11,176 research outputs found
Eta Carinae across the 2003.5 Minimum: Analysis in the visible and near infrared spectral region
We present an analysis of the visible through near infrared spectrum of Eta
Carinae and its ejecta obtained during the "Eta Carinae Campaign with the UVES
at the ESO VLT". This is a part of larger effort to present a complete Eta
Carinae spectrum, and extends the previously presented analyses with the
HST/STIS in the UV (1240-3159 A) to 10,430 A. The spectrum in the mid and near
UV is characterized by the ejecta absorption. At longer wavelengths, stellar
wind features from the central source and narrow emission lines from the
Weigelt condensations dominate the spectrum. However, narrow absorption lines
from the circumstellar shells are present. This paper provides a description of
the spectrum between 3060 and 10,430 A, including line identifications of the
ejecta absorption spectrum, the emission spectrum from the Weigelt
condensations and the P-Cygni stellar wind features. The high spectral
resolving power of VLT/UVES enables equivalent width measurements of atomic and
molecular absorption lines for elements with no transitions at the shorter
wavelengths. However, the ground based seeing and contributions of nebular
scattered radiation prevent direct comparison of measured equivalent widths in
the VLT/UVES and HST/STIS spectra. Fortunately, HST/STIS and VLT/UVES have a
small overlap in wavelength coverage which allows us to compare and adjust for
the difference in scattered radiation entering the instruments' apertures. This
paper provides a complete online VLT/UVES spectrum with line identifications
and a spectral comparison between HST/STIS and VLT/UVES between 3060 and 3160
A.Comment: 13 pages, 11 figures + atlas. The paper accepted for the ApJS and is
accompanied with an atlas in the online edition pape
Electron trapping and acceleration by the plasma wakefield of a self-modulating proton beam
It is shown that co-linear injection of electrons or positrons into the
wakefield of the self-modulating particle beam is possible and ensures high
energy gain. The witness beam must co-propagate with the tail part of the
driver, since the plasma wave phase velocity there can exceed the light
velocity, which is necessary for efficient acceleration. If the witness beam is
many wakefield periods long, then the trapped charge is limited by beam loading
effects. The initial trapping is better for positrons, but at the acceleration
stage a considerable fraction of positrons is lost from the wave. For efficient
trapping of electrons, the plasma boundary must be sharp, with the density
transition region shorter than several centimeters. Positrons are not
susceptible to the initial plasma density gradient.Comment: 9 pages, 9 figures, 1 table, 44 reference
Spin dynamics of the S=1/2 antiferromagnetic zig-zag ladder with anisotropy
We use exact diagonalization and the modified Lanczos method to study the
finite energy and finite momentum spectral weight of the longitudinal and
transverse spin excitations of the anisotropic zig-zag ladder. We find that the
spin excitations form continua of gapless or gapped spinons in the different
regions of the phase diagram. The results obtained are consistent with a
picture previously proposed that in the anisotropic case there is a transition
from a gapped regime to a gapless regime, for small interchain coupling. In
this regime we find a sharp low-energy peak in the structure function for the
transverse spin excitations, consistent with a finite stiffness.Comment: 17 figure
A simple deterministic self-organized critical system
We introduce a new continuous cellular automaton that presents self-organized
criticality. It is one-dimensional, totally deterministic, without any kind of
embedded randomness, not even in the initial conditions. This system is in the
same universality class as the Oslo rice pile system, boundary driven interface
depinning and the train model for earthquakes. Although the system is chaotic,
in the thermodynamic limit chaos occurs only in a microscopic level.Comment: System slightly modified. New results on Liapunov exponents.
Submitted for publication (8 pages
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A Certification Framework for Cloud Security Properties: The Monitoring Path
In this paper we describe the structure and functionality of a certification integrated framework aimed to support the certification of security properties of a Cloud infrastructure (IaaS), a platform (PaaS), or the software layer (SaaS). Such framework will bring service users, service providers and cloud suppliers to work together with certification authorities in order to ensure security properties and certificates validity in the continuously evolving cloud environment. For this purpose, the framework relies on multiple types of evidence gathering with respect to security, e.g., testing services, monitoring agents or trusted computing proofs. In this paper we will focus only on the monitoring case and will illustrate its use. Yet, this framework is designed to be able to follow models for hybrid, incremental and multi-layer security certification since cloud security has to build upon the entire cloud stack
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