365 research outputs found
Stochastic excitation of gravity waves in rapidly rotating massive stars
Stochastic gravity waves have been recently detected and characterised in
stars thanks to space asteroseismology and they may play an important role in
the evolution of stellar angular momentum. In this context, the observational
study of the CoRoT hot Be star HD 51452 suggests a potentially strong impact of
rotation on stochastic excitation of gravito-inertial waves in rapidly rotating
stars. In this work, we present our results on the action of the Coriolis
acceleration on stochastic wave excitation by turbulent convection. We study
the change of efficiency of this mechanism as a function of the waves' Rossby
number and we demonstrate that the excitation presents two different regimes
for super-inertial and sub-inertial frequencies. Consequences for rapidly
rotating early-type stars and the transport of angular momentum in their
interiors are discussed.Comment: 2 pages, 1 figure, IAU 307 symposium proceedings, 'New windows on
massive stars: asteroseismology, interferometry, and spectropolarimetry',
Eds. G. Meynet, C. Georgy, J.H. Groh & Ph. Ste
Asteroseismology and spectropolarimetry: opening new windows on the internal dynamics of massive stars
In this article, we show how asteroseismology and spectropolarimetry allow to
probe dynamical processes in massive star interiors. First, we give a summary
of the state-of-the-art. Second, we recall the MHD mechanisms that take place
in massive stars. Next, we show how asteroseimology gives strong constraints on
the internal mixing and transport of angular momentum while spectropolarimetry
allows to unravel the role played by magnetic fields.Comment: 5 pages, 1 figure, IAU 307 symposium proceedings, New windows on
massive stars: asteroseismology, interferometry, and spectropolarimetry, Eds.
G. Meynet, C. Georgy, J. H. Groh, and P. Ste
Magnetic fields in early-type stars
For several decades we have been cognizant of the presence of magnetic fields
in early-type stars, but our understanding of their magnetic properties has
recently (over the last decade) expanded due to the new generation of
high-resolution spectropolarimeters (ESPaDOnS at CFHT, Narval at TBL, HARPSpol
at ESO). The most detailed surface magnetic field maps of intermediate-mass
stars have been obtained through Doppler imaging techniques, allowing us to
probe the small-scale structure of these stars. Thanks to the effort of large
programmes (e.g. the MiMeS project), we have, for the first time, addressed key
issues regarding our understanding of the magnetic properties of massive (M > 8
M_sun) stars, whose magnetic fields were only first detected about fifteen
years ago. In this proceedings article we review the spectropolarimetric
observations and statistics derived in recent years that have formed our
general understanding of stellar magnetism in early-type stars. We also discuss
how these observations have furthered our understanding of the interactions
between the magnetic field and stellar wind, as well as the consequences and
connections of this interaction with other observed phenomena.Comment: 8 pages, 2 figures. To appear in the proceedings of the IAUS 305 -
Polarimetry: From the sun to stars and stellar environment
Discovery of a very weak magnetic field on the Am star Alhena
Alhena ( Gem) was observed in the frame of the BRITE (BRIght Target
Explorer) spectropolarimetric survey, which gathers high resolution, high
signal-to-noise, high sensitivity, spectropolarimetric observations of all
stars brighter than V=4 to combine seismic and spectropolarimetric studies of
bright stars.
We present here the discovery of a very weak magnetic field \textbf{on} the
Am star Alhena, thanks to very high signal-to-noise spectropolarimetric data
obtained with Narval at T\'elescope Bernard Lyot (TBL). All previously studied
Am stars show the presence of ultra-weak (sub-Gauss) fields with Zeeman
signatures with an unexpected prominent positive lobe. However, Alhena presents
a slightly stronger (but still very weak, only a few Gauss) field with normal
Zeeman signatures with a positive and negative lobe, as found in stronger field
(hundreds or thousands of Gauss) stars. It is the first detection of a normal
magnetic signature in an Am star.
Alhena is thus a very interesting object, which might provide the clue to
understanding the peculiar shapes of the magnetic signatures of the other Am
stars.Comment: letter accepted by MNRA
Combining seismology and spectropolarimetry of hot stars
Asteroseismology and spectropolarimetry have allowed us to progress
significantly in our understanding of the physics of hot stars over the last
decade. It is now possible to combine these two techniques to learn even more
information about hot stars and constrain their models. While only a few
magnetic pulsating hot stars are known as of today and have been studied with
both seismology and spectropolarimetry, new opportunities - in particular
Kepler2 and BRITE - are emerging and will allow us to rapidly obtain new
combined results.Comment: proceedings of the IAU Symposium 307 held in Geneva in June 201
Discovery of two new bright magnetic B stars: i Car and Atlas
The BRITE (BRIght Target Explorer) constellation of nano-satellites performs
seismology of bright stars via high precision photometry. In this context, we
initiated a high resolution, high signal-to-noise, high sensitivity,
spectropolarimetric survey of all stars brighter than V=4. The goal of this
survey is to detect new bright magnetic stars and provide prime targets for
both detailed magnetic studies and asteroseismology with BRITE. Circularly
polarised spectra were acquired with Narval at TBL (France) and HarpsPol at ESO
in La Silla (Chile). We discovered two new magnetic B stars: the B3V star i Car
and the B8V component of the binary star Atlas. Each star was observed twice to
confirm the magnetic detections and check for variability. These bright
magnetic B stars are prime targets for asteroseismology and for flux-demanding
techniques, such as interferometry.Comment: accepted in MNRAS Letters, 5 pages, 3 figure
Static spectropolarimeter concept adapted to space conditions and wide spectrum constraints
The issues related to moving elements in space and instruments working in
broader wavelength ranges lead to a need for robust polarimeters, efficient on
a wide spectral domain, and adapted to space conditions. As part of the UVMag
consortium, created to develop spectropolarimetric UV facilities in space, such
as the Arago mission project, we present an innovative concept of static
spectropolarimetry. We studied a static and polychromatic method for
spectropolarimetry, applicable to stellar physics. Instead of modulating the
polarization information temporally, as usually done in spectropolarimeters,
the modulation is performed in a spatial direction, orthogonal to the spectral
one. Thanks to the proportionality between phase retardance imposed by a
birefringent material and its thickness, birefringent wedges can be used to
create this spatial modulation. The light is then spectrally cross-dispersed,
and a full-Stokes determination of the polarization over the whole spectrum can
be obtained with a single-shot measurement. The use of Magnesium Fluoride
wedges, for example, could lead to a compact, static polarimeter working at
wavelengths from 0.115 mm up to 7 mm. We present the theory and simulations of
this concept, as well as laboratory validation and a practical application to
Arago.Comment: Article accepted for publication in Applied Optics on 20 July 201
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