126 research outputs found
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
An infrared diagnostic for magnetism in hot stars
Magnetospheric observational proxies are used for indirect detection of
magnetic fields in hot stars in the X-ray, UV, optical, and radio wavelength
ranges. To determine the viability of infrared (IR) hydrogen recombination
lines as a magnetic diagnostic for these stars, we have obtained low-resolution
(R~1200), near-IR spectra of the known magnetic B2V stars HR 5907 and HR 7355,
taken with the Ohio State Infrared Imager/Spectrometer (OSIRIS) attached to the
4.1m Southern Astrophysical Research (SOAR) Telescope. Both stars show definite
variable emission features in IR hydrogen lines of the Brackett series, with
similar properties as those found in optical spectra, including the derived
location of the detected magnetospheric plasma. These features also have the
added advantage of a lowered contribution of stellar flux at these wavelengths,
making circumstellar material more easily detectable. IR diagnostics will be
useful for the future study of magnetic hot stars, to detect and analyze
lower-density environments, and to detect magnetic candidates in areas obscured
from UV and optical observations, increasing the number of known magnetic stars
to determine basic formation properties and investigate the origin of their
magnetic fields.Comment: 4 pages, accepted for publication in A&
Long-term spectropolarimetric monitoring of the cool supergiant Betelgeuse
We report on a long-term monitoring of the cool supergiant Betelgeuse, using
the NARVAL and ESPaDOnS high-resolution spectropolarimeters, respectively
installed at Telescope Bernard Lyot (Pic du Midi Observatory, France) and at
the Canada-France-Hawaii Telescope (Mauna Kea Observatory, Hawaii). The data
set, constituted of circularly polarized (Stokes V) and intensity (Stokes I)
spectra, was collected between 2010 and 2012. We investigate here the temporal
evolution of magnetic field, convection and temperature at photospheric level,
using simultaneous measurements of the longitudinal magnetic field component,
the core emission of the Ca II infrared triplet, the line-depth ratio of
selected photospheric lines and the radial velocity of the star.Comment: Proceedings of the Betelgeuse Workshop, Paris, 26-29 Nov 201
The constant magnetic field of xi 1 CMa: geometry or slow rotation?
We report recent observations of the sharp-lined magnetic beta Cep pulsator
xi 1 CMa (= HD 46328). The longitudinal magnetic field of this star is detected
consistently, but it is not observed to vary strongly, during nearly 5 years of
observation. In this poster we evaluate whether the nearly constant
longitudinal field is due to intrinsically slow rotation, or rather if the
stellar or magnetic geometry is responsible
Discovery of new magnetic early-B stars within the MiMeS HARPSpol survey
To understand the origin of the magnetic fields in massive stars as well as
their impact on stellar internal structure, evolution, and circumstellar
environment, within the MiMeS project, we searched for magnetic objects among a
large sample of massive stars, and build a sub-sample for in-depth follow-up
studies required to test the models and theories of fossil field origins,
magnetic wind confinement and magnetospheric properties, and magnetic star
evolution.
We obtained high-resolution spectropolarimetric observations of a large
number of OB stars thanks to three large programs that have been allocated on
the high-resolution spectropolarimeters ESPaDOnS, Narval, and the polarimetric
module HARPSpol of the HARPS spectrograph. We report here on the methods and
first analysis of the HARPSpol magnetic detections. We identified the magnetic
stars using a multi-line analysis technique. Then, when possible, we monitored
the new discoveries to derive their rotation periods, which are critical for
follow-up and magnetic mapping studies. We also performed a first-look analysis
of their spectra and identified obvious spectral anomalies (e.g., abundance
peculiarities, Halpha emission), which are also of interest for future studies.
In this paper, we focus on eight of the 11 stars in which we discovered or
confirmed a magnetic field from the HARPSpol LP sample (the remaining three
were published in a previous paper). Seven of the stars were detected in
early-type Bp stars, while the last star was detected in the Ap companion of a
normal early B-type star. We report obvious spectral and multiplicity
properties, as well as our measurements of their longitudinal field strengths,
and their rotation periods when we are able to derive them. We also discuss the
presence or absence of Halpha emission with respect to the theory of
centrifugally-supported magnetospheres. (Abriged)Comment: 19 pages, 8 figures, accepted for publication in A&
First HARPSpol discoveries of magnetic fields in massive stars
In the framework of the Magnetism in Massive Stars (MiMeS) project, a
HARPSpol Large Program at the 3.6m-ESO telescope has recently started to
collect high-resolution spectropolarimetric data of a large number of Southern
massive OB stars in the field of the Galaxy and in many young clusters and
associations. In this Letter, we report on the first discoveries of magnetic
fields in two massive stars with HARPSpol - HD 130807 and HD 122451, and
confirm the presence of a magnetic field at the surface of HD 105382 that was
previously observed with a low spectral resolution device. The longitudinal
magnetic field measurements are strongly varying for HD 130807 from -100
G to 700 G. Those of HD 122451 and HD 105382 are less variable with
values ranging from -40 to -80 G, and from -300 to -600 G,
respectively. The discovery and confirmation of three new magnetic massive
stars, including at least two He-weak stars, is an important contribution to
one of the MiMeS objectives: the understanding of origin of magnetic fields in
massive stars and their impacts on stellar structure and evolution.Comment: 4 pages, 2 figures, accepted for publication in A&A Lette
Discovery of the magnetic field of the B1/B2V star \sigma Lupi
In our search for new magnetic massive stars we use the strongest indirect
indicator of a magnetic field in B stars, which is periodic variability of UV
stellar wind lines occurring in a velocity range symmetric around zero. Our aim
is to obtain follow-up spectropolarimetry to search for a magnetic field in
magnetic candidate stars. We quantify UV wind line variability, and analyse its
time behaviour. The B1/B2V star sigma Lup emerged as a new magnetic candidate
star. AAT spectropolarimetric measurements with SEMPOL were obtained.
The stellar wind line variations of sigma Lup are similar to what is known in
magnetic B stars, but no periodicity could be determined. We detected a
longitudinal magnetic field with varying strength and amplitude of about 100 G
with error bars of typically 20 G, which supports an oblique magnetic-rotator
configuration. The equivalent width variations of the UV lines, the magnetic
and the optical line variations are consistent with the well-known photometric
period of 3.02 days, which we identify with the rotation period of the star.
Additional observations with ESPaDOnS at CFHT strongly confirmed this
discovery, and allowed to determine a precise magnetic period. Further analysis
revealed that Lupi is a helium-strong star, with an enhanced nitrogen
abundance and an underabundance of carbon, and has a spotted surface.
We conclude that sigma Lup is a magnetic oblique rotator, and is a He-strong
star. It is the 4th B star for which a magnetic field is discovered from
studying only its wind variability. Like in the other magnetic B stars the wind
emission originates in the magnetic equator, with maximum emission occurring
when a magnetic pole points towards the Earth. The 3.02 d magnetic rotation
period is consistent with the photometric period, with maximum light
corresponding to maximum magnetic field. A full paper will be submitted to A&A.Comment: 4 pages, 5 figures, to appear in proceedings with AIP. Stellar
polarimetry: From birth to death, Eds. Jennifer Hoffman, Barb Whitney, and
Jon Bjorkma
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