264 research outputs found
The Incidence of Magnetic Fields in Massive Stars: An Overview of the MiMeS Survey Component
With only a handful of known magnetic massive stars, there is a troubling
deficit in the scope of our knowledge of the influence of magnetic fields on
stellar evolution, and almost no empirical basis for understanding how fields
modify mass loss and rotation in massive stars. Most remarkably, there is still
no solid consensus regarding the origin physics of these fields - whether they
are fossil remnants, or produced by contemporaneous dynamos, or some
combination of these mechanisms. This article will present an overview of the
Survey Component of the MiMeS Large Programs, the primary goal of which is to
search for Zeeman signatures in the circular polarimetry of massive stars
(stars with spectral types B3 and hotter) that were previously unknown to host
any magnetic field. To date, the MiMeS collaboration has collected more than
550 high-resolution spectropolarimetric observations with ESPaDOnS and Narval
of nearly 170 different stars, from which we have discovered 14 new magnetic
stars.Comment: 7 pages (+1 for questions), 3 figures, to appear in proceedings of
Stellar polarimetry: From birth to deat
On the Stability of Non Force-Free Magnetic Equilibria in Stars
The existence of stable magnetic configurations in white dwarfs, neutron
stars and various non-convective stellar {regions} is now well recognized. It
has recently been shown numerically that various families of equilibria,
including axisymmetric mixed poloidal-toroidal configurations, are stable. Here
we test the stability of an analytically-derived non force-free magnetic
equilibrium, using three-dimensional magnetohydrodynamic simulations: the mixed
configuration is compared with the dynamical evolution of its purely poloidal
and purely toroidal components, both known to be unstable. The mixed
equilibrium shows no sign of instability under white noise perturbations. {This
configuration therefore provides a good description of magnetic equilibrium
topology inside non-convective stellar objects and will be useful to initialize
magneto-rotational transport in stellar evolution codes.Comment: Accepted for publication in ApJ
Search for magnetic fields in particle-accelerating colliding-wind binaries
Some colliding-wind massive binaries, called particle-accelerating
colliding-wind binaries (PACWB), exhibit synchrotron radio emission, which is
assumed to be generated by a stellar magnetic field. However, no measurement of
magnetic fields in these stars has ever been performed. We aim at quantifying
the possible stellar magnetic fields present in PACWB to provide constraints
for models. We gathered 21 high-resolution spectropolarimetric observations of
9 PACWB available in the ESPaDOnS, Narval and HarpsPol archives. We analysed
these observations with the Least Squares Deconvolution method. We separated
the binary spectral components when possible. No magnetic signature is detected
in any of the 9 PACWB stars and all longitudinal field measurements are
compatible with 0 G. We derived the upper field strength of a possible field
that could have remained hidden in the noise of the data. While the data are
not very constraining for some stars, for several stars we could derive an
upper limit of the polar field strength of the order of 200 G. We can therefore
exclude the presence of strong or moderate stellar magnetic fields in PACWB,
typical of the ones present in magnetic massive stars. Weak magnetic fields
could however be present in these objects. These observational results provide
the first quantitative constraints for future models of PACWB.Comment: Accepted in A&
Critical evaluation of magnetic field detections reported for pulsating B-type stars in the light of ESPaDOnS, Narval and reanalyzed FORS1/2 observations
Recent spectropolarimetric studies of 7 SPB and Cep stars have
suggested that photospheric magnetic fields are more common in B-type pulsators
than in the general population of B stars, suggesting a significant connection
between magnetic and pulsational phenomena. We present an analysis of new and
previously published spectropolarimetric observations of these stars. New
Stokes observations obtained with the high-resolution ESPaDOnS and Narval
instruments confirm the presence of a magnetic field in one of the stars
( Lup), but find no evidence of magnetism in 5 others. A re-analysis
of the published longitudinal field measurements obtained with the
low-resolution FORS1/2 spectropolarimeters finds that the measurements of all
stars show more scatter from zero than can be attributed to Gaussian noise,
suggesting the presence of a signal and/or systematic under-estimation of error
bars. Re-reduction and re-measurement of the FORS1/2 spectra from the ESO
archive demonstrates that small changes in reduction procedure lead to
substantial changes in the inferred longitudinal field, and substantially
reduces the number of field detections at the 3 level. Furthermore, we
find that the published periods are not unique solutions to the time series of
either the original or the revised FORS1/2 data. We conclude that the reported
field detections, proposed periods and field geometry models for Pyx,
15 CMa, 33 Eri and V1449 Aql are artefacts of the data analysis and reduction
procedures, and that magnetic fields at the reported strength are no more
common in SPB/ Cep stars than in the general population of B stars.Comment: 10 pages, 5 figures, accepted for publication in ApJ, 2012, typo
correcte
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