289 research outputs found
Cool Stars and Space Weather
Stellar flares, winds and coronal mass ejections form the space weather. They
are signatures of the magnetic activity of cool stars and, since activity
varies with age, mass and rotation, the space weather that extra-solar planets
experience can be very different from the one encountered by the solar system
planets. How do stellar activity and magnetism influence the space weather of
exoplanets orbiting main-sequence stars? How do the environments surrounding
exoplanets differ from those around the planets in our own solar system? How
can the detailed knowledge acquired by the solar system community be applied in
exoplanetary systems? How does space weather affect habitability? These were
questions that were addressed in the splinter session "Cool stars and Space
Weather", that took place on 9 Jun 2014, during the Cool Stars 18 meeting. In
this paper, we present a summary of the contributions made to this session.Comment: Proceedings of the 18th Cambridge Workshop on Cool Stars, Stellar
Systems, and the Sun, Eds G. van Belle & H. Harris, 13 pages, 1 figur
Modelling the Corona of HD 189733 in 3D
The braking of main sequence stars originates mainly from their stellar wind. The efficiency of this angular momentum extraction depends on the rotation rate of the star, the acceleration profile of the wind and the coronal magnetic field. The derivation of scaling laws parametrizing the stellar wind torque is important for our understanding of gyro-chronology and the evolution of the rotation rates of stars. In order to understand the impact of complex magnetic topologies on the stellar wind torque, we present three-dimensional, dynamical simulations of the corona of HD 189733. Using the observed complex topology of the magnetic field, we estimate how the torque associated with the wind scales with model parameters and compare those trends to previously published scaling laws.AS thank A. Vidotto for discussions about the modelling of the corona of HD 189733. This work was supported by the ANR 2011
Blanc Toupies and the ERC project STARS2 (207430). The authors acknowledge CNRS INSU/PNST and CNES/Solar Orbiter
fundings. AS acknowledges support from the Canada’s Natural Sciences and Engineering Research Council and from the Canadian
Institute of Theoretical Astrophysics (National fellow). We acknowledge access to supercomputers through GENCI (project 1623),
Prace, and ComputeCanada infrastructures
Ultra-high energy Inverse Compton emission from Galactic electron accelerators
It is generally held that >100 TeV emission from astrophysical objects
unambiguously demonstrates the presence of PeV protons or nuclei, due to the
unavoidable Klein-Nishina suppression of inverse Compton emission from
electrons. However, in the presence of inverse Compton dominated cooling, hard
high-energy electron spectra are possible. We show that the environmental
requirements for such spectra can naturally be met in spiral arms, and in
particular in regions of enhanced star formation activity, the natural
locations for the most promising electron accelerators: powerful young pulsars.
Our scenario suggests a population of hard ultra-high energy sources is likely
to be revealed in future searches, and may also provide a natural explanation
for the 100 TeV sources recently reported by HAWC.Comment: Accepted for publication in ApJ
Proteolysis in salmon ( Salmo salar ) during cold storage : Effects of storage time and smoking process
Multiwavelength study of Cygnus A II. X-ray inverse-Compton emission from a relic counterjet and implications for jet duty-cycles
The duty-cycle of powerful radio galaxies and quasars such as the prototype
Cygnus A is poorly understood. X-ray observations of inverse-Compton scattered
Cosmic Microwave Background (ICCMB) photons probe lower Lorentz-factor
particles than radio observations of synchrotron emission. Comparative studies
of the nearer and further lobes, separated by many 10s of kpc and thus 10s of
thousands of years in light-travel time, yield additional temporal resolution
in studies of the lifecycles. We have co-added all archival Chandra ACIS-I data
and present a deep 200 ks image of Cygnus A. This deep image reveals the
presence of X-ray emission from a counterjet i.e. a jet receding from Earth and
related to a previous episode of jet activity. The non-thermal X-ray emission,
we interpret as ICCMB radiation. There is an absence of any discernible X-ray
emission associated with a jet flowing towards Earth. We conclude that: (1) The
emission from a relic jet, indicates a previous episode of jet activity, that
took place earlier than the current jet activity appearing as synchrotron radio
emission. (2) The presence of X-ray emission from a relic counterjet of Cygnus
A and the absence of X-ray emission associated with any relic approaching jet
constrains the timescale between successive episodes of jet activity to ~10^6
years. (3) Transverse expansion of the jet causes expansion losses which shifts
the energy distribution to lower energies. (4) Assuming the electrons cooled
due to adiabatic expansion, the required magnetic field strength is
substantially smaller than the equipartition magnetic field strength. (5) A
high minimum Lorentz factor for the distribution of relativistic particles in
the current jet, of a few 10^3, is ejected from the central nucleus of this
active galaxy. Abridged.Comment: Accepted for publication by MNRAS, 8 pages Dates in Table 1 correcte
Gamma-ray emission of accelerated particles escaping a supernova remnant in a molecular cloud
We present a model of gamma-ray emission from core-collapse supernovae
originating from the explosions of massive young stars. The fast forward shock
of the supernova remnant (SNR) can accelerate particles by diffusive shock
acceleration (DSA) in a cavern blown by a strong, pre-supernova stellar wind.
As a fundamental part of nonlinear DSA, some fraction of the accelerated
particles escape the shock and interact with a surrounding massive dense shell
producing hard photon emission. To calculate this emission, we have developed a
new Monte Carlo technique for propagating the cosmic rays (CRs) produced by the
forward shock of the SNR, into the dense, external material. This technique is
incorporated in a hydrodynamic model of an evolving SNR which includes the
nonlinear feedback of CRs on the SNR evolution, the production of escaping CRs
along with those that remain trapped within the remnant, and the broad-band
emission of radiation from trapped and escaping CRs. While our combined
CR-hydro-escape model is quite general and applies to both core collapse and
thermonuclear supernovae, the parameters we choose for our discussion here are
more typical of SNRs from very massive stars whose emission spectra differ
somewhat from those produced by lower mass progenitors directly interacting
with a molecular cloud.Comment: Accepted in Ap
The theory of pulsar winds and nebulae
We review current theoretical ideas on pulsar winds and their surrounding
nebulae. Relativistic MHD models of the wind of the aligned rotator, and of the
striped wind, together with models of magnetic dissipation are discussed. It is
shown that the observational signature of this dissipation is likely to be
point-like, rather than extended, and that pulsed emission may be produced. The
possible pulse shapes and polarisation properties are described. Particle
acceleration at the termination shock of the wind is discussed, and it is
argued that two distinct mechanisms must be operating, with the first-order
Fermi mechanism producing the high-energy electrons (above 1 TeV) and either
magnetic annihilation or resonant absorption of ion cyclotron waves responsible
for the 100 MeV to 1 TeV electrons. Finally, MHD models of the morphology of
the nebula are discussed and compared with observation.Comment: 33 pages, to appear in Springer Lecture Notes on "Neutron stars and
pulsars, 40 years after the discovery", ed W.Becke
Proton imaging of an electrostatic field structure formed in laser-produced counter-streaming plasmas
We report the measurements of electrostatic field structures associated with an electrostatic shock formed in laser-produced counter-streaming plasmas with proton imaging. The thickness of the electrostatic structure is estimated from proton images with different proton kinetic energies from 4.7 MeV to 10.7 MeV. The width of the transition region is characterized by electron scale length in the laser-produced plasma, suggesting that the field structure is formed due to a collisionless electrostatic shock
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