12,345 research outputs found
Thin-shell wormholes: Linearization stability
The class of spherically-symmetric thin-shell wormholes provides a
particularly elegant collection of exemplars for the study of traversable
Lorentzian wormholes. In the present paper we consider linearized (spherically
symmetric) perturbations around some assumed static solution of the Einstein
field equations. This permits us to relate stability issues to the (linearized)
equation of state of the exotic matter which is located at the wormhole throat.Comment: 4 pages; ReV_TeX 3.0; one postscript figur
Bound Modes in Dielectric Microcavities
We demonstrate how exactly bound cavity modes can be realized in dielectric
structures other than 3d photonic crystals. For a microcavity consisting of
crossed anisotropic layers, we derive the cavity resonance frequencies, and
spontaneous emission rates. For a dielectric structure with dissipative loss
and central layer with gain, the beta factor of direct spontaneous emission
into a cavity mode and the laser threshold is calculated.Comment: 5 pages, 3 figure
Smelling out predators is innate in birds
The role of olfaction for predation risk assessment remains barely explored in birds, although predator chemical cues could be useful in predator detection under low visibility conditions for many bird species. We examine whether Great Tits Parus major are able to use the odour of mustelids to assess predation risk when selecting cavities for roosting. We analysed whether the response to predator chemical cues is innate and assessed whether the antipredatory response is associated with exploratory behaviour, a proxy for the personality of birds. In a choice experiment in aviaries, we offered naĂŻve adult Great Tits of known personality two nest-boxes, one control and one experimental. The experimental nest-box had the odour of a mustelid predator or a strong new odour without biological significance, the control nest-box contained no odour. When one of the cavities contained the odour of a predator, birds avoided the use of either of the two offered nest-boxes, whereas there was no avoidance of boxes when one of the nest-boxes contained a control odour. There was no relationship with exploratory behaviour. We show that the ability to use the chemical cues of predators is innate in birds, but individual differences in the response to predator chemical cues cannot be explained by the personality of the bird.
Chemical tracers of episodic accretion in low-mass protostars
Aims: Accretion rates in low-mass protostars can be highly variable in time.
Each accretion burst is accompanied by a temporary increase in luminosity,
heating up the circumstellar envelope and altering the chemical composition of
the gas and dust. This paper aims to study such chemical effects and discusses
the feasibility of using molecular spectroscopy as a tracer of episodic
accretion rates and timescales.
Methods: We simulate a strong accretion burst in a diverse sample of 25
spherical envelope models by increasing the luminosity to 100 times the
observed value. Using a comprehensive gas-grain network, we follow the chemical
evolution during the burst and for up to 10^5 yr after the system returns to
quiescence. The resulting abundance profiles are fed into a line radiative
transfer code to simulate rotational spectra of C18O, HCO+, H13CO+, and N2H+ at
a series of time steps. We compare these spectra to observations taken from the
literature and to previously unpublished data of HCO+ and N2H+ 6-5 from the
Herschel Space Observatory.
Results: The bursts are strong enough to evaporate CO throughout the
envelope, which in turn enhances the abundance of HCO+ and reduces that of
N2H+. After the burst, it takes 10^3-10^4 yr for CO to refreeze and for HCO+
and N2H+ to return to normal. The chemical effects of the burst remain visible
in the rotational spectra for as long as 10^5 yr after the burst has ended,
highlighting the importance of considering luminosity variations when analyzing
molecular line observations in protostars. The spherical models are currently
not accurate enough to derive robust timescales from single-dish observations.
As follow-up work, we suggest that the models be calibrated against spatially
resolved observations in order to identify the best tracers to be used for
statistically significant source samples.Comment: Accepted by A&A; 12 pages, 7 figure
Static and symmetric wormholes respecting energy conditions in Einstein-Gauss-Bonnet gravity
Properties of -dimensional static wormhole solutions are
investigated in Einstein-Gauss-Bonnet gravity with or without a cosmological
constant . We assume that the spacetime has symmetries corresponding
to the isometries of an -dimensional maximally symmetric space with the
sectional curvature . It is also assumed that the metric is at
least and the -dimensional maximally symmetric subspace is
compact. Depending on the existence or absence of the general relativistic
limit , solutions are classified into general relativistic (GR)
and non-GR branches, respectively, where is the Gauss-Bonnet coupling
constant. We show that a wormhole throat respecting the dominant energy
condition coincides with a branch surface in the GR branch, otherwise the null
energy condition is violated there. In the non-GR branch, it is shown that
there is no wormhole solution for . For the matter field with
zero tangential pressure, it is also shown in the non-GR branch with
and that the dominant energy condition holds at the
wormhole throat if the radius of the throat satisfies some inequality. In the
vacuum case, a fine-tuning of the coupling constants is shown to be necessary
and the radius of a wormhole throat is fixed. Explicit wormhole solutions
respecting the energy conditions in the whole spacetime are obtained in the
vacuum and dust cases with and .Comment: 10 pages, 2 tables; v2, typos corrected, references added; v3,
interpretation of the solution for n=5 in section IV corrected; v4, a very
final version to appear in Physical Review
Cylindrically symmetric wormholes
This paper discusses traversable wormholes that differ slightly but
significantly from those of the Morris-Thorne type under the assumption of
cylindrical symmetry. The throat is a piecewise smooth cylindrical surface
resulting in a shape function that is not differentiable at some value. It is
proposed that the regular derivative be replaced by a one-sided derivative at
this value. The resulting wormhole geometry satisfies the weak energy
condition.Comment: Supplied missing figures; 15 pages AMSTe
Cylindrical thin-shell wormholes and energy conditions
We prove the impossibility of cylindrical thin-shell wormholes supported by
matter satisfying the energy conditions everywhere, under reasonable
assumptions about the asymptotic behaviour of the - in general different -
metrics at each side of the throat. In particular, we reproduce for singular
sources previous results corresponding to flat and conical asymptotics, and
extend them to a more general asymptotic behaviour. Besides, we establish
necessary conditions for the possibility of non exotic cylindrical thin-shell
wormholes.Comment: 9 pages; slightly improved version of the article accepted in Int. J.
Mod. Phys.
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