157 research outputs found
The transport of cosmic rays in self-excited magnetic turbulence
The process of diffusive shock acceleration relies on the efficacy with which
hydromagnetic waves can scatter charged particles in the precursor of a shock.
The growth of self-generated waves is driven by both resonant and non-resonant
processes. We perform high-resolution magnetohydrodynamic simulations of the
non-resonant cosmic-ray driven instability, in which the unstable waves are
excited beyond the linear regime. In a snapshot of the resultant field,
particle transport simulations are carried out. The use of a static snapshot of
the field is reasonable given that the Larmor period for particles is typically
very short relative to the instability growth time. The diffusion rate is found
to be close to, or below, the Bohm limit for a range of energies. This provides
the first explicit demonstration that self-excited turbulence reduces the
diffusion coefficient and has important implications for cosmic ray transport
and acceleration in supernova remnants.Comment: 8 pages, 8 figures, accepted for publication in MNRA
VISIR-VLT Images of the Water Maser Emitting Planetary Nebula K 3-35
K3-35 is an extremely young bipolar planetary nebula that contains a
precessing bipolar jet and a small (radius 80 AU) water maser equatorial ring.
We have obtained VISIR- VLT images of K 3-35 in the PAH1 ({\lambda}=8.6
{\mu}m), [S iv] ({\lambda}=10.6 {\mu}m), and SiC ({\lambda}=11.85 {\mu}m)
filters to analize the mid-IR morphology and the temperature structure of its
dust emission. The images show the innermost nebular regions undetected at
optical wavelegths and the precessing bipolar jets. The temperature map shows
variations in the temperature in the equatorial zone and in regions associated
to its jets.Comment: 2 pages, 2 figures, 283 IAU Symp. Planetary Nebulae an Eye to the
Futur
The Molecular Gas Environment around Two Herbig Ae/Be Stars: Resolving the Outflows of LkHa 198 and LkHa 225S
Observations of outflows associated with pre-main-sequence stars reveal
details about morphology, binarity and evolutionary states of young stellar
objects. We present molecular line data from the Berkeley-Illinois-Maryland
Association array and Five Colleges Radio Astronomical Observatory toward the
regions containing the Herbig Ae/Be stars LkHa 198 and LkHa 225S. Single dish
observations of 12CO 1-0, 13CO 1-0, N2H+ 1-0 and CS 2-1 were made over a field
of 4.3' x 4.3' for each species. 12CO data from FCRAO were combined with high
resolution BIMA array data to achieve a naturally-weighted synthesized beam of
6.75'' x 5.5'' toward LkHa 198 and 5.7'' x 3.95'' toward LkHa 225S,
representing resolution improvements of factors of approximately 10 and 5 over
existing data. By using uniform weighting, we achieved another factor of two
improvement. The outflow around LkHa 198 resolves into at least four outflows,
none of which are centered on LkHa 198-IR, but even at our resolution, we
cannot exclude the possibility of an outflow associated with this source. In
the LkHa 225S region, we find evidence for two outflows associated with LkHa
225S itself and a third outflow is likely driven by this source. Identification
of the driving sources is still resolution-limited and is also complicated by
the presence of three clouds along the line of sight toward the Cygnus
molecular cloud. 13CO is present in the environments of both stars along with
cold, dense gas as traced by CS and (in LkHa 225S) N2H+. No 2.6 mm continuum is
detected in either region in relatively shallow maps compared to existing
continuum observations.Comment: 14 pages, 10 figures (5 color), accepted for publication in Ap
Formation of hard VHE gamma-ray spectra of blazars due to internal photon-photon absorption
The energy spectra of TeV gamma-rays from blazars, after being corrected for
intergalatic absorption in the Extragalactic Background Light (EBL), appear
unusually hard, a fact that poses challenges to the conventional models of
particle acceleration in TeV blazars and/or to the EBL models. In this paper we
show that the internal absorption of gamma-rays caused by interactions with
dense narrow-band radiation fields in the vicinity of compact gamma-ray
production regions can lead to the formation of gamma-ray spectra of an almost
arbitrary hardness. This allows significant relaxation of the current tight
constraints on particle acceleration and radiation models, although at the
expense of enhanced requirements to the available nonthermal energy budget. The
latter, however, is not a critical issue, as long as it can be largely
compensated by the Doppler boosting, assuming very large () Doppler
factors of the relativistically moving gamma-ray production regions. The
suggested scenario of formation of hard gamma-ray spectra predicts detectable
synchrotron radiation of secondary electron-positron pairs which might require
a revision of the current ``standard paradigm'' of spectral energy
distributions of gamma-ray blazars. If the primary gamma-rays are of hadronic
origin related to or interactions, the ``internal gamma-ray
absorption'' model predicts neutrino fluxes close to the detection threshold of
the next generation high energy neutrino detectors.Comment: 10 pages, 8 figures, submitted to MNRA
Cosmic Rays X. The cosmic ray knee and beyond: Diffusive acceleration at oblique shocks
Our purpose is to evaluate the rate of the maximum energy and the
acceleration rate that cosmic rays acquire in the non-relativistic diffusive
shock acceleration as it could apply during their lifetime in various
astrophysical sites, where highly oblique shocks exist. We examine numerically
(using Monte Carlo simulations) the effect of the diffusion coefficients on the
energy gain and the acceleration rate, by testing the role between the
obliquity of the magnetic field to the shock normal, and the significance of
both perpendicular cross-field diffusion and parallel diffusion coefficients to
the acceleration rate. We find (and justify previous analytical work - Jokipii
1987) that in highly oblique shocks the smaller the perpendicular diffusion
gets compared to the parallel diffusion coefficient values, the greater the
energy gain of the cosmic rays to be obtained. An explanation of the cosmic ray
spectrum in high energies, between eV and about eV is
claimed, as we estimate the upper limit of energy that cosmic rays could gain
in plausible astrophysical regimes; interpreted by the scenario of cosmic rays
which are injected by three different kind of sources, (a) supernovae which
explode into the interstellar medium, (b) Red Supergiants, and (c) Wolf-Rayet
stars, where the two latter explode into their pre-supernovae winds.Comment: Accepted in Astronomy and Astrophysics, 9 pages, 8 figures (for the
'Cosmic Rays' series papers
On-sky results of the adaptive optics MACAO for the new IR-spectrograph CRIRES at VLT
The adaptive optics MACAO has been implemented in 6 focii of the VLT
observatory, in three different flavors. We present in this paper the results
obtained during the commissioning of the last of these units, MACAO-CRIRES.
CRIRES is a high-resolution spectrograph, which efficiency will be improved by
a factor two at least for point-sources observations with a NGS brighter than
R=15. During the commissioning, Strehl exceeding 60% have been observed with
fair seeing conditions, and a general description of the performance of this
curvature adaptive optics system is done.Comment: SPIE conference 2006, Advances in adaptive optics, 12 pages, 11
figure
Imaging the outward motions of clumpy dust clouds around the red supergiant Antares with VLT/VISIR
We present a 0.5"-resolution diffraction-limited 17.7 micron image of the red
supergiant Antares obtained with the VLT mid-infrared instrument VISIR. The
VISIR image shows six clumpy dust clouds located at 0.8"--1.8" (43--96 stellar
radii = 136--306 AU) away from the star. We also detected compact emission
within a radius of 0.5" from the star. Comparison of our VISIR image taken in
2010 and the 20.8 micron image taken in 1998 with the Keck telescope reveals
the outward motions of four dust clumps. The proper motions of these dust
clumps amount to 0.2"--0.6" in 12 years. This translates into expansion
velocities (projected onto the plane of the sky) of 13--40 km/s with an
uncertainty of +/-7 km/s. The velocities of the dust clumps cannot be explained
by a simple accelerating outflow, implying the possible random nature of the
dust cloud ejection mechanism. The inner compact emission seen in the 2010
VISIR image is presumably newly formed dust, because it is not detected in the
image taken in 1998. The mass of the dust clouds is estimated to be
(3-6)x10^{-9} Msun. These values are lower by a factor of 3--7 than the amount
of dust ejected in one year estimated from the (gas+dust) mass-loss rate of
2x10^{-6} Msun/yr, suggesting that the continuous mass loss is superimposed on
the clumpy dust cloud ejection.Comment: 10 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
The influence of the Alfv\'enic drift on the shape of cosmic ray spectra in SNRs
Cosmic ray acceleration in SNRs in the presence of the Alfv\'enic drift is
considered. It is shown that spectra of accelerated particles may be
considerably softer in the presence of amplified magnetic fields.Comment: 4 pages, 4 figures, poster talk at 4-th Gamma-ray Symposium
(Heidelberg, Germany, 7-11th of July 2008
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