14,710 research outputs found
Supernova Remnants as the Sources of Galactic Cosmic Rays
The origin of cosmic rays holds still many mysteries hundred years after they
were first discovered. Supernova remnants have for long been the most likely
sources of Galactic cosmic rays. I discuss here some recent evidence that
suggests that supernova remnants can indeed efficiently accelerate cosmic rays.
For this conference devoted to the Astronomical Institute Utrecht I put the
emphasis on work that was done in my group, but placed in a broader context:
efficient cosmic-ray acceleration and the im- plications for cosmic-ray escape,
synchrotron radiation and the evidence for magnetic- field amplification,
potential X-ray synchrotron emission from cosmic-ray precursors, and I conclude
with the implications of cosmic-ray escape for a Type Ia remnant like Tycho and
a core-collapse remnant like Cas A.Comment: Proceedings of the Meeting "370 years of astronomy in Utrecht",
Noordwijkerhout, The Netherlands, April 2-5, 2012 (ASPCS Conference Series
High Energy Emission from Supernova Remnants
This paper discusses several aspects of current research on high energy
emission from supernova remnants, covering the following main topics: 1) The
recent evidence for magnetic field amplification near supernova remnant shocks,
which makes that cosmic rays are more efficiently accelerated than previously
thought. 2) The evidence that ions and electrons in some remnants have very
different temperatures, and only equilibrate through Coulomb interactions. 3)
The evidence that the explosion that created Cas A was asymmetric, and seems to
have involved a jet/counter jet structure. And finally, 4), I will argue that
the unremarkable properties of supernova remnants associated with magnetars
candidates, suggest that magnetars are not formed from rapidly (P ~ 1 ms)
rotating proto-neutron stars, but that it is more likely that they are formed
from massive progenitors stars with high magnetic fields.Comment: Invited review, to be published in the proceedings of IAUS230:
"Populations of High Energy Sources in Galaxies", 14-19 August 2005, Dublin,
Eds E.J.A. Meurs and G. Fabbian
Making chiral fermion actions (almost) gauge invariant using Laplacian gauge fixing
Straight foreward lattice descriptions of chiral fermions lead to actions
that break gauge invariance. I describe a method to make such actions gauge
invariant (up to global gauge transformations) with the aid of gauge fixing. To
make this prescription unambiguous, Laplacian gauge fixing is used, which is
free from Gribov ambiguities.Comment: 3 p., Latex, (proc. Lattice '93, Dallas), 2 figs. appended, UCSD/PTH
93-4
Non-thermal X-ray Emission from Supernova Remnants
Recent studies of narrow, X-ray synchrotron radiating filaments surrounding
young supernova remnants indicate that magnetic fields strengths are relatively
high, B ~ 0.1 mG, or even higher, and that diffusion is close to the Bohm
limit. I illustrate this using Cas A as an example. Also older remnants such as
RCW 86 appear to emit X-ray synchrotron radiation, but the emission is more
diffuse, and not always confined to a region close to the shock front. I argue
that for RCW 86 the magnetic field is likely to be low (B ~ 17 microGauss), and
at the location where the shell emits X-ray synchrotron radiation the shock
velocity is much higher than the average shock velocity of ~600 km/s.Comment: To be published in the proceedings of the "International Symposium on
High Energy Gamma-Ray Astronomy" (Gamma-2004), Heidelberg, edited by F.A.
Aharonian and H. Voelk (AIP, NY). This preprint version contains 13 pages, 5
figures (4 in color
Very Massive Stars: a metallicity-dependent upper-mass limit, slow winds, and the self-enrichment of Globular Clusters
One of the key questions in Astrophysics concerns the issue of whether there
exists an upper-mass limit to stars, and if so, what physical mechanism sets
this limit, which might also determine if the upper-mass limit is metallicity
(Z) dependent. We argue that mass loss by radiation-driven winds mediated by
line opacity is one of the prime candidates setting the upper-mass limit. We
present mass-loss predictions (dM/dt_wind) from Monte Carlo radiative transfer
models for relatively cool (Teff = 15kK) inflated very massive stars (VMS) with
large Eddington Gamma factors in the mass range 100-1000 Msun as a function of
metallicity down to 1/100 Z/Zsun. We employ a hydrodynamic version of our Monte
Carlo method, allowing us to predict the rate of mass loss (dM/dt_wind) and the
terminal wind velocity (vinf) simultaneously. Interestingly, we find wind
terminal velocities (vinf) that are low (100-500 km/s) over a wide Z-range, and
we propose that the slow winds from VMS are an important source of
self-enrichment in globular clusters. We also find mass-loss rates
(dM/dt_wind), exceeding the typical mass-accretion rate (dM/dt_accr) of 0.001
Msun/yr during massive-star formation. We express our mass-loss predictions as
a function of mass and Z, finding log dM/dt = -9.13 + 2.1 log(M/Msun) + 0.74
log(Z/Zsun) (Msun/yr). Even if stellar winds would not directly halt & reverse
mass accretion during star formation, if the most massive stars form by stellar
mergers stellar wind mass loss may dominate over the rate at which stellar
growth takes place. We therefore argue that the upper-mass limit is effectively
Z-dependent due to the nature of radiation-driven winds. This has dramatic
consequences for the most luminous supernovae, gamma-ray bursts, and other
black hole formation scenarios at different Cosmic epochs.Comment: 9 pages, 3 figures. Accepted by Astronomy & Astrophysics. Small
textual change
Cosmic-Ray Acceleration Efficiency vs Temperature Equilibration: the Case of SNR 0509-67.5
We study the 0509-67.5 supernova remnant in the Large Magellanic Cloud with
the VLT/FORS2 spectrograph. We detect a broad component in the H-alpha emission
with a FWHM of 2680 \pm 70 km/s and 3900 \pm 800 km/s for the southwest (SW)
and northeast (NE) shocks respectively. For the SW, the proton temperature
appears to be too low for the shock velocity, which we attribute to a
cosmic-ray pressure behind the shock front of at least 20% of the total
pressure. For the NE, the post-shock proton temperature and the shock velocity
are compatible, only if the plasma behind the shock front has a degree of
thermal equilibrium of over 20%, which is at odds with current models for
temperature equilibration behind fast shocks, which do not accelerate cosmic
rays. If we assume the electron temperature to be less than 10% of the proton
temperature, we find a post-shock cosmic-ray pressure of at least 7%.Comment: Accepted for publication in ApJ Letters, 5 pages, 4 figures and 1
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Linear Spectropolarimetry and the Circumstellar Media of Young and Massive Stars
Linear spectropolarimetry is a powerful tool to probe circumstellar
structures on spatial scales that cannot yet be achieved through direct
imaging. In this review I discuss the role that emission-line polarimetry can
play in constraining geometrical and physical properties of a wide range of
circumstellar environments, varying from the accretion disks around pre-main
sequence T Tauri and Herbig Ae/Be stars, to the issue of stellar wind clumping,
and the aspherical outflows from the massive star progenitors of supernovae and
long gamma-ray bursts at low metallicity.Comment: 12 pages, 12 figures, Invited Review in Stellar Polarimetry: From
Birth to Deat
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