328 research outputs found
Beta-fibrinogen gene promoter A-455 allele associated with poor longterm survival among 55-71 years old Caucasian women in Finnish stroke cohort
Peer reviewe
Revisiting the Rigidly Rotating Magnetosphere model for Ori E - II. Magnetic Doppler imaging, arbitrary field RRM, and light variability
The initial success of the Rigidly Rotating Magnetosphere (RRM) model
application to the B2Vp star sigma OriE by Townsend, Owocki & Groote (2005)
triggered a renewed era of observational monitoring of this archetypal object.
We utilize high-resolution spectropolarimetry and the magnetic Doppler imaging
(MDI) technique to simultaneously determine the magnetic configuration, which
is predominately dipolar, with a polar strength Bd = 7.3-7.8 kG and a smaller
non-axisymmetric quadrupolar contribution, as well as the surface distribution
of abundance of He, Fe, C, and Si. We describe a revised RRM model that now
accepts an arbitrary surface magnetic field configuration, with the field
topology from the MDI models used as input. The resulting synthetic Ha emission
and broadband photometric observations generally agree with observations,
however, several features are poorly fit. To explore the possibility of a
photospheric contribution to the observed photometric variability, the MDI
abundance maps were used to compute a synthetic photospheric light curve to
determine the effect of the surface inhomogeneities. Including the computed
photospheric brightness modulation fails to improve the agreement between the
observed and computed photometry. We conclude that the discrepancies cannot be
explained as an effect of inhomogeneous surface abundance. Analysis of the UV
light variability shows good agreement between observed variability and
computed light curves, supporting the accuracy of the photospheric light
variation calculation. We thus conclude that significant additional physics is
necessary for the RRM model to acceptably reproduce observations of not only
sigma Ori E, but also other similar stars with significant stellar
wind-magnetic field interactions.Comment: 16 pages, 17 figures, accepted for publication in MNRA
Chandra HETGS Multiphase Spectroscopy Of The Young Magnetic O Star Theta(1) Orionis C
We report on four Chandra grating observations of the oblique magnetic rotator theta(1) Ori C (O5.5 V), covering a wide range of viewing angles with respect to the star\u27s 1060 G dipole magnetic field. We employ line-width and centroid analyses to study the dynamics of the X-ray - emitting plasma in the circumstellar environment, as well as line-ratio diagnostics to constrain the spatial location, and global spectral modeling to constrain the temperature distribution and abundances of the very hot plasma. We investigate these diagnostics as a function of viewing angle and analyze them in conjunction with new MHD simulations of the magnetically channeled wind shock mechanism on theta(1) Ori C. This model fits all the data surprisingly well, predicting the temperature, luminosity, and occultation of the X-ray - emitting plasma with rotation phase
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&
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&
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
First detections of 610 MHz radio emission from hot magnetic stars
We have carried out a study of radio emission from a small sample of magnetic
O- and B-type stars using the Giant Metrewave Radio Telescope, with the goal of
investigating their magnetospheres at low frequencies. These are the lowest
frequency radio measurements ever obtained of hot magnetic stars. The
observations were taken at random rotational phases in the 1390 and the 610 MHz
bands. Out of the 8 stars, we detect five B-type stars in both the 1390 and the
610 MHz bands. The O-type stars were observed only in the 1390 MHz band, and no
detections were obtained. We explain this result as a consequence of free-free
absorption by the free-flowing stellar wind exterior to the closed
magnetosphere. We also study the variability of individual stars. One star - HD
133880 - exhibits remarkably strong and rapid variability of its low frequency
flux density. We discuss the possibility of this emission being coherent
emission as reported for CU Vir by Trigilio et al. (2000).Comment: 9 pages, 4 figures, 4 tables, submitted to MNRA
Activity and Magnetic Field Structure of the Sun-Like Planet Hosting Star HD 1237
We analyse the magnetic activity characteristics of the planet hosting
Sun-like star, HD 1237, using HARPS spectro-polarimetric time-series data. We
find evidence of rotational modulation of the magnetic longitudinal field
measurements consistent with our ZDI analysis, with a period of 7 days. We
investigate the effect of customising the LSD mask to the line depths of the
observed spectrum and find that it has a minimal effect on shape of the
extracted Stokes V profile but does result in a small increase in the S/N
( 7%). We find that using a Milne-Eddington solution to describe the
local line profile provides a better fit to the LSD profiles in this slowly
rotating star, which also impacts the recovered ZDI field distribution. We also
introduce a fit-stopping criterion based on the information content (entropy)
of the ZDI maps solution set. The recovered magnetic field maps show a strong
(+90 G) ring-like azimuthal field distribution and a complex radial field
dominating at mid latitudes (45 degrees). Similar magnetic field maps are
recovered from data acquired five months apart. Future work will investigate
how this surface magnetic field distribution impacts the coronal magnetic field
and extended environment around this planet-hosting star.Comment: Accepted for publication in A&
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