar is in spatial coincidence with an enhancement in X-rays
and TeV gamma-rays, which could be due to its putative pulsar wind nebula
(PWN).
The purpose of this study is to search for diffuse non-thermal X-ray emission
around PSR J0855-4644 to test for the presence of a PWN and to estimate the
distance to the pulsar. An X-ray observation was carried out with the
XMM-Newton satellite to constrain the properties of the pulsar and its nebula.
The absorption column density derived in X-rays from the pulsar and from
different regions of the rim of the SNR was compared with the absorption
derived from the atomic (HI) and molecular (12CO) gas distribution along the
corresponding lines of sight to estimate the distance of the pulsar and of the
SNR.
The observation has revealed the X-ray counterpart of the pulsar together
with surrounding extended emission thus confirming the existence of a PWN. The
comparison of column densities provided an upper limit to the distance of the
pulsar PSR J0855-4644 and the SNR RX J0852.0-4622 (d<900 pc). Although both
objects are at compatible distances, we rule out that the pulsar and the SNR
are associated. With this revised distance, PSR J0855-4644 is the second most
energetic pulsar, after the Vela pulsar, within a radius of 1 kpc and could
therefore contribute to the local cosmic-ray e-/e+ spectrum.Comment: 10 pages, 9 Figures. Accepted for publication in A&
Comparaison des qualités sensorielles de la viande et de la carcasse d’agneaux élevés au pâturage en production biologique ou conventionnelle à deux niveaux de disponibilités en herbe
Nous avons comparé les qualités sensorielles des carcasses et des viandes d’agneaux engraissés au pâturage en élevage biologique ou conventionnel (O vs. C) à deux niveaux de disponibilités en herbe (Haut H vs. Bas L). Le profil de croissance a été maintenu similaire entre les deux systèmes de production. L’expérimentation a été conduite pendant deux années avec 12 agneaux mâles castrés de race Limousine dans chaque groupe OH, OL, CH et CL chaque année. Les traitements O et C différaient par le niveau de fertilisation azotée minérale épandu sur les parcelles. Les parcelles expérimentales étaient des repousses après fauche et elles étaient conduites en pâturage tournant pour conduire à un âge moyen des agneaux à l’abattage de 5 et 6 mois dans les lots H et L respectivement. Les côtelettes O ont été moins appréciées que les côtelettes C. L’indice de rouge du muscle longissimus thoracis et lumborum après 2h d’exposition à l’air a été plus élevé chez les agneaux L que chez les agneaux H, indiquant les effets possibles d’une intensification de l’élevage biologique à travers une augmentation du chargement
Theoretical Foundations and the Book's Roadmap
This collection places children's issues at the centre of understandings of human development. Using Amartya Sen's 'Capability Approach', the contributors to this book draw on new tools and theoretical perspectives to understand the role of children in human development. Looking at a wide range of themes including child poverty, microfinance, disability, education, the built environment, the role of emotions and promoting children's active participation, this study furthers the capability approach as a key theoretical perpsective in understanding children and development
Spatial identification of the overionized plasma in W49B
Recent Suzaku X-ray observations of the ejecta-dominated supernova remnant
W49B have shown that in the global spectrum there is a clear indication for the
presence of overionized plasma whose physical origin is still under debate. In
order to ascertain the physical origin of such a rapidly cooling plasma, we
focus on the study of its spatial localization within the X-ray emitting
ejecta. We confirm the presence of a saw-edged excess (interpreted as a strong
radiative recombination continuum) in the global spectrum above 8 keV, emerging
above the ionization-equilibrium model. We produce a hardness ratio map to
determine where the plasma is overionized and we perform a spectral analysis of
the regions with and without strong overionization. We find that the
overionized plasma is localized in the center of the remnant and in its western
jet, while it is not detected in the bright eastern jet, where the expansion of
the ejecta is hampered by their interaction with a dense interstellar cloud.
The location of overionized plasma suggests that the inner ejecta are rapidly
cooling by expansion, unlike the outer ejecta, for which expansion is hampered
by interstellar clouds seen in H2Comment: Accepted for publication as a Letter in Astronomy and Astrophysics
Disentangling hadronic from leptonic emission in the composite SNR G326.31.8
G326.31.8 (also known as MSH 1556) has been detected in radio as a
middle-aged composite supernova remnant (SNR) consisting of an SNR shell and a
pulsar wind nebula (PWN), which has been crushed by the SNR's reverse shock.
Previous -ray studies of SNR G326.31.8 revealed bright and extended
emission with uncertain origin. Understanding the nature of the -ray
emission allows probing the population of high-energy particles (leptons or
hadrons) but can be challenging for sources of small angular extent. With the
recent Large Area Telescope data release Pass 8, we
investigate the morphology of this SNR to disentangle the PWN from the SNR
contribution. We perform a morphological and spectral analysis from 300 MeV to
300 GeV. We use the reconstructed events with the best angular resolution to
separately investigate the PWN and the SNR emissions, which is crucial to
accurately determine the spectral properties of G326.31.8 and understand its
nature. The centroid of the -ray emission evolves with energy and is
spatially coincident with the radio PWN at high energies (E 3 GeV). The
morphological analysis reveals that a model considering two contributions from
the SNR and the PWN reproduces the -ray data better than a
single-component model. The associated spectral analysis using power laws shows
two distinct spectral features, a softer spectrum for the remnant ( =
2.17 0.06) and a harder spectrum for the PWN ( = 1.79
0.12), consistent with hadronic and leptonic origin for the SNR and the PWN
respectively. Focusing on the SNR spectrum, we use one-zone models to derive
some physical properties and, in particular, we find that the emission is best
explained with a hadronic scenario in which the large target density is
provided by radiative shocks in HI clouds struck by the SNR
3D simulations of supernova remnants evolution including non-linear particle acceleration
If a sizeable fraction of the energy of supernova remnant shocks is channeled into energetic particles (commonly identified with Galactic cosmic rays), then the morphological evolution of the remnants must be distinctly modified. Evidence of such modifications has been recently obtained with the Chandra and XMM-Newton X-ray satellites. To investigate these effects, we coupled a semi-analytical kinetic model of shock acceleration with a 3D hydrodynamic code (by means of an effective adiabatic index). This enables us to study the time-dependent compression of the region between the forward and reverse shocks due to the back reaction of accelerated particles, concomitantly with the development of the Rayleigh-Taylor hydrodynamic instability at the contact discontinuity. Density profiles depend critically on the injection level η of particles: for η ≲ 10-4 modifications are weak and progressive, for η ˜ 10-3 modifications are strong and immediate. Nevertheless, the extension of the Rayleigh-Taylor unstable region does not depend on the injection rate. A first comparison of our simulations with observations of Tycho's remnant strengthens the case for efficient acceleration of protons at the forward shock
- …