4,680 research outputs found
Asteroseismological study of massive ZZ Ceti stars with fully evolutionary models
We present the first asteroseismological study for 42 massive ZZ Ceti stars
based on a large set of fully evolutionary carbonoxygen core DA white dwarf
models characterized by a detailed and consistent chemical inner profile for
the core and the envelope. Our sample comprise all the ZZ Ceti stars with
spectroscopic stellar masses between 0.72 and known to date.
The asteroseismological analysis of a set of 42 stars gives the possibility to
study the ensemble properties of the massive pulsating white dwarf stars with
carbonoxygen cores, in particular the thickness of the hydrogen envelope and
the stellar mass. A significant fraction of stars in our sample have stellar
mass high enough as to crystallize at the effective temperatures of the ZZ Ceti
instability strip, which enables us to study the effects of crystallization on
the pulsation properties of these stars. Our results show that the phase
diagram presented in Horowitz et al. (2010) seems to be a good representation
of the crystallization process inside white dwarf stars, in agreement with the
results from white dwarf luminosity function in globular clusters.Comment: 58 pages, 11 figures, accepted in Ap
Dissipation and memory effects in pure glue deconfinement
We investigate the effects of dissipation in the deconfining transition for a
pure SU(2) gauge theory. Using an effective model for the order parameter, we
study its Langevin evolution numerically, and compare results from local
additive noise dynamics to those obtained considering an exponential non-local
kernel for early times.Comment: 4 pages, 2 figures, to appear in the proceedings of Strong and
Electroweak Matter (SEWM06), BNL, May 200
Hydration-induced anisotropic spin fluctuations in Na_{x}CoO_{2}\cdot1.3H_{2}O superconductor
We report ^{59}Co NMR studies in single crystals of cobalt oxide
superconductor Na_{0.42}CoO_{2}\cdot1.3H_{2}O (T_c=4.25K) and its parent
compound Na_{0.42}CoO_{2}. We find that both the magnitude and the temperature
(T) dependence of the Knight shifts are identical in the two compounds above
T_c. The spin-lattice relaxation rate (1/T_1) is also identical above T_0
\sim60 K for both compounds. Below T_0, the unhydrated sample is found to be a
non-correlated metal that well conforms to Fermi liquid theory, while spin
fluctuations develop in the superconductor. These results indicate that water
intercalation does not change the density of states but its primary role is to
bring about spin fluctuations. Our result shows that, in the hydrated
superconducting compound, the in-plane spin fluctuation around finite wave
vector is much stronger than that along the c-axis, which indicates that the
spin correlation is quasi-two-dimensional.Comment: 4 pages, 5 figure
The sdA problem - II. Photometric and Spectroscopic Follow-up
Subdwarf A star (sdA) is a spectral classification given to objects showing
H-rich spectra and sub-main sequence surface gravities, but effective
temperature lower than the zero-age horizontal branch. Their evolutionary
origin is an enigma. In this work, we discuss the results of follow-up
observations of selected sdAs. We obtained time resolved spectroscopy for 24
objects, and time-series photometry for another 19 objects. For two targets, we
report both spectroscopy and photometry observations. We confirm seven objects
to be new extremely-low mass white dwarfs (ELMs), one of which is a known
eclipsing star. We also find the eighth member of the pulsating ELM class.Comment: Accepted for publication in MNRAS. 19 pages, 30 figures, 6 table
From QCD lattice calculations to the equation of state of quark matter
We describe two-flavor QCD lattice data for the pressure at finite
temperature and zero chemical potential within a quasiparticle model. Relying
only on thermodynamic selfconsistency, the model is extended to nonzero
chemical potential. The results agree with lattice calculations in the region
of small chemical potential.Comment: 5 eps figure
Formation of ions by high energy photons
We calculate the electron energy spectrum of ionization by a high energy
photon, accompanied by creation of electron-positron pair. The total cross
section of the process is also obtained. The asymptotics of the cross section
does not depend on the photon energies. At the photon energies exceeding a
certain value this appeares to to be the dominant mechanism of
formation of the ions. The dependence of on the value of nuclear
charge is obtained. Our results are consistent with experimental data.Comment: 16 pages, 6 figure
Heating Strategies in Cellulose Pyrolysis as an Alternative For Targeting Energy Efficient Product Distribution
Energy generation and platform chemicals production from biomass are a potential route towards an oil-free economy. Pyrolysis is one of the key technologies for transforming biomass into both fuels and chemicals. However, pyrolysis is a complex and energy-intensive process, and optimizing the operation for reducing its energy requirements is critical for the design of competitive biorefineries. This work presents a model to describe cellulose pyrolysis based on mass, energy and momentum conservation of solid and gaseous species. Lumped and detailed kinetic models are used to investigate how heating conditions impact pyrolysis product distribution. The resulting complex system was solved using gPROMS. Results suggest that pyrolysis mainly occurs in the boundary of the modelled particles. The developed model presents flexibility to use lumped and detailed kinetic models and provided both a general perspective of the pyrolysis process and detailed information on product distribution. Using this model, the results show that an initial high heating rate, followed by a lower heating rate, could reduce energy requirements by 10 % without changing the product distribution. There is also a trade-off between the yield of high added-value products, such as levoglucosan, and the overall energy requirement
The Role of Fermions in Bubble Nucleation
We present a study of the role of fermions in the decay of metastable states
of a scalar field via bubble nucleation. We analyze both one and
three-dimensional systems by using a gradient expansion for the calculation of
the fermionic determinant. The results of the one-dimensional case are compared
to the exact results of previous work.Comment: 15 pages, revtex, 9 figure
Micobiota do solo de uma ĂĄrea de duna na restinga da Marambaia, Rio de Janeiro, RJ.
Este trabalho avaliou a micobiota do solo de uma duna nos terços inferior, mĂ©dio e superior de encosta, localizada na Restinga da Marambaia, RJ. A ĂĄrea de estudo com aproximadamente 500 m2 estĂĄ dividida em trĂȘs terços, com a vegetação variando de arbustiva a arbĂłrea. Foi delimitada uma gleba de dimensĂ”es de 50 x 50 m, onde foram abertas cinco minitrincheiras atĂ© a profundidade de 10 cm para coleta, sendo coletadas trĂȘs amostras compostas formadas a partir da reuniĂŁo de cinco amostras simples para anĂĄlise do solo e caracterização da micobiota. A fertilidade do solo indicou que o terço inferior apresentou maiores teores de alumĂnio e menores de nutrientes. As unidades formadoras de colĂŽnias de fungos variaram de 4,6 x 103 a 9 x 104 ufc.g-1. Quarenta e nove fungos, num total de 85 isolados, pertencentes a 13 gĂȘneros e 32 espĂ©cies foram identificados. Os gĂȘneros mais frequentes foram Penicillium, Aspergillus e Trichoderma. A maior riqueza ocorreu no terço mĂ©dio, porĂ©m uma espĂ©cie (Trichoderma pseudokoningi) foi comum entre as ĂĄreas. Este trabalho sugere que a Restinga da Marambaia apresenta grande diversidade fĂșngica
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