15,441 research outputs found
Causal Structure and Birefringence in Nonlinear Electrodynamics
We investigate the causal structure of general nonlinear electrodynamics and
determine which Lagrangians generate an effective metric conformal to
Minkowski. We also proof that there is only one analytic nonlinear
electrodynamics presenting no birefringence.Comment: 11 pages, no figure
Bopp-Podolsky black holes and the no-hair theorem
Bopp-Podolsky electrodynamics is generalized to curved space-times. The
equations of motion are written for the case of static spherically symmetric
black holes and their exterior solutions are analyzed using Bekenstein's
method. It is shown the solutions split-up into two parts, namely a
non-homogeneous (asymptotically massless) regime and a homogeneous
(asymptotically massive) sector which is null outside the event horizon. In
addition, in the simplest approach to Bopp-Podolsky black holes, the
non-homogeneous solutions are found to be Maxwell's solutions leading to a
Reissner-Nordstr\"om black hole. It is also demonstrated that the only exterior
solution consistent with the weak and null energy conditions is the Maxwell's
one. Thus, in light of energy conditions, it is concluded that only Maxwell
modes propagate outside the horizon and, therefore, the no-hair theorem is
satisfied in the case of Bopp-Podolsky fields in spherically symmetric
space-times.Comment: 9 pages, updated to match published versio
Bioprospecção de actinobactérias da rizosfera para produção de metabólitos bioativos.
Actinobactérias rizosféricas, têm mostrado grande potencial na produção de metabólitos bioativos, sendo o gênero das Streptomyces o mais bem estudado e com melhor potencial para produção destes compostos. Assim, o presente trabalho visou realizar a bioprospecção de actinobactérias rizosféricas e avaliar o potencial de seus metabólitos bioativos no controle de bactérias patogênicas. Foram isoladas linhagens de actinobactérias de plantas sadias de milho, cultivado em diferentes regiões edafoclimáticas do Estado de São Paulo. As linhagens foram avaliadas quanto ao potencial de controle de Bacillus megaterium, Escherichia coli, Klebsiella spp., Pseudomonnas aeruginosa e Sthaphylococcus aureus. Os isolados que se mostraram eficientes em testes de antagonismo foram utilizados em ensaios de antibiose para verificação do potencial do composto bioativo no controle destes patógenos. Foram isolados 60 actinobactérias, identificadas morfologicamente como sendo do gênero Streptomyces. Dentre estas, 31 inibiram pelo menos 1 dos 5 patógenos em testes de antagonismos. Nos ensaios de antibiose, foi verificada a eficiência dos metabólitos de 25 actinobactérias no controle dos patógenos testados
Effective interaction between molecules in the BEC regime of a superfluid Fermi gas
We investigate the effective interaction between Cooper-pair molecules in the
st rong-coupling BEC regime of a superfluid Fermi gas with a Feshbach
resonance. Our work uses a path integral formulation and a renormalization
group (RG) analy sis of fluctuations in a single-channel model. We show that a
physical cutoff en ergy originating from the finite molecular
binding energy is the key to understanding the interaction between molecules in
the BEC regime. Our work t hus clarifies recent results by showing that is a {\it ba re} molecular scattering length while is the low energy molecular scattering length
renormalized to include high-energy scat tering up to (here is the scattering length between Fermi atoms). We also include many-body
effects at finite temperatures. We find that is strongly dependent
on temperature, vanishing at , consistent with the earlier Bose gas
results of Bijlsma and Stoof.Comment: 10 pages, 3 figure
Search for associations containing young stars (SACY). V. Is multiplicity universal? Tight multiple systems
Context: Dynamically undisrupted, young populations of stars are crucial to
study the role of multiplicity in relation to star formation. Loose nearby
associations provide us with a great sample of close (150 pc) Pre-Main
Sequence (PMS) stars across the very important age range (5-70 Myr) to
conduct such research.
Aims: We characterize the short period multiplicity fraction of the SACY
(Search for Associations Containing Young stars) accounting for any
identifiable bias in our techniques and present the role of multiplicity
fractions of the SACY sample in the context of star formation.
Methods: Using the cross-correlation technique we identified double-lined
spectroscopic systems (SB2), in addition to this we computed Radial Velocity
(RV) values for our subsample of SACY targets using several epochs of FEROS and
UVES data. These values were used to revise the membership of each association
then combined with archival data to determine significant RV variations across
different data epochs characteristic of multiplicity; single-lined multiple
systems (SB1).
Results: We identified 7 new multiple systems (SB1s: 5, SB2s: 2). We find no
significant difference between the short period multiplicity fraction
() of the SACY sample and that of nearby star forming regions
(1-2 Myr) and the field (10%) both as a function of
age and as a function of primary mass, , in the ranges [1:200 day] and
[0.08 -].
Conclusions: Our results are consistent with the picture of universal star
formation, when compared to the field and nearby star forming regions (SFRs).
We comment on the implications of the relationship between increasing
multiplicity fraction with primary mass, within the close companion range, in
relation to star formation.Comment: 14 pages, 18 figures, published, A&A
http://dx.doi.org/10.1051/0004-6361/20142385
How can one probe Podolsky Electrodynamics?
We investigate the possibility of detecting the Podolsky generalized
electrodynamics constant . First we analyze an ion interferometry apparatus
proposed by B. Neyenhuis, et al (Phys. Rev. Lett. 99, (2007) 200401) who looked
for deviations from Coulomb's inverse-square law in the context of Proca model.
Our results show that this experiment has not enough precision for measurements
of . In order to set up bounds for we investigate the influence of
Podolsky's electrostatic potential on the ground state of the Hydrogen atom.
The value of the ground state energy of the Hydrogen atom requires Podolsky's
constant to be smaller than 5.6 fm, or in energy scales larger than 35.51 MeV.Comment: 12 pages, 2 figure
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