1,675 research outputs found
Brownian motion of black holes in stellar systems with non-Maxwellian distribution for the stars field
A massive black hole at the center of a dense stellar system, such as a
globular cluster or a galactic nucleus, is subject to a random walk due
gravitational encounters with nearby stars. It behaves as a Brownian particle,
since it is much more massive than the surrounding stars and moves much more
slowly than they do. If the distribution function for the stellar velocities is
Maxwellian, there is a exact equipartition of kinetic energy between the black
hole and the stars in the stationary state. However, if the distribution
function deviates from a Maxwellian form, the strict equipartition cannot be
achieved. The deviation from equipartition is quantified in this work by
applying the Tsallis q-distribution for the stellar velocities in a
q-isothermal stellar system and in a generalized King model.Comment: Presented at XXVI Int. Astronomical Union General Assembly, Symp.
238, Prague, Czech Republic, Aug 21-25 200
Gravity with extra dimensions and dark matter interpretation: A straightforward approach
Any connection between dark matter and extra dimensions can be cognizably
evinced from the associated effective energy-momentum tensor. In order to
investigate and test such relationship, a higher dimensional spacetime endowed
with a factorizable general metric is regarded to derive a general expression
for the stress tensor -- from the Einstein-Hilbert action -- and to elicit the
effective gravitational potential. A particular construction for the case of
six dimensions is provided, and it is forthwith revealed that the missing mass
phenomenon may be explained, irrespective of the dark matter existence.
Moreover, the existence of extra dimensions in the universe accrues the
possibility of a straightforward mechanism for such explanation. A
configuration which density profile coincides with the Newtonian potential for
spiral galaxies is constructed, from a 4-dimensional isotropic metric plus
extra-dimensional components. A Miyamoto-Nagai \emph{ansatz} is used to solve
Einstein equations. The stable rotation curves associated to such system are
computed, in full compliance to the observational data, without fitting
techniques. The density profiles are reconstructed and compared to that ones
obtained from the Newtonian potential.Comment: 13 pages, 6 figure
Multi-messenger particles as a probe for UHECR luminosity
Very-high energy (GeV-TeV) gamma rays in the universe suggest the presence of
an accelerator in the source. Neutrinos and gamma rays are intriguing
astrophysical messengers. Multi-messenger particle emission produced by
interactions of cosmic rays with radiation fields and interstellar matter is a
probe of luminosity of sources of cosmic rays with EeV energies, known as
Ultra-High Energy Cosmic Rays (UHECRs). This work estimates the neutrino flux
and suggests that gamma-ray emission is primarily caused by cosmic-ray
interactions during propagation. As energy loss processes occur, secondary
fluxes are generated, primarily by pion decay. We provide UHECR luminosity of
galaxies from multi-messenger particles. These findings not only highlight the
potential of certain galaxies as sources of UHECR, but also underscore the
intricate interplay of various astrophysical processes within them. By
understanding the luminosity patterns and multi-messenger particle emissions,
we can gain valuable insights into the environmental conditions, acceleration
mechanisms, and other intrinsic properties that position these galaxies as
candidates for UHECR production.Comment: 10 pages, 4 figure
Características espectrais da radiação solar em Igarapé-Açu, PA: variáveis associadas a respostas biológicas.
São apresentados resultados preliminares da caracterização espectral da radiação solar na região nordeste do Estado do Pará, realizada mediante espectroradiômetro portátil (LI-1800, Li-Cor), no município de Igarapé-Açu, PA. São abordadas as seguintes variáveis, que interferem de modo diferenciado em processos de crescimento e desenvolvimento em plantas: radiação ultravioleta A (UV-A); radiação ativa ao pigmento azul (RAA); radiação ativa na fotossíntese (RAF); radiação no infra-vermelho próximo (IVP); e razão vermelho/vermelho extremo (V:Ve). Foi observado que, dentro de cada faixa espectral estudada, há pouca variabilidade ao longo do ano, em sua contribuição relativa à radiação total, sendo apenas observadas variações marcantes em seus valores absolutos. V:Ve exibiu valores crescentes acompanhando classes de irradiância total (IT), variando entre 1,28 (±0,03) para IT400 W m-2.Disponível o artigo completo em CD
Hydrogen sulfide inhibits preoptic prostaglandin E2 production during endotoxemia
AbstractHydrogen sulfide (H2S) is a gaseous neuromodulator endogenously produced in the brain by the enzyme cystathionine β-synthase (CBS). We tested the hypothesis that H2S acts within the anteroventral preoptic region of the hypothalamus (AVPO) modulating the production of prostaglandin (PG) E2 (the proximal mediator of fever) and cyclic AMP (cAMP). To this end, we recorded deep body temperature (Tb) of rats before and after pharmacological modulation of the CBS–H2S system combined or not with lipopolysaccharide (LPS) exposure, and measured the levels of H2S, cAMP, and PGE2 in the AVPO during systemic inflammation. Intracerebroventricular (icv) microinjection of aminooxyacetate (AOA, a CBS inhibitor; 100pmol) did not affect basal PGE2 production and Tb, but enhanced LPS-induced PGE2 production and fever, indicating that endogenous H2S plays an antipyretic role. In agreement, icv microinjection of a H2S donor (Na2S; 260nmol) reduced the LPS-induced PGE2 production and fever. Interestingly, we observed that the AVPO levels of H2S were decreased following the immunoinflammatory challenge. Furthermore, fever was associated with decreased levels of AVPO cAMP and increased levels of AVPO PGE2. The LPS-induced decreased levels of cAMP were reduced to a lesser extent by the H2S donor. The LPS-induced PGE2 production was potentiated by AOA (the CBS inhibitor) and inhibited by the H2S donor. Our data are consistent with the notion that the gaseous messenger H2S synthesis is downregulated during endotoxemia favoring PGE2 synthesis and lowering cAMP levels in the preoptic hypothalamus
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