arXiv subjects

A. Bonanno

Publications and source records attributed to A. Bonanno.

At least 73 records · Page 4Linked to original sources

arXiv2005

Protoneutron star dynamos and pulsar magnetism

We have investigated the turbulent mean-field dynamo action in protoneutron stars that are subject to convective and neutron finger instabilities during the early evolutionary phase. While the first one develops mostly in the inner regions of the star, the second one is favored in the outer regions, where the Rossby number is much smaller and a mean-field dynamo action is more efficient. By solving the mean-field induction equation we have computed the critical spin period below which no dynamo action is possible and found it to be $\sim 1$ s for a wide range of stellar models and for both axisymmetric and non-axisymmetric magnetic fields. Because this critical period is substantially longer than the characteristic spin period of very young pulsars, we expect that a mean-field dynamo will be effective for most protoneutron stars. The saturation dipole field estimated by making use of the model of ``global'' quenching fits well the pulsar magnetic fields inferred from the spin-down data. Apart from the large scale magnetic field, our model predicts also a generation of small scale fields which are typically stronger than the poloidal field and can survive during the lifetime of pulsars. Extremely rapidly rotating protoneutron stars ($P \sim 1$ ms) may have the dipole field $\sim (3-6) \times 10^{14}$ G.

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arXiv2004

Asteroseismology of Procyon A with SARG at TNG

We present high precision radial velocity measurements on the F5 IV star alpha CMi obtained by the SARG spectrograph at TNG (Telescopio Nazionale Galileo) exploiting the iodine cell technique. The time series of about 950 spectra of Procyon A taken during 6 observation nights are affected by an individual error of 1.3 m/s. Thanks to the iodine cell technique, the spectrograph contribution to the Doppler shift measurement error is quite negligible and our error is dominated by the photon statistics Brown et al 1994. An excess of power between 0.5 and 1.5 mHz, detected also by Martic et al. 2004 has been found. We determined a large separation frequency Delta nu0 = 56\pm 2 microHz, consistent with both theoretical estimates Chaboyer et al. 1999 and previous observations Martic et al. 2004.

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arXiv2004

Asteroseismology of sdB stars with FUSE

The hot subdwarf B (sdB) stars form an homogeneous group populating an extension of the horizontal branch (HB) in the ($T_{\rm eff}$--$\log g$) diagram towards temperatures up to 40,000 K. The recent discovery that many of them are multimode pulsators has triggered a large observational and theoretical effort. We discuss the possibility of performing space-based asteroseismology with {\em FUSE}, and we demonstrate that periodic luminosity variations are already detectable in archival TTAG data of sdB stars. In particular, we report on the {\em FUSE} observation of the pulsator PF 1219+534, which shows the presence of periodic variations at 6.9 mHz and 7.8 mHz, consistent with those reported from ground--based observations.

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arXiv2004

Proper Time Flow Equation for Gravity

We analyze a proper time renormalization group equation for Quantum Einstein Gravity in the Einstein-Hilbert truncation and compare its predictions to those of the conceptually different exact renormalization group equation of the effective average action. We employ a smooth infrared regulator of a special type which is known to give rise to extremely precise critical exponents in scalar theories. We find perfect consistency between the proper time and the average action renormalization group equations. In particular the proper time equation, too, predicts the existence of a non-Gaussian fixed point as it is necessary for the conjectured nonperturbative renormalizability of Quantum Einstein Gravity.

hep-th↗

arXiv2004

Spontaneous Symmetry Breaking and Proper-Time Flow Equations

We discuss the phenomenon of spontaneous symmetry breaking by means of a class of non-perturbative renormalization group flow equations which employ a regulating smearing function in the proper-time integration. We show, both analytically and numerically, that the convexity property of the renormalized local potential is obtained by means of the integration of arbitrarily low momenta in the flow equation. Hybrid Monte Carlo simulations are performed to compare the lattice Effective Potential with the numerical solution of the renormalization group flow equation. We find very good agreement both in the strong and in the weak coupling regime.

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arXiv2003

Confronting the IR Fixed Point Cosmology with High Redshift Observations

We use high-redshift type Ia supernova and compact radio source data in order to test the infrared (IR) fixed point model of the late Universe which was proposed recently. It describes a cosmology with a time dependent cosmological constant and Newton constant whose dynamics arises from an underlying renormalization group flow near an IR-attractive fixed point. Without any finetuning or quintessence field it yields $Ω_{\rm M}=Ω_Λ=1/2$. Its characteristic $t^{4/3}$-dependence of the scale factor leads to a distance-redshift relation whose predictions are compared both to the supernova and to the radio source data. According to the $χ^2$ test, the fixed point model reproduces the data at least as well as the best-fit (Friedmann-Robertson-Walker) standard cosmology.Furthermore, we extend the original fixed point model by assuming that the fixed point epoch is preceded by an era with constant $G$ and $Λ$. By means of a Monte Carlo simulation we show that the data expected from the forthcoming SNAP satellite mission could detect the transition to the fixed point regime provided it took place at a redshift of less than about 0.5.

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arXiv2003

A Class of Renormalization Group Invariant Scalar Field Cosmologies

We present a class of scalar field cosmologies with a dynamically evolving Newton parameter $G$ and cosmological term $Λ$. In particular, we discuss a class of solutions which are consistent with a renormalization group scaling for $G$ and $Λ$ near a fixed point. Moreover, we propose a modified action for gravity which includes the effective running of $G$ and $Λ$ near the fixed point. A proper understanding of the associated variational problem is obtained upon considering the four-dimensional gradient of the Newton parameter.

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arXiv2003

Cosmological Perturbations in Renormalization Group Derived Cosmologies

A linear cosmological perturbation theory of an almost homogeneous and isotropic perfect fluid Universe with dynamically evolving Newton constant $G$ and cosmological constant $Λ$ is presented. A gauge-invariant formalism is developed by means of the covariant approach, and the acoustic propagation equations governing the evolution of the comoving fractional spatial gradients of the matter density, $G$, and $Λ$ are thus obtained. Explicit solutions are discussed in cosmologies where both $G$ and $Λ$ vary according to renormalization group equations in the vicinity of a fixed point.

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arXiv2002

PG 1613+426: a new sdB pulsator

We report the detection of short period oscillations in the hot subdwarf B (sdB) star PG 1613+426 from time-series photometry carried out with the 91-cm Cassegrain telescope of the Catania Astrophysical Observatory. This star, which is brighter than the average of the presently known sdB pulsators, with B = 14.14 mag, has $T_{\rm eff}=34 400 {\rm K}$ and $\log g = 5.97$, its position is near the hot end of the sdB instability strip, and it is a pulsator with a well observed peak in the power spectrum at $144.18\pm 0.06 \rm s$. This star seems to be well suited for high precision measurements, which could detect a possible multi-mode pulsation behaviour

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arXiv2002

Cosmology with Self-Adjusting Vacuum Energy Density from a Renormalization Group Fixed Point

Cosmologies with a time dependent Newton constant and cosmological constant are investigated. The scale dependence of $G$ and $Λ$ is governed by a set of renormalization group equations which is coupled to Einstein's equation in a consistent way. The existence of an infrared attractive renormalization group fixed point is postulated, and the cosmological implications of this assumption are explored. It turns out that in the late Universe the vacuum energy density is automatically adjusted so as to equal precisely the matter energy density, and that the deceleration parameter approaches $q = -1/4$. This scenario might explain the data from recent observations of high redshift type Ia Supernovae and the cosmic microwave background radiation without introducing a quintessence field.

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arXiv2002

The age of the Sun and the relativistic corrections in the EOS

We show that the inclusion of special relativistic corrections in the revised OPAL and MHD equations of state has a significant impact on the helioseismic determination of the solar age. Models with relativistic corrections included lead to a reduction of about $0.05 - 0.08 \gi $ with respect to those obtained with the old OPAL or MHD EOS. Our best-fit value is $t_\mathrm{seis} = (4.57 \pm 0.11) \gi $ which is in remarkably good agreement with the meteoritic value for the solar age. We argue that the inclusion of relativistic corrections is important for probing the evolutionary state of a star by means of the small frequency separations $δν_{\ell,n}=ν_{\ell,n}-ν_{\ell+2,n-1}$, for spherical harmonic degrees $\ell = 0,1$ and radial order $n \gg \ell$.

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arXiv2002

Parity properties of an advection-dominated solar $α^2\Om$-dynamo

We have developed a high-precision code which solves the kinematic dynamo problem both for given rotation law and meridional flow in the case of a low eddy diffusivity of the order of $10^{11}$ cm$^2$/s known from the sunspot decay. All our models work with an \alf-effect which is positive (negative) in the northern (southern) hemisphere. It is concentrated in radial layers located either at the top or at the bottom of the convection zone. We have also considered an \alf-effect uniformly distributed in all the convection zone. In the present paper the main attention is focused on i) the parity of the solution, ii) the form of the butterfly diagram and iii) the phase relation of the resulting field components. If the helioseismologically derived internal solar rotation law is considered, a model without meridional flow of high magnetic Reynolds number (corresponding to low eddy diffusivity) fails in all the three issues in comparison with the observations. However, a meridional flow with equatorial drift at the bottom of the convection zone of few meters by second can indeed enforce the equatorward migration of the toroidal magnetic field belts similar to the observed butterfly diagram but, the solution has only a dipolar parity if the (positive) \alf-effect is located at the base of the convection zone rather than at the top. We can, therefore, confirm the main results of a similar study by Dikpati & Gilman (2001).

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arXiv2001

Relativistic effects in the solar EOS

We study the sensitivity of the sound speed to relativistic corrections of the equation of state (EOS) in the standard solar model by means of a helioseismic forward analysis. We use the latest GOLF/SOHO data for $\ell = 0,1,2,3$ modes to confirm that the inclusion of the relativistic corrections to the adiabatic exponent $Γ_1$ computed from both OPAL and MHD EOS leads to a more reliable theoretical modelling of the innermost layers of the Sun.

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arXiv2001

Cosmology of the Planck Era from a Renormalization Group for Quantum Gravity

Homogeneous and isotropic cosmologies of the Planck era before the classical Einstein equations become valid are studied taking quantum gravitational effects into account. The cosmological evolution equations are renormalization group improved by including the scale dependence of Newton's constant and of the cosmological constant as it is given by the flow equation of the effective average action for gravity. It is argued that the Planck regime can be treated reliably in this framework because gravity is found to become asymptotically free at short distances. The epoch immediately after the initial singularity of the Universe is described by an attractor solution of the improved equations which is a direct manifestation of an ultraviolet attractive renormalization group fixed point. It is shown that quantum gravity effects in the very early Universe might provide a resolution to the horizon and flatness problem of standard cosmology, and could generate a scale-free spectrum of primordial density fluctuations.

hep-th↗

arXiv2000

Towards an accurate determination of the critical exponents with the Renormalization Group flow equations

The determination of the critical exponents by means of the Exact Renormalizion Group approach is still a topic of debate. The general flow equation is by construction scheme independent, but the use of the truncated derivative expansion generates a model dependence in the determination of the universal quantities. We derive new nonperturbative flow equations for the one-component, $Z_2$ symmetric scalar field to the next-to-leading order of the derivative expansion by means of a class of proper time regulators. The critical exponents $η$, $ν$ and $ω$ for the Wilson-Fisher fixed point are computed by numerical integration of the flow equations, without resorting to polynomial truncations. We show that by reducing the width of the cut-off employed, the critical exponents become rapidly insensitive to the cut-off width and their values are in good agreement with the results of entirely different approaches.

hep-th↗

arXiv2000

Chiral symmetry breaking in the Wegner-Houghton approach

The Wegner-Houghton formulation of the exact renormalization group evolution equation is used in order to study the chiral symmetry breaking of the linear sigma model coupled to an isospin doublet of quarks. A numerical investigation for a particular truncation of the equation which includes the scalar field renormalization function is presented.

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arXiv2000

Renormalization group improved black hole spacetimes

We study the quantum gravitational effects in spherically symmetric black hole spacetimes. The effective quantum spacetime felt by a point-like test mass is constructed by ``renormalization group improving'' the Schwarzschild metric. The key ingredient is the running Newton constant which is obtained from the exact evolution equation for the effective average action. The conformal structure of the quantum spacetime depends on its ADM-mass M and it is similar to that of the classical Reissner-Nordstrom black hole. For M larger than, equal to, and smaller than a certain critical mass $M_{\rm cr}$ the spacetime has two, one and no horizon(s), respectively. Its Hawking temperature, specific heat capacity and entropy are computed as a function of M. It is argued that the black hole evaporation stops when M approaches $M_{\rm cr}$ which is of the order of the Planck mass. In this manner a ``cold'' soliton-like remnant with the near-horizon geometry of $AdS_2\times S^2$ is formed. As a consequence of the quantum effects, the classical singularity at r=0 is either removed completely or it is at least much milder than classically; in the first case the quantum spacetime has a smooth de Sitter core which would be in accord with the cosmic censorship hypothesis even if $M<M_{\rm cr}$.

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arXiv1999

Signature of the efficiency of solar nuclear reactions in the neutrino experiments

In the framework of the neutrino oscillation scenario, we discuss the influence of the uncertainty on the efficiency of the neutrino emitting reactions 1H(p,e+ ν_e)2H and 7Be(p,γ)8B for the neutrino oscillation parameters. We consider solar models with zero-energy astrophysical S-factors S_11 and S_17 varied within nuclear physics uncertainties, and we test them by means of helioseismic data. We then analyse the neutrino mixing parameters and recoil electron spectra for the presently operating neutrino experiments and we predict the results which can be obtained from the recoil electron spectra in SNO and Borexino experiments. We suggest that it should be possible to determine tight bounds on S_17 from the results of the future neutrino experiment, in the case of matter-enhanced oscillations of active neutrinos.

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