138 research outputs found
Investigating the origin of cyclical wind variability in hot, massive stars - II. Hydrodynamical simulations of co-rotating interaction regions using realistic spot parameters for the O giant Persei
OB stars exhibit various types of spectral variability historically
associated with wind structures, including the apparently ubiquitous discrete
absorption components (DACs). These features have been proposed to be caused
either by magnetic fields or non-radial pulsations. In this second paper of
this series, we revisit the canonical phenomenological hydrodynamical modelling
used to explain the formation of DACs by taking into account modern
observations and more realistic theoretical predictions. Using constraints on
putative bright spots located on the surface of the O giant Persei
derived from high precision space-based broadband optical photometry obtained
with the Microvariability and Oscillations of STars (MOST) space telescope, we
generate two-dimensional hydrodynamical simulations of co-rotating interaction
regions in its wind. We then compute synthetic ultraviolet (UV) resonance line
profiles using Sobolev Exact Integration and compare them with historical
timeseries obtained by the International Ultraviolet Explorer (IUE) to evaluate
if the observed behaviour of Persei's DACs is reproduced. Testing three
different models of spot size and strength, we find that the classical pattern
of variability can be successfully reproduced for two of them: the model with
the smallest spots yields absorption features that are incompatible with
observations. Furthermore, we test the effect of the radial dependence of
ionization levels on line driving, but cannot conclusively assess the
importance of this factor. In conclusion, this study self-consistently links
optical photometry and UV spectroscopy, paving the way to a better
understanding of cyclical wind variability in massive stars in the context of
the bright spot paradigm.Comment: 16 pages, 10 figures, accepted for publication by MNRA
Investigating the Magnetospheres of Rapidly Rotating B-type Stars
Recent spectropolarimetric surveys of bright, hot stars have found that ~10%
of OB-type stars contain strong (mostly dipolar) surface magnetic fields (~kG).
The prominent paradigm describing the interaction between the stellar winds and
the surface magnetic field is the magnetically confined wind shock (MCWS)
model. In this model, the stellar wind plasma is forced to move along the
closed field loops of the magnetic field, colliding at the magnetic equator,
and creating a shock. As the shocked material cools radiatively it will emit
X-rays. Therefore, X-ray spectroscopy is a key tool in detecting and
characterizing the hot wind material confined by the magnetic fields of these
stars. Some B-type stars are found to have very short rotational periods. The
effects of the rapid rotation on the X-ray production within the magnetosphere
have yet to be explored in detail. The added centrifugal force due to rapid
rotation is predicted to cause faster wind outflows along the field lines,
leading to higher shock temperatures and harder X-rays. However, this is not
observed in all rapidly rotating magnetic B-type stars. In order to address
this from a theoretical point of view, we use the X-ray Analytical Dynamical
Magnetosphere (XADM) model, originally developed for slow rotators, with an
implementation of new rapid rotational physics. Using X-ray spectroscopy from
ESA's XMM-Newton space telescope, we observed 5 rapidly rotating B-type stars
to add to the previous list of observations. Comparing the observed X-ray
luminosity and hardness ratio to that predicted by the XADM allows us to
determine the role the added centrifugal force plays in the magnetospheric
X-ray emission of these stars.Comment: IAUS Conference Proceeding
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
The effects of surface fossil magnetic fields on massive star evolution. III:The case of τ Sco
Sco, a well-studied magnetic B-type star in the Upper Sco association,
has a number of surprising characteristics. It rotates very slowly and shows
nitrogen excess. Its surface magnetic field is much more complex than a purely
dipolar configuration which is unusual for a magnetic massive star. We employ
the CMFGEN radiative transfer code to determine the fundamental parameters and
surface CNO and helium abundances. Then, we employ MESA and GENEC stellar
evolution models accounting for the effects of surface magnetic fields. To
reconcile Sco's properties with single-star models, an increase is
necessary in the efficiency of rotational mixing by a factor of 3 to 10 and in
the efficiency of magnetic braking by a factor of 10. The spin down could be
explained by assuming a magnetic field decay scenario. However, the
simultaneous chemical enrichment challenges the single-star scenario. Previous
works indeed suggested a stellar merger origin for Sco. However, the
merger scenario also faces similar challenges as our magnetic single-star
models to explain Sco's simultaneous slow rotation and nitrogen excess.
In conclusion, the single-star channel seems less likely and versatile to
explain these discrepancies, while the merger scenario and other potential
binary-evolution channels still require further assessment as to whether they
may self-consistently explain the observables of Sco.Comment: Accepted for publication in MNRAS. A full reproduction package is
shared on zenodo in accordance with the Research Data Management plan of the
Anton Pannekoek Institute for Astronomy at the University of Amsterdam:
10.5281/zenodo.463340
Investigating the origin of cyclical wind variability in hot, massive stars - I. On the dipolar magnetic field hypothesis
OB stars exhibit various types of spectral variability associated with wind
structures, including the apparently ubiquitous discrete absorption components
(DACs). These are proposed to be caused by either magnetic fields or non-radial
pulsations (NRPs). In this paper, we evaluate the possible relation between
large-scale, dipolar magnetic fields and the DAC phenomenon by investigating
the magnetic properties of a sample of 13 OB stars exhibiting well-documented
DAC behaviour.
Using high-precision spectropolarimetric data acquired in part in the context
of the Magnetism in Massive Stars (MiMeS) project, we find no evidence for
surface dipolar magnetic fields in any of these stars. Using Bayesian
inference, we compute upper limits on the strengths of the fields and use these
limits to assess two potential mechanisms by which the field may influence wind
outflow: magnetic wind confinement and local photospheric brightness
enhancements. Within the limits we derive, both mechanisms fail to provide a
systematic process capable of producing DACs in all of the stars of our sample.
Therefore, this implies that dipolar fields are highly unlikely to be
responsible for these structures in all massive stars, meaning that some other
mechanism must come into play.Comment: 17 pages, 6 figures, accepted for publication in MNRA
The magnetic characteristics of Galactic OB stars from the MiMeS survey of magnetism in massive stars
The Magnetism in Massive Stars (MiMeS) project represents the largest
systematic survey of stellar magnetism ever undertaken. Based on a sample of
over 550 Galactic B and O-type stars, the MiMeS project has derived the basic
characteristics of magnetism in hot, massive stars. Herein we report
preliminary results.Comment: Proceedings of IAUS 302: Magnetic fields throughout stellar evolutio
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