32,463 research outputs found
Gauge Symmetry and Consistent Spin-Two Theories
We study Lagrangians with the minimal amount of gauge symmetry required to
propagate spin-two particles without ghosts or tachyons. In general, these
Lagrangians also have a scalar mode in their spectrum. We find that, in two
cases, the symmetry can be enhanced to a larger group: the whole group of
diffeomorphisms or a enhancement involving a Weyl symmetry. We consider the
non-linear completions of these theories. The intuitive completions yield the
usual scalar-tensor theories except for the pure spin-two cases, which
correspond to two inequivalent Lagrangians giving rise to Einstein's equations.
A more constructive self-consistent approach yields a background dependent
Lagrangian.Comment: 7 pages, proceedings of IRGAC'06; typo correcte
Monitoring luminous yellow massive stars in M33: new yellow hypergiant candidates
The evolution of massive stars surviving the red supergiant (RSG) stage
remains unexplored due to the rarity of such objects. The yellow hypergiants
(YHGs) appear to be the warm counterparts of post-RSG classes located near the
Humphreys-Davidson upper luminosity limit, which are characterized by
atmospheric instability and high mass-loss rates. We aim to increase the number
of YHGs in M33 and thus to contribute to a better understanding of the
pre-supernova evolution of massive stars. Optical spectroscopy of five
dust-enshrouded YSGs selected from mid-IR criteria was obtained with the goal
of detecting evidence of extensive atmospheres. We also analyzed BVI photometry
for 21 of the most luminous YSGs in M33 to identify changes in the spectral
type. To explore the properties of circumstellar dust, we performed SED-fitting
of multi-band photometry of the 21 YSGs. We find three luminous YSGs in our
sample to be YHG candidates, as they are surrounded by hot dust and are
enshrouded within extended, cold dusty envelopes. Our spectroscopy of star 2
shows emission of more than one H component, as well as emission of
CaII, implying an extended atmospheric structure. In addition, the long-term
monitoring of the star reveals a dimming in the visual light curve of amplitude
larger than 0.5 mag that caused an apparent drop in the temperature that
exceeded 500 K. We suggest the observed variability to be analogous to that of
the Galactic YHG Cas. Five less luminous YSGs are suggested as post-RSG
candidates showing evidence of hot or/and cool dust emission. We demonstrate
that mid-IR photometry, combined with optical spectroscopy and time-series
photometry, provide a robust method for identifying candidate YHGs. Future
discovery of YHGs in Local Group galaxies is critical for the study of the late
evolution of intermediate-mass massive stars.Comment: 24 pages, 12 figures, 7 Tables. A&A in pres
Revised Pulsar Spindown
We address the issue of electromagnetic pulsar spindown by combining our
experience from the two limiting idealized cases which have been studied in
great extent in the past: that of an aligned rotator where ideal MHD conditions
apply, and that of a misaligned rotator in vacuum. We construct a spindown
formula that takes into account the misalignment of the magnetic and rotation
axes, and the magnetospheric particle acceleration gaps. We show that near the
death line aligned rotators spin down much slower than orthogonal ones. In
order to test this approach, we use a simple Monte Carlo method to simulate the
evolution of pulsars and find a good fit to the observed pulsar distribution in
the P-Pdot diagram without invoking magnetic field decay. Our model may also
account for individual pulsars spinning down with braking index n < 3, by
allowing the corotating part of the magnetosphere to end inside the light
cylinder. We discuss the role of magnetic reconnection in determining the
pulsar braking index. We show, however, that n ~ 3 remains a good approximation
for the pulsar population as a whole. Moreover, we predict that pulsars near
the death line have braking index values n > 3, and that the older pulsar
population has preferentially smaller magnetic inclination angles. We discuss
possible signatures of such alignment in the existing pulsar data.Comment: 8 pages, 7 figures; accepted to Ap
The Equivalence Principle in the Non-baryonic Regime
We consider the empirical validity of the equivalence principle for
non-baryonic matter. Working in the context of the TH\epsilon\mu formalism, we
evaluate the constraints experiments place on parameters associated with
violation of the equivalence principle (EVPs) over as wide a sector of the
standard model as possible. Specific examples include new parameter constraints
which arise from torsion balance experiments, gravitational red shift,
variation of the fine structure constant, time-dilation measurements, and
matter/antimatter experiments. We find several new bounds on EVPs in the
leptonic and kaon sectors.Comment: 22 pages, late
Biodiversidade de espécies arbóreas do sub-bosque de reflorestamentos no Brasil
Este estudo teve como objetivo avaliar diversidade de espécies arbustivo-arbóreas que ocorrem no sub-bosque de plantios comerciais de Eucalyptus spp e Pinus spp no Brasil por meio da revisão de 40 trabalhos científicos. Ao todo foram identificadas 79 famílias, 294 gêneros e 761 espécies nativas. As cinco espécies de maior ocorrência foram: Casearia sylvestris (Sw.); Copaifera langsdorffii (Desf.); Zanthoxylum rhoifolium (Lam.); Siparuna guianensis (Aubl.); Piptadenia gonoacantha (Mart.) Macbr. De todas as espécies relacionadas somente 20% são pioneiras. Das espécies encontradas nesse ambiente, 233 ocorrem somente no bioma mata atlântica e 108 somente em fisionomias do bioma cerrado. As demais espécies ocorrem em ambos os biomas. A forma de dispersão de sementes predominante é a zoocoria, onde 62% das espécies apresentam essa característica. Das espécies arbóreas encontradas nos sub-bosque de Eucalyptus cerca de 130 espécies não foram encontrados em outros dois estudos que revisaram centenas de trabalhos de espécies arbóreas com ocorrência em mata ciliar e vegetação de cerrado em grande parte do Brasil. Desenvolvendo-se sob a copa do plantio de Eucalyptus spp foram encontradas 38 espécies com algum grau de ameaça de extinção. Apesar da carência de trabalhos e do pequeno tamanho dos fragmentos estudados, a biodiversidade de espécies nativas arbóreas e arbustivas encontradas é relevante e qualifica esse tipo de fragmento florestal como útil para o surgimento, desenvolvimento e manutenção de espécies florestais nativas
Dielectric branes in non-trivial backgrounds
We present a procedure to evaluate the action for dielectric branes in
non-trivial backgrounds. These backgrounds must be capable to be taken into a
Kaluza-Klein form, with some non-zero wrapping factor. We derive the way this
wrapping factor is gauged away. Examples of this are AdS_5xS^5 and
AdS_3xS^3xT^4, where we perform the construction of different stable systems,
which stability relies in its dielectric character.Comment: 14 pages, published versio
A role for TSPO in mitochondrial Ca2+ homeostasis and redox stress signaling
The 18 kDa translocator protein TSPO localizes on the outer mitochondrial membrane (OMM). Systematically overexpressed at sites of neuroinflammation it is adopted as a biomarker of brain conditions. TSPO inhibits the autophagic removal of mitochondria by limiting PARK2-mediated mitochondrial ubiquitination via a peri-organelle accumulation of reactive oxygen species (ROS). Here we describe that TSPO deregulates mitochondrial Ca2+ signaling leading to a parallel increase in the cytosolic Ca2+ pools that activate the Ca2+-dependent NADPH oxidase (NOX) thereby increasing ROS. The inhibition of mitochondrial Ca2+ uptake by TSPO is a consequence of the phosphorylation of the voltage-dependent anion channel (VDAC1) by the protein kinase A (PKA), which is recruited to the mitochondria, in complex with the Acyl-CoA binding domain containing 3 (ACBD3). Notably, the neurotransmitter glutamate, which contributes neuronal toxicity in age-dependent conditions, triggers this TSPO-dependent mechanism of cell signaling leading to cellular demise. TSPO is therefore proposed as a novel OMM-based pathway to control intracellular Ca2+ dynamics and redox transients in neuronal cytotoxicity
Quantum corrections to Higher-Dimensional Theories
This is a non-technical summary of the subtleties of quantum corrections on
extra-dimensional theories: should one first renormalize and then mode expand,
or first expand in four-dimensional modes and then renormalize?Comment: 9 pages, based on a talk at IRGAC 2006, Barcelon
Long-term Dynamics of the Electron-nuclear Spin System of a Semiconductor Quantum Dot
A quasi-classical theoretical description of polarization and relaxation of
nuclear spins in a quantum dot with one resident electron is developed for
arbitrary mechanisms of electron spin polarization. The dependence of the
electron-nuclear spin dynamics on the correlation time of electron
spin precession, with frequency , in the nuclear hyperfine field is
analyzed. It is demonstrated that the highest nuclear polarization is achieved
for a correlation time close to the period of electron spin precession in the
nuclear field. For these and larger correlation times, the indirect hyperfine
field, which acts on nuclear spins, also reaches a maximum. This maximum is of
the order of the dipole-dipole magnetic field that nuclei create on each other.
This value is non-zero even if the average electron polarization vanishes. It
is shown that the transition from short correlation time to
does not affect the general structure of the equation for nuclear spin
temperature and nuclear polarization in the Knight field, but changes the
values of parameters, which now become functions of . For
correlation times larger than the precession time of nuclei in the electron
hyperfine field, it is found that three thermodynamic potentials (,
, ) characterize the polarized electron-nuclear spin
system. The values of these potentials are calculated assuming a sharp
transition from short to long correlation times, and the relaxation mechanisms
of these potentials are discussed. The relaxation of the nuclear spin potential
is simulated numerically showing that high nuclear polarization decreases
relaxation rate.Comment: RevTeX 4, 12 pages, 9 figure
First Results from Viper: Detection of Small-Scale Anisotropy at 40 GHZ
Results of a search for small-scale anisotropy in the cosmic microwave
background (CMB) are presented. Observations were made at the South Pole using
the Viper telescope, with a .26 degree (FWHM) beam and a passband centered at
40 GHz. Anisotropy band-power measurements in bands centered at l = 108, 173,
237, 263, 422 and 589 are reported. Statistically significant anisotropy is
detected in all bands.Comment: 5 pages, 4 figures, uses emulateapj.sty, submitted to ApJ Letter
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