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ABSTRACT: Aims. During the first 40 s after their birth, proto-neutron stars are expected to be subject to at least two types of instability. The first one, the convective instability, is excited in the inner regions, where the entropy gradient produces a Rayleigh-type convection. The second one, the neutron-finger instability, is instead excited in the outer layers where the lepton gradients are large. Both instabilities involve convective motions and hence can trigger dynamo actions that may be responsible for the large magnetic fields in neutron stars and magnetars. However, because they have rather different mean turbulent velocities, they are also likely to give rise to different types of dynamo.
Methods. We have solved the mean-field induction equation in a simplified one-dimensional model of both the convective and the neutron-finger instability zones. Although very idealized, the model includes the nonlinearities introduced by the feedback processes that tend to saturate the growth of the magnetic field (α-quenching) and suppress its turbulent diffusion (η-quenching). The possibility
of a dynamo action is studied within a dynamical model of turbulent diffusivity where the boundary of the unstable zone is allowed to move. A large number of numerical simulations have been performed in which the relevant parameters, such as the spin-period, the strength of the differential rotation, the intensity of the initial magnetic field, and the extent of the neutron finger instability zone, have been suitably varied.
Results. We show that the dynamo action can also be operative within a dynamical model of turbulent diffusivity and that the amplification of the magnetic field can still be very effective. Furthermore, we confirm the existence of a critical spin-period, below which the dynamo is always excited independently of the degree of differential rotation, and whose value is related to the size of the neutron-finger instability zone. We provide a relation for the intensity of the final field as a function of the spin of the star and of its
differential rotation.
Conclusions. Although they were obtained by using a toy model, we expect that our results are able to capture the qualitative and asymptotic behaviour of a mean-field dynamo action developing in the neutron-finger instability zone. Overall, we find that such a dynamo is very efficient in producing magnetic fields well above equipartition, and thus that it could represent a possible explanation
for the large surface magnetic fields observed in neutron stars.
Astronomy and Astrophysics 01/2008; 479:167-176. · 4.59 Impact Factor
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ABSTRACT: High-resolution spectroscopic observations of the rapidly rotating delta Scuti star gamma Bootis have been carried out on 2005, over 6 consecutive nights, in order to search for line-profile variability. Time series, consisting of flux measurements at each wavelength bin across the TiII 4571.917 A line profile as a function of time, have been Fourier analyzed. The results confirm the early detection reported by Kennelly et al. (1992) of a dominant periodic component at frequency 21.28 c/d in the observer's frame, probably due to a high azimuthal order sectorial mode. Moreover, we found other periodicities at 5.06 c/d, 12.09 c/d, probably present but not secure, and at 11.70 c/d and 18.09 c/d, uncertain. The latter frequency, if present, should be identifiable as another high azimuthal order sectorial mode and three additional terms, probably due to low-l modes, as proved by the analysis of the first three moments of the line. Owing to the short time baseline and the one-site temporal sampling we consider our results only preliminary but encouraging for a more extensive multisite campaign. A refinement of the atmospheric physical parameters of the star has been obtained from our spectroscopic data and adopted for preliminary computations of evolutionary models of gamma Bootis. Comment: 9 pages, 7 figures, to be published in MNRAS
08/2007;
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ABSTRACT: The F5 IV-V star Procyon A (aCMi) was observed in January 2001 by means of the high resolution spectrograph SARG operating with the TNG 3.5m Italian telescope (Telescopio Nazionale Galileo) at Canary Islands, exploiting the iodine cell technique. The time-series of about 950 spectra carried out during 6 observation nights and a preliminary data analysis were presented in Claudi et al. 2005. These measurements showed a significant excess of power between 0.5 and 1.5 mHz, with ~ 1 m/s peak amplitude. Here we present a more detailed analysis of the time-series, based on both radial velocity and line equivalent width analyses. From the power spectrum we found a typical p-mode frequency comb-like structure, identified with a good margin of certainty 11 frequencies in the interval 0.5-1400 mHz of modes with l=0,1,2 and 7< n < 22, and determined large and small frequency separations, Dn = 55.90 \pm 0.08 muHz and dnu_02=7.1 \pm 1.3 muHz, respectively. The mean amplitude per mode (l=0,1) at peak power results to be 0.45 \pm 0.07 m/s, twice larger than the solar one, and the mode lifetime 2 \pm 0.4 d, that indicates a non-coherent, stochastic source of mode excitation. Line equivalent width measurements do not show a significant excess of power in the examined spectral region but allowed us to infer an upper limit to the granulation noise. Comment: 10 pages, 15 figures, 4 tables. Accepted for publication in A&A
12/2006;
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ABSTRACT: The differential rotation of the F5V-IV star Procyon A is computed for a class of models which are consistent with recent astrometric and asteroseismic data. The rotation pattern is determined by solving the Reynolds equation for motion, including the convective energy transport, where the latter is anisotropic owing to the Coriolis force action which produces a horizontal temperature gradient between the poles and the equator. All the models show a decrease of the rotation rate with increasing radius and solar-type isorotation surfaces with the equator rotating faster than the poles, the horizontal rotational shear being much smaller for models with a less extended convective envelope. The meridional flow circulation can be either clockwise or counter-clockwise, and in some cases a double latitudinal cell appears. The rotational splittings are calculated for low degree $p$-modes with $l=1, m=1$ and $l=2, m=1,2$, and it is shown that, for modes with $m=1$, the stronger is the horizontal differential rotation shear the weaker the effect on the average rotational splitting expected, whilst the opposite happens for the mode with $m=2$. On the basis of the present study, a resolution of $10 \rm nHz$ in individual oscillation frequencies seems to be necessary to test the different dynamical behaviour of the proposed models, that appears barely achievable even in the forthcoming space missions. However, the average over several observed splittings could produce the required accuracy.
11/2006;
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ABSTRACT: 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. Comment: 4 pages, 5 figures, accepted to be published in A&A Letters
11/2004;
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ABSTRACT: 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 Comment: 3 pages, 4 figures. to appear on A&A
12/2002;
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ABSTRACT: 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 $\delta\nu_{{\ell,n}}=\nu_{{\ell,n}}-\nu_{{\ell+2,n-1}}$, for spherical harmonic degrees $\ell = 0,1$ and radial order $n \gg \ell$.
05/2002;
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ABSTRACT: 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 $\Gamma_1$ computed from both OPAL and MHD EOS leads to a more reliable theoretical modelling of the innermost layers of the Sun.
08/2001;
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ABSTRACT: 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+ \nu_e)2H and 7Be(p,\gamma)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.
03/1999;
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ABSTRACT: Mechanisms which produce a mixing in the solar core, based on the
original idea of the ``solar spoon'' (Dilke & Gough
1972Natur.240..262D), for explaining the neutrino problem have recently
been ruled out by Bahcall & Kumar (1993ApJ...409L..73B). We
therefore revisit the original work of Rosenbluth & Bahcall
(1973ApJ...184....9R) on the nonspherical thermal instabilities in the
solar core, introducing some improvements. The authors found that their
solar model was stable against these perturbations and they concluded
that the turbulence in the solar core appeared unlikely to be the
explanation of the lack of solar neutrinos. Our analysis is motived by
the fact that the updated standard solar models are sensibly different
from those used in the 70's, and therefore instabilities of the kind
postulated by Rosenbluth & Bahcall might be excited. However our
results fully confirm the previous ones that the present Sun is stable
against nonradial thermal disturbances, at least in the linear analysis,
therefore no solutions for growing thermal modes are possible. The
confirmation of the stability of the solar core against the mixing and
the impressive agreement of the standard solar models with helioseismic
data indicate that the present neutrino problem is not astrophysical in
origin but it relies upon non standard neutrino properties.
Astronomy and Astrophysics 05/1997; 322:340-346. · 4.59 Impact Factor
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ABSTRACT: The rotation of the Sun's core, below 0.3Rsun_, is inferred
from two independent new results. The first is based on the recent
oblateness measurements carried out by the Solar Disk Sextant (SDS)
instrument outside the Earth's atmosphere, and the second on the very
accurate measurements of rotational splittings of the lowest degree
acoustic modes, carried out in the framework of the helioseismic network
IRIS. By using the theory of slowly rotating stars applied to a solar
standard model, we deduce a set of rotational laws for the innermost
layers, which are consistent with both the measured oblateness value and
the results of the inversion of helioseismic data. The SDS and IRIS
results indicate that the Sun's central regions rotate at a rate in
between 1.5 and 2 times the surface equatorial angular velocity. As a
result of our analysis, we deduce a quadrupole moment J_2_=2.22x10^-7^,
which implies an advance of Mercury's perihelion of 42.98arcsec/c, in
agreement with the theory of General Relativity and the measurements of
Mercury's orbit by means of planetary radar ranging. However, very
recent results obtained by the helioseismic network BISON indicate that
core rotation is even slower than the polar surface rotation and
therefore imply a completely different scenario than that proposed here.
If we assume the intermediate solution of rigid body rotation, an
alternate source of the oblateness may be attributed to a magnetic field
of the order of 10^5^Gauss in the interior of the Sun.
Astronomy and Astrophysics 09/1996; 314:940-946. · 4.59 Impact Factor
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ABSTRACT: We use the horizontal momentum balance equation to infer the strength of the meridional circulation (MC) and Reynolds stresses (RS) at the sun's surface from the observed properties of the differential rotation (DR). Both MC and RS are important for maintaining the equatorial acceleration. The results indicate that the average value of MC is about 1.1 m/s, with circulation directed towards the poles in both the hemispheres, and the average value of RS is 3.6 10 exp 7 sq cm2/s, with transport of angular momentum directed towards the equator in both hemispheres, this latter in good agreement with observations. With the above values of MC and RS, we integrate the momentum equation in time, starting from a state of rigid rotation, to investigate the competitive role of MC and RS in producing the presently observed average DR. Our results show that DR is consistent with observations only if both MC and RS have opposite effects, with the strengths given above, in order to balance the viscous torque.
08/1992;
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ABSTRACT: The frequencies of some 400 identified solar p-modes of degree 0 ≤ l
≤ 14 have been compared with the corresponding eigenfrequencies
deduced from two standard solar models in order to investigate on the
frequency difference δν between observation and theory as a
function of frequency ν and degree l. The behaviour of
δν(ν, l) permits to have information on the correctness of
the internal structure of the Sun. The results of this preliminary
analysis show that errors in the computed solar structure mainly arise
from the treatment of the surface layers, though small errors in the
innermost layers cannot be ruled out. A subsequent comparison between
the eigenfrequencies of the two models clarifies the method of this
analysis and points out that few per cent changes in sound speed in the
deep interior affect the eigenfrequencies by few μHz.
Astronomy and Astrophysics 06/1988; 200:213-217. · 4.59 Impact Factor
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ABSTRACT: Observed radial velocity amplitudes are used to determine the energies
of the identified solar gravity modes. The energies of some modes are of
the same order of magnitude of the total energy of the convective
motions in the solar envelope. Therefore the authors argue that this
fact rules out the possibility of direct or indirect excitation of these
modes of convection. After having reviewed some possibilities for a
driving mechanism acting in the solar core, they suggest that the most
plausible driving mechanism is the effect of the magnetic torque caused
by the presence of a global magnetic field and mild turbulence in the
core.
Astronomy and Astrophysics 09/1985; 151:47-51. · 4.59 Impact Factor
