Article

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

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(Impact Factor: 4.48). 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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Available from: Pascal Petit, Jun 11, 2015
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##### Article: Magnetorotational instability in decretion disks of critically rotating stars and the outer structure of Be and Be/X-ray disks
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Astronomy and Astrophysics 10/2014; DOI:10.1051/0004-6361/201424867 · 4.48 Impact Factor
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##### Article: B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type stars β CMa and ϵ CMa
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ABSTRACT: Within the context of the "B fields in OB stars (BOB)" collaboration, we used the HARPSpol spectropolarimeter to observe the early B-type stars beta CMa (HD44743; B1 II/III) and epsilon CMa (HD52089; B1.5 II). For both stars, we consistently detected the signature of a weak (<30 G in absolute value) longitudinal magnetic field. We determined the physical parameters of both stars and characterise their X-ray spectrum. For beta CMa, our mode identification analysis led to determining a rotation period of 13.6+/-1.2 days and of an inclination angle of the rotation axis of 57.6+/-1.7 degrees, with respect to the line of sight. On the basis of these measurements and assuming a dipolar field geometry, we derived a best fitting obliquity of ~22 degrees and a dipolar magnetic field strength (Bd) of ~100 G (60<Bd<230 G within 1 sigma), below what is typically found for other magnetic massive stars. For epsilon CMa we could only determine a lower limit on the dipolar magnetic field strength of 13 G. For this star, we determine that the rotation period ranges between 1.3 and 24 days. Both stars are expected to have a dynamical magnetosphere. We also conclude that both stars are most likely core hydrogen burning and that they have spent more than 2/3 of their main sequence lifetime. A histogram of the distribution of the dipolar magnetic field strength for the magnetic massive stars known to date does not show the magnetic field "desert" observed instead for intermediate-mass stars. The biases involved in the detection of (weak) magnetic fields in massive stars with the currently available instrumentation and techniques imply that weak fields might be more common than currently observed. Our results show that, if present, even relatively weak magnetic fields are detectable in massive stars and that more observational effort is probably still needed to properly access the magnetic field incidence.
Astronomy and Astrophysics 11/2014; 574. DOI:10.1051/0004-6361/201424986 · 4.48 Impact Factor
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##### Article: Weak magnetic fields of intermediate-mass stars
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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.
Astronomische Nachrichten 12/2011; 332(9-10). DOI:10.1002/asna.201111608 · 1.12 Impact Factor