283 research outputs found
Protostars: Forges of cosmic rays?
Galactic cosmic rays (CR) are particles presumably accelerated in supernova
remnant shocks that propagate in the interstellar medium up to the densest
parts of molecular clouds, losing energy and their ionisation efficiency
because of the presence of magnetic fields and collisions with molecular
hydrogen. Recent observations hint at high levels of ionisation and at the
presence of synchrotron emission in protostellar systems, which leads to an
apparent contradiction. We want to explain the origin of these CRs accelerated
within young protostars as suggested by observations. Our modelling consists of
a set of conditions that has to be satisfied in order to have an efficient CR
acceleration through diffusive shock acceleration. We analyse three main
acceleration sites, then we follow the propagation of these particles through
the protostellar system up to the hot spot region. We find that jet shocks can
be strong accelerators of CR protons, which can be boosted up to relativistic
energies. Other promising acceleration sites are protostellar surfaces, where
shocks caused by impacting material during the collapse phase are strong enough
to accelerate CR protons. In contrast, accretion flow shocks are too weak to
efficiently accelerate CRs. Though CR electrons are weakly accelerated, they
can gain a strong boost to relativistic energies through re-acceleration in
successive shocks. We suggest a mechanism able to accelerate both CR protons
and electrons through the diffusive shock acceleration mechanism, which can be
used to explain the high ionisation rate and the synchrotron emission observed
towards protostellar sources. The existence of an internal source of energetic
particles can have a strong and unforeseen impact on the ionisation of the
protostellar disc, on the star and planet formation processes, and on the
formation of pre-biotic molecules.Comment: 22 pages, 15 figures, accepted by Astronomy and Astrophysic
Spatial distribution of interstellar gas in the innermost 3 kpc of our Galaxy
We review the present observational knowledge on the spatial distribution and
the physical state of the different (molecular, atomic and ionized) components
of the interstellar gas in the innermost 3 kpc of our Galaxy -- a region which
we refer to as the interstellar Galactic bulge, to distinguish it from its
stellar counterpart. We try to interpret the observations in the framework of
recent dynamical models of interstellar gas flows in the gravitational
potential of a barred galaxy. Finally, relying on both the relevant
observations and their theoretical interpretation, we propose a model for the
space-averaged density of each component of the interstellar gas in the
interstellar Galactic bulge.Comment: 19 pages, 11 figure
Galaxies in box: A simulated view of the interstellar medium
We review progress in the development of physically realistic three
dimensional simulated models of the galaxy.We consider the scales from star
forming molecular clouds to the full spiral disc. Models are computed using
hydrodynamic (HD) or magnetohydrodynamic (MHD) equations and may include cosmic
ray or tracer particles. The range of dynamical scales between the full galaxy
structure and the turbulent scales of supernova (SN) explosions and even cloud
collapse to form stars, make it impossible with current computing tools and
resources to resolve all of these in one model. We therefore consider a
hierarchy of models and how they can be related to enhance our understanding of
the complete galaxy.Comment: Chapter in Large Scale Magnetic Fields in the Univers
Inflation-Produced Magnetic Fields in Nonlinear Electrodynamics
We study the generation of primeval magnetic fields during inflation era in
nonlinear theories of electrodynamics. Although the intensity of the produced
fields strongly depends on characteristics of inflation and on the form of
electromagnetic Lagrangian, our results do not exclude the possibility that
these fields could be astrophysically interesting.Comment: 6 page
Galactic interstellar filaments as probed by LOFAR and Planck
Recent Low Frequency Array (LOFAR) observations at 115-175 MHz of a field at
medium Galactic latitudes (centered at the bright quasar 3C196) have shown
striking filamentary structures in polarization that extend over more than 4
degrees across the sky. In addition, the Planck satellite has released full sky
maps of the dust emission in polarization at 353GHz. The LOFAR data resolve
Faraday structures along the line of sight, whereas the Planck dust
polarization maps probe the orientation of the sky projected magnetic field
component. Hence, no apparent correlation between the two is expected. Here we
report a surprising, yet clear, correlation between the filamentary structures,
detected with LOFAR, and the magnetic field orientation, probed by the Planck
satellite. This finding points to a common, yet unclear, physical origin of the
two measurements in this specific area in the sky. A number of follow-up multi-
frequency studies are proposed to shed light on this unexpected finding.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letter
Observational constraints on models for the interstellar magnetic field in the Galactic disk
Our purpose is to place firm observational constraints on the three most
widely used theoretical models for the spatial configuration of the large-scale
interstellar magnetic field in the Galactic disk, namely, the ring, the
axisymmetric and the bisymmetric field models.
We use the rotation measures (RMs) of low-latitude Galactic pulsars and
combine them with their dispersion measures and estimated distances to map out
the line-of-sight component of the interstellar magnetic field in the near half
of the Galactic disk. We then fit our map of the line-of-sight field to the
three aforementioned theoretical field models and discuss the acceptability of
each fit, in order to determine whether the considered field model is allowed
by the pulsar data or not.
Strictly speaking, we find that all three field models are ruled out by the
pulsar data. Furthermore, none of them appears to perform significantly better
than the others. From this we conclude that the large-scale interstellar
magnetic field in the Galactic disk has a more complex pattern than just
circular, axisymmetric or bisymmetric.Comment: 11 pages, 8 figure
X-ray and UV spectroscopy of Galactic diffuse hot gas along the LMC X--3 sight line
We present Suzaku spectra of X-ray emission in the fields just off the LMC
X-3 sight line. OVII, OVIII, and NeIX emission lines are clearly detected,
suggesting the presence of an optically thin thermal plasma with an average
temperature of 2.4E6. This temperature is significantly higher than that
inferred from existing X-ray absorption line data obtained with Chandra grating
observations of LMC X-3, strongly suggesting that the gas is not isothermal. We
then jointly analyze these data to characterize the spatial and temperature
distributions of the gas. Assuming a vertical exponential Galactic disk model,
we estimate the gas temperature and density at the Galactic plane and their
scale heights as 3.6(2.9, 4.7)E6 K and 1.4(0.3, 3.4)E-3 cm^{-3} and 1.4(0.2,
5.2) kpc and kpc, respectively. This characterization can
account for all the \ovi line absorption, as observed in a FUSE spectrum of LMC
X-3, but only predicts less than one tenth of the OVI line emission intensity
typically detected at high Galactic latitudes. The bulk of the OVI emission
most likely arises at interfaces between cool and hot gases.Comment: 10 pages, 7 figures, 3 tables, accepted for publication in ApJ, 200
On the Origin of Cosmic Magnetic Fields
We review the literature concerning how the cosmic magnetic fields pervading
nearly all galaxies actually got started. some observational evidence involves
the chemical abundance of the light elements Be and B, while another one is
based on strong magnetic fields seen in high red shift galaxies. Seed fields,
whose strength is of order 10^{-20} gauss, easily sprung up in the era
preceding galaxy formation. Several mechanisms are proposed to amplify these
seed fields to microgauss strengths. The standard mechanism is the Alpha-Omega
dynamo theory. It has a major difficulty that makes unlikely to provide the
sole origin. The difficulty is rooted in the fact that the total flux is
constant. This implies that flux must be removed from the galactic discs. This
requires that the field and flux be separated, for otherwise interstellar mass
must be removed from the deep galactic gravitational and then their strength
increased by the alpha omega theory.Comment: 90 pages and 6 figures; accepted for publication in Reports of
Progress in Physics as an invited revie
Multiwavelength Campaign on Mrk 509 X. Lower limit on the distance of the absorber from HST COS and STIS spectroscopy
Active Galactic Nuclei often show evidence of photoionized outflows. A major
uncertainty in models for these outflows is the distance () to the gas from
the central black hole. In this paper we use the HST/COS data from a massive
multi-wavelength monitoring campaign on the bright Seyfert I galaxy Mrk 509, in
combination with archival HST/STIS data, to constrain the location of the
various kinematic components of the outflow. We compare the expected response
of the photoionized gas to changes in ionizing flux with the changes measured
in the data using the following steps: 1) We compare the column densities of
each kinematic component measured in the 2001 STIS data with those measured in
the 2009 COS data; 2) We use time-dependent photionization calculations with a
set of simulated lightcurves to put statistical upper limits on the hydrogen
number density that are consistent with the observed small changes in the ionic
column densities; 3) From the upper limit on the number density, we calculate a
lower limit on the distance to the absorber from the central source via the
prior determination of the ionization parameter. Our method offers two
improvements on traditional timescale analysis. First, we account for the
physical behavior of AGN lightcurves. Second, our analysis accounts for the
quality of measurement in cases where no changes are observed in the absorption
troughs. The very small variations in trough ionic column densities (mostly
consistent with no change) between the 2001 and 2009 epochs allow us to put
statistical lower limits on the distance between 100--200 pc for all the major
UV absorption components at a confidence level of 99%. These results are mainly
consistent with the independent distance estimates derived for the warm
absorbers from the simultaneous X-ray spectra.Comment: Accepted to A&A (06 APR 2012
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