128 research outputs found
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
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 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&
Chandra HETGS Multi-Phase 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's 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.Comment: 52 pages, 14 figures (1 color), 6 tables. To appear in the
Astrophysical Journal, 1 August 2005, v628, issue 2. New version corrects
e-mail address, figure and table formatting problem
Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198478
Blue supergiant stars are known to display photometric and spectroscopic
variability that is suggested to be linked to stellar pulsations. Pulsational
activity in massive stars strongly depends on the star's evolutionary stage and
is assumed to be connected with mass-loss episodes, the appearance of
macroturbulent line broadening, and the formation of clumps in the wind. To
investigate a possible interplay between pulsations and mass-loss, we carried
out an observational campaign of the supergiant 55 Cyg over a period of five
years to search for photospheric activity and cyclic mass-loss variability in
the stellar wind. We modeled the H, He I, Si II and Si III lines using the
nonlocal thermal equilibrium atmosphere code FASTWIND and derived the
photospheric and wind parameters. In addition, we searched for variability in
the intensity and radial velocity of photospheric lines and performed a moment
analysis of the line profiles to derive frequencies and amplitudes of the
variations. The Halpha line varies with time in both intensity and shape,
displaying various types of profiles: P Cygni, pure emission, almost complete
absence, and double or multiple peaked. The star undergoes episodes of variable
mass-loss rates that change by a factor of 1.7-2 on different timescales. We
also observe changes in the ionization rate of Si II and determine a
multiperiodic oscillation in the He I absorption lines, with periods ranging
from a few hours to 22.5 days. We interpret the photospheric line variations in
terms of oscillations in p-, g-, and strange modes. We suggest that these
pulsations can lead to phases of enhanced mass loss. Furthermore, they can
mislead the determination of the stellar rotation. We classify the star as a
post-red supergiant, belonging to the group of alpha Cyg variables.Comment: 20 pages, 18 figures, 3 tables, accepted to Astronomy & Astrophysic
The MiMeS Project: Overview and Current Status
The Magnetism in Massive Stars (MiMeS) Project is a consensus collaboration
among many of the foremost international researchers of the physics of hot,
massive stars, with the basic aim of understanding the origin, evolution and
impact of magnetic fields in these objects. At the time of writing, MiMeS Large
Programs have acquired over 950 high-resolution polarised spectra of about 150
individual stars with spectral types from B5-O4, discovering new magnetic
fields in a dozen hot, massive stars. The quality of this spectral and magnetic
mat\'eriel is very high, and the Collaboration is keen to connect with
colleagues capable of exploiting the data in new or unforeseen ways. In this
paper we review the structure of the MiMeS observing programs and report the
status of observations, data modeling and development of related theory.Comment: Proceedings of IAUS272: Active OB star
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