Article

Detection of a weak surface magnetic field on Sirius A: are all tepid stars magnetic ?

Astronomy and Astrophysics (Impact Factor: 5.08). 06/2011; 532. DOI: 10.1051/0004-6361/201117573
Source: arXiv

ABSTRACT We aim at a highly sensitive search for weak magnetic fields in main sequence
stars of intermediate mass, by scanning classes of stars with no previously
reported magnetic members. After detecting a weak magnetic field on the normal,
rapidly rotating A-type star Vega, we concentrate here on the bright star
Sirius A, taken as a prototypical, chemically peculiar, moderately rotating Am
star. We employed the NARVAL and ESPaDOnS high-resolution spectropolarimeters
to collect 442 circularly polarized spectra, complemented by 60 linearly
polarized spectra. Using a list of about 1,100 photospheric spectral lines, we
computed a cross correlation line profile from every spectrum, leading to a
signal-to-noise ratio of up to 30,000 in the polarized profile. We report the
repeated detection of circularly polarized, highly asymmetric signatures in the
line profiles, interpreted as Zeeman signatures of a large-scale photospheric
magnetic field, with a line-of-sight component equal to $0.2 \pm 0.1$ G. This
is the first polarimetric detection of a surface magnetic field on an Am star.
Using rough estimates of the physical properties of the upper layers of Sirius
A, we suggest that a dynamo operating in the shallow convective envelope cannot
account for the field strength reported here. Together with the magnetic field
of Vega, this result confirms that a new class of magnetic objects exists among
non Ap/Bp intermediate-mass stars, and it may indicate that a significant
fraction of tepid stars are magnetic.

0 Bookmarks
 · 
62 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The interferometric observations of dwarf stars in the solar neighbourhood, combined with Hipparcos parallaxes provide very precise values of their linear diameters. In this paper, we report the direct measurement of the angular diameter of the bright star Sirius A with the VINCI/VLTI instrument. We obtain a uniform disk angular diameter of $\theta_{\rm UD} = 5.936 \pm 0.016$ mas in the $K$ band and a limb darkened value of $\theta_{\rm LD} = 6.039 \pm 0.019$ mas. In combination with the Hipparcos parallax of $379.22 \pm 1.58$ mas, this translates into a linear diameter of $1.711 \pm 0.013~D_{\odot}$. Using the VINCI/VLTI interferometric diameter and the published properties of Sirius A, we derive internal structure models corresponding to ages between $200$ and $250 \pm 12$ Myr. This range is defined mainly by the hypothesis on the mass of the star, the overshoot and the metallicity.
    Astronomy and Astrophysics 01/2003; · 5.08 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: New solar models are presented, which have been computed with the most recent physical inputs (nuclear reaction rates, equation of state, opacities, microscopic diffusion). Rotation-induced mixing has been introduced in a way which includes the feed-back effect of the μ-gradient induced by helium settling. A parametrization of the tachocline region below the convective zone has also been added in the computations. The sound velocities have been computed in the models and compared to the seismic Sun. Our best model is described in some detail. Besides the new physical inputs, the most important improvement concerns the computations of μ-gradients during the solar evolution and their influence in slowing down rotation-induced mixing. This process can explain why lithium is depleted in the present Sun while beryllium is not, and meanwhile why 3He has not increased at the solar surface for at least 3 Gyrs.
    Solar Physics 01/2004; 220(2):243-259. · 3.26 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We observe the slowly-rotating, active, single giant, EK Eri, to study and infer the nature of its magnetic field directly. We used the spectropolarimeter NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least Square Deconvolution method to create high signal-to-noise ratio Stokes V profiles. We fitted the Stokes V profiles with a model of the large-scale magnetic field. We studied the classical activity indicators, the CaII H and K lines, the CaII infrared triplet, and H\alpha line. We detected the Stokes V signal of EK Eri securely and measured the longitudinal magnetic field Bl for seven individual dates spanning 60% of the rotational period. The measured longitudinal magnetic field of EK Eri reached about 100 G and was as strong as fields observed in RSCVn or FK Com type stars: this was found to be extraordinary when compared with the weak fields observed at the surfaces of slowly-rotating MS stars or any single red giant previously observed with NARVAL. From our modeling, we infer that the mean surface magnetic field is about 270 G, and that the large scale magnetic field is dominated by a poloidal component. This is compatible with expectations for the descendant of a strongly magnetic Ap star. Comment: 8 pages, 6 figures. Accepted for publication in A&A
    Astronomy and Astrophysics 10/2008; · 5.08 Impact Factor

Full-text (2 Sources)

View
17 Downloads
Available from
Jun 1, 2014