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.

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