arXiv subjects

J. A. de Freitas Pacheco

Publications and source records attributed to J. A. de Freitas Pacheco.

44 records · Page 3Linked to original sources

arXiv1998

Do Soft Gamma Repeaters Emit Gravitational Waves?

Soft gamma repeaters are identified as highly magnetized (B$\approx 10^{14}$ Gauss) neutron stars. Magnetic stresses induce tectonic activity, and field annihilation in faults is the ultimate energy source for the observed $γ$-ray emission. As a consequence of the crustal cracking, the stored elastic energy is converted into high frequency (kHz) shear waves, that excite nonradial oscillation modes damped by gravitational wave emission. This class of objects should certainly be considered as potential sources of gravitational waves that could be detected by the present planned interferometric antennas like VIRGO or LIGO.

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arXiv1997

Gravitational Wave Emission from Galactic Radio Pulsars

We consider in this work continuous gravitational wave (GW) emission from non-axisymmetric radio pulsars. We treat in some detail the observational issues related to the known radio pulsar sample with the aim of unveiling the actual number of sources contributing to GW, which are likely to be the main contributors of GWs. It is shown that the operation of spheroidal GW detectors and full-size interferometers could detect this component of the radiation or impose useful limits on the effective oblateness of young radio pulsars.

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arXiv1996

Equilibrium configurations for quark-diquark stars and the problem of Her X-1 mass

We report new calculations of the physical properties of a quark-diquark plasma. A vacuum contribution is taken into account and is responsible for the appearance of a stable state at zero pressure and at a baryon density of about 2.2 times the nuclear matter density in this model. The resulting equation of state was used to integrate numerically the Tolman-Oppenheimer-Volkoff equations. The mass-radius relationship has been derived from a series of equilibrium configurations constituted by a mixture of quarks and diquarks. These stellar models, which are representative of a whole class, may be helpful to understand the possible compactness of the X-ray source Her X-1 and related objects.

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arXiv1995

The role of thermal instabilities on the theoretical mass function of MACHOs

We analyze the primordial protogalactic gas to investigate the role of thermal instabilities in the formation of condensations that could contribute to the MACHO mass function. We find that the previrialized protogalaxy could have undergone a thermally unstable regime which defines a peak in mass condensations starting around $0.1 \, M_{\odot}$ after being reprocessed by the environment which may be identified as giving origin to the MACHOs. Effects of magnetic fields on this instability-driven MACHOs scenario are also addressed.

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arXiv1995

Metal-rich globular clusters of the Galaxy: morphology of the helium-burning "clump," and the determination of relative ages through the "$ΔV$" method

Morphological aspects of the $V, \bv$ diagram of metal-rich globular clusters are analyzed, on the basis of stellar evolution models for horizontal-branch (HB) and red giant branch stars. These have been incorporated into detailed synthetic HB models, the influence of differential reddening upon which is also discussed. The synthetic HB models are found to be very clumpy in the colour-magnitude diagram, though more extended structures occasionally result due to the effect of evolution away from the zero-age HB. In particular, "sloped" clumps may be naturally expected for sufficiently high helium abundances. Differential reddening is found to have a smaller influence than evolution itself upon the morphology of the clump. The synthetic HB models are also used to study the age difference between metal-rich clusters on the basis of the "$ΔV$" method. We emphasize the importance of knowing in advance the relative abundances of helium and metals in order to estimate this age difference quantitatively. We present chemical evolution models that serve to illustrate the effect, and show that, for two clusters of identical [Fe/H] and $ΔV$, the one enriched predominantly by ejecta from supernovae type II explosions will appear younger than the other which also suffered enrichment from supernovae type Ia ejecta. The implied locations of the red giant branch feature known as the "bump" are also discussed on the basis of recent observational and theoretical results. Our corresponding predictions should be checked against accurate observational data obtained with the Hubble Space Telescope.

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arXiv1995

The Helium-Core Mass at the Helium Flash in Low-Mass Red Giant Stars: Observations and Theory

The method developed by Raffelt (1990a,b,c) to estimate a possible increase in the standard values of the helium-core mass at the tip of the red giant branch, $\Mc$, from properties of the color-magnitude diagrams of Galactic globular clusters is employed. In the present study, we revise and update Raffelt's database, including also constraints from RR Lyrae pulsation, and find that a small increase, by $Δ\Mc \approx 0.01\pm 0.015 \Msun$, cannot be ruled out with the present data and evolutionary models. Our new upper limits on $Δ\Mc$ are less restrictive than those previously obtained by Raffelt, as are the corresponding constraints on novel astroparticle phenomena which may affect the evolution of low-mass red giants. Within the estimated uncertainties, however, the standard values of $\Mc$ may also be acceptable. Raffelt's method does not rule out a low envelope helium abundance in globular cluster giants, though again the standard values are compatible with the available constraints. The influence of a non-solar ratio for the $α$-capture elements upon these results is also investigated. In addition, we review several aspects of the input physics employed in red giant stellar evolutionary calculations, with the purpose of evaluating possible sources of uncertainty in the value of the helium-core mass at the helium flash that is obtained from evolutionary computations, such as: heat conduction by electrons in the degenerate core; Coulomb effects upon the Equation of State; triple-$α$ reaction rates and screening factors; neutrino emission rates, both standard and enhanced by a possible non-zero magnetic moment; stellar rotation; microscopic element diffusion; and energy losses by axions and Weakly Interacting Massive Particles.

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arXiv1995

Deformation of Rapidly Rotating Compact Stars

We have developed a numerical code to study the deformation ($\varepsilon = (I_{zz}-I_{xx})/I_{zz}$, where $I_{ii}$ are the moments of inertia) of neutron stars in rapidly rotation in a fully general relativistic calculation. We have found that the deformation is larger, depending on the angular velocity, than is generally assumed for gravitational wave estimations. Calculations were performed by employing the Bethe-Johnson I EOS (equation of state) and a new set of models by the Frankfurt group including $Λ$ hyperons for several choices of their coupling constants to ordinary nucleons. Possible implications for gravitational wave searches are briefly discussed.

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arXiv1994

Gravitational Waves from Wobbling Pulsars

The prospects for detection of gravitational waves from precessing pulsars have been considered by constructing fully relativistic rotating neutron star models and evaluating the expected wave amplitude $h$ from a galactic source. For a "typical" neutron matter equation of state and observed rotation rates, it is shown that moderate wobble angles may render an observable signal from a nearby source once the present generation of interferometric antennas becomes operative.

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