10,722 research outputs found
Cannibalism as a life boat mechanism
Under certain conditions a cannibalistic population can survive when food for the adults is too scarce to support a non-cannibalistic population. Cannibalism can have this lifeboat effect if (i) the juveniles feed on a resource inaccessible to the adults; and (ii) the adults are cannibalistic and thus incorporate indirectly the inaccessible resource. Using a simple model we conclude that the mechanism works when, at low population densities, the average yield, in terms of new offspring, due to the energy provided by one cannibalized juvenile is larger than one
Optical and ROSAT X-ray observations of the dwarf nova OY Carinae in superoutburst and quiescence
We present ROSAT X-ray and optical light curves of the 1994 February
superoutburst of the eclipsing SU UMa dwarf nova OY Carinae. There is no
eclipse of the flux in the ROSAT HRI light curve. Contemporaneous `wide B' band
optical light curves show extensive superhump activity and dips at superhump
maximum. Eclipse mapping of these optical light curves reveals a disc with a
considerable physical flare, even three days into the superoutburst decline.
We include a later (1994 July) ROSAT PSPC observation of OY Car that allows
us to put constraints on the quiescent X-ray spectrum. We find that while there
is little to choose between OY Car and its fellow high inclination systems with
regard to the temperature of the emitting gas and the emission measure, we have
difficulties reconciling the column density found from our X-ray observation
with the column found in HST UV observations by Horne et al. (1994). The
obvious option is to invoke time variability.Comment: 16 pages, 14 figures, accepted for publication in MNRA
A Window On The Earliest Star Formation: Extreme Photoionization Conditions of a High-Ionization, Low-Metallicity Lensed Galaxy at z~2
We report new observations of SL2SJ021737-051329, a lens system consisting of
a bright arc at z=1.84435, magnified ~17x by a massive galaxy at z=0.65.
SL2SJ0217 is a low-mass (M <10^9 M*), low-metallicity (Z~1/20 Z*) galaxy, with
extreme star-forming conditions that produce strong nebular UV emission lines
in the absence of any apparent outflows. Here we present several notable
features from rest-frame UV Keck/LRIS spectroscopy: (1) Very strong narrow
emission lines are measured for CIV 1548,1550, HeII 1640, OIII] 1661,1666,
SiIII] 1883,1892, and CIII] 1907,1909. (2) Double-peaked LyA emission is
observed with a dominant blue peak and centered near the systemic velocity. (3)
The low- and high-ionization absorption features indicate very little or no
outflowing gas along the sightline to the lensed galaxy. The relative emission
line strengths can be reproduced with a very high-ionization, low-metallicity
starburst with binaries, with the exception of He \ii, which indicates an
additional ionization source is needed. We rule out large contributions from
AGN and shocks to the photoionization budget, suggesting that the emission
features requiring the hardest radiation field likely result from extreme
stellar populations that are beyond the capabilities of current models.
Therefore, SL2S0217 serves as a template for the extreme conditions that are
important for reionization and thought to be more common in the early Universe.Comment: 28 pages, 16 figures, 8 tables, re-submitted to ApJ, comments welcom
The Fe XXII I(11.92 A)/I(11.77 A) Density Diagnostic Applied to the Chandra High Energy Transmission Grating Spectrum of EX Hydrae
Using the Livermore X-ray Spectral Synthesizer, which calculates spectral
models of highly charged ions based primarily on HULLAC atomic data, we
investigate the temperature, density, and photoexcitation dependence of the
I(11.92 A)/I(11.77 A) line ratio of Fe XXII. We find that this line ratio has a
critical density n_c \approx 5x10^13 cm^-3, is approximately 0.3 at low
densities and 1.5 at high densities, and is very insensitive to temperature and
photoexcitation, so is a useful density diagnostic for sources like magnetic
cataclysmic variables in which the plasma densities are high and the efficacy
of the He-like ion density diagnostic is compromised by the presence of a
bright ultraviolet continuum. Applying this diagnostic to the Chandra High
Energy Transmission Grating spectrum of the intermediate polar EX Hya, we find
that the electron density of its T_e \approx 12 MK plasma is n_e =
1.0^{+2.0}_{-0.5} x 10^14 cm^-3, orders of magnitude greater than that
typically observed in the Sun or other late-type stars.Comment: 11 pages including 3 encapsulated postscript figures; LaTeX format,
uses aastex.cls; accepted on 2003 April 3 for publication in The
Astrophysical Journa
Screening of charged spheroidal colloidal particles
We study the effective screened electrostatic potential created by a
spheroidal colloidal particle immersed in an electrolyte, within the mean field
approximation, using Poisson--Botzmann equation in its linear and nonlinear
forms, and also beyond the mean field by means of Monte Carlo computer
simulation. The anisotropic shape of the particle has a strong effect on the
screened potential, even at large distances (compared to the Debye length) from
it. To quantify this anisotropy effect, we focus our study on the dependence of
the potential on the position of the observation point with respect with the
orientation of the spheroidal particle. For several different boundary
conditions (constant potential, or constant surface charge) we find that, at
large distance, the potential is higher in the direction of the large axis of
the spheroidal particle
A New Test of the Einstein Equivalence Principle and the Isotropy of Space
Recent research has established that nonsymmetric gravitation theories like
Moffat's NGT predict that a gravitational field singles out an orthogonal pair
of polarization states of light that propagate with different phase velocities.
We show that a much wider class of nonmetric theories encompassed by the formalism predict such violations of the Einstein equivalence principle.
This gravity-induced birefringence of space implies that propagation through a
gravitational field can alter the polarization of light. We use data from
polarization measurements of extragalactic sources to constrain birefringence
induced by the field of the Galaxy. Our new constraint is times sharper
than previous ones.Comment: 21 pages, Latex, 3 Postscript figure
Interdependent networks with correlated degrees of mutually dependent nodes
We study a problem of failure of two interdependent networks in the case of
correlated degrees of mutually dependent nodes. We assume that both networks (A
and B) have the same number of nodes connected by the bidirectional
dependency links establishing a one-to-one correspondence between the nodes of
the two networks in a such a way that the mutually dependent nodes have the
same number of connectivity links, i.e. their degrees coincide. This implies
that both networks have the same degree distribution . We call such
networks correspondently coupled networks (CCN). We assume that the nodes in
each network are randomly connected. We define the mutually connected clusters
and the mutual giant component as in earlier works on randomly coupled
interdependent networks and assume that only the nodes which belong to the
mutual giant component remain functional. We assume that initially a
fraction of nodes are randomly removed due to an attack or failure and find
analytically, for an arbitrary , the fraction of nodes which
belong to the mutual giant component. We find that the system undergoes a
percolation transition at certain fraction which is always smaller than
the for randomly coupled networks with the same . We also find that
the system undergoes a first order transition at if has a finite
second moment. For the case of scale free networks with , the
transition becomes a second order transition. Moreover, if we find
as in percolation of a single network. For we find an exact
analytical expression for . Finally, we find that the robustness of CCN
increases with the broadness of their degree distribution.Comment: 18 pages, 3 figure
Derived Categories of Coherent Sheaves and Triangulated Categories of Singularities
In this paper we establish an equivalence between the category of graded
D-branes of type B in Landau-Ginzburg models with homogeneous superpotential W
and the triangulated category of singularities of the fiber of W over zero. The
main result is a theorem that shows that the graded triangulated category of
singularities of the cone over a projective variety is connected via a fully
faithful functor to the bounded derived category of coherent sheaves on the
base of the cone. This implies that the category of graded D-branes of type B
in Landau-Ginzburg models with homogeneous superpotential W is connected via a
fully faithful functor to the derived category of coherent sheaves on the
projective variety defined by the equation W=0.Comment: 26 pp., LaTe
Lorentz invariance of entanglement classes in multipartite systems
We analyze multipartite entanglement in systems of spin-1/2 particles from a
relativistic perspective. General conditions which have to be met for any
classification of multipartite entanglement to be Lorentz invariant are
derived, which contributes to a physical understanding of entanglement
classification. We show that quantum information in a relativistic setting
requires the partition of the Hilbert space into particles to be taken
seriously. Furthermore, we study exemplary cases and show how the spin and
momentum entanglement transforms relativistically in a multipartite setting.Comment: v2: 5 pages, 4 figures, minor changes to main body, journal
references update
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