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

Astronomy and Astrophysics (Impact Factor: 4.38). 06/2011; 532. DOI: 10.1051/0004-6361/201117573
Source: arXiv


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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    ABSTRACT: We present the result of a highly sensitive spectropolarimetric study dedicated to intermediate mass stars. We report the detection of sub-gauss surface magnetic fields on the normal, rapidly-rotating A-type star Vega and on the moderately-rotating Am star Sirius A. These magnetic detections constitute the first evidence that tepid stars that do not belong to the class of Ap/Bp stars can also host magnetized photospheres, suggesting that a significant fraction of stars in this mass regime are magnetic. We present here the observational clues gathered so far to progress towards understanding the physical processes at the origin of this newly identified Vega-like magnetism.
    Full-text · Article · Dec 2011 · Astronomische Nachrichten
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    ABSTRACT: Strong magnetic fields play a crucial role in the removal of angular momentum from collapsing clouds and protostellar discs, and are necessary for the formation of disc winds as well as jets from the inner disc and, indeed, strong large-scale poloidal magnetic fields are observed in protostellar discs at all radii down to ∼10 R⊙. Nevertheless, by the time the star is visible virtually, all of the original magnetic flux has vanished. We explore mechanisms for removing this flux during the formation of the protostar once it is magnetically disconnected from the parent cloud, looking at both radiative and convective protostars. This includes a numerical investigation of buoyant magnetic field removal from convective stars. It is found that if the star goes through a fully convective phase, all remaining flux can easily be removed from the protostar, essentially on an Alfvén time-scale. If, on the other hand, the protostar has no fully convective phase, then some flux can be retained, the quantity depending on the net magnetic helicity, which is probably quite small. Only some fraction of this flux is visible at the stellar surface. We also look at how the same mechanisms could prevent flux from accreting on to the star at all, meaning that mass would only accrete as fast as it is able to slip past the flux.
    Preview · Article · Jan 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Weak magnetic fields have recently been detected in Vega and Sirius. Here, we explore the possibility that these fields are the remnants of some field inherited or created during or shortly after star formation and are still evolving dynamically as we observe them. The time-scale of this evolution is given in terms of the Alfvén time-scale and the rotation frequency by τevol ∼ τ2A Ω, which is then comparable to the age of the star. According to this theory, all intermediate- and high-mass stars should contain fields of at least the strength found so far in Vega and Sirius. Faster rotators are expected to have stronger magnetic fields. Stars may experience an increase in surface field strength during their early main sequence, but for most of their lives field strength will decrease slowly. The length scale of the magnetic structure on the surface may be small in very young stars but should quickly increase to at least very approximately a fifth of the stellar radius. The field strength may be higher at the poles than at the equator.
    Full-text · Article · Jan 2012 · Monthly Notices of the Royal Astronomical Society
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