121 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
Étude du système binaire CV Ser à l'aide du satellite MOST
Ce mémoire s’intéresse au système binaire massif CV Serpentis, composé d’une Wolf-
Rayet riche en carbone et d’une étoile de la séquence principale, de type spectral O
(WC8d + O8-9IV).
D’abord, certains phénomènes affectant les étoiles massives sont mentionnés, de leur
passage sur la séquence principale à leur mort (supernova). Au cours du premier cha-
pitre, un rappel est fait concernant certaines bases de l’astrophysique stellaire observa-
tionnelle (diagramme Hertzsprung-Russell, phases Ă©volutives, etc...).
Au chapitre suivant, un des aspects les plus importants de la vie des Ă©toiles massives est
abordé : la perte de masse sous forme de vents stellaires. Un historique de la découverte
des vents ouvre le chapitre, suivi des fondements théoriques permettant d’expliquer ce
phénomène. Ensuite, différents aspects propres aux vents stellaires sont présentés.
Au troisième chapitre, un historique détaillé de CV Ser est présenté en guise d’introduc-
tion à cet objet singulier. Ses principales caractéristiques connues y sont mentionnées.
Finalement, le cœur de ce mémoire se retrouve au chapitre 4. Des courbes de lumière
ultra précises du satellite MOST (2009 et 2010) montrent une variation apparente du
taux de perte de masse de la WR de l’ordre de 62% sur une période orbitale de 29.701
jours. L’analyse des résidus permet de trouver une signature suggérant la présence de
régions d’interaction en corotation (en anglais corotating interaction regions, ou CIR)
dans le vent WR. Une nouvelle solution orbitale est présentée ainsi que les paramètres
de la région de collision des vents et les types spectraux sont confirmés.This thesis focuses on the massive binary CV Serpentis, consisting of a carbon-rich
Wolf-Rayet star and a main-sequence O-type star (WC8d + O8-9IV).
First off, different phenomena linked to massive stars throughout their existence - from
main sequence to the supernova explosion - are mentioned. The first chapter offers a
brief overview of some of the basics of observational stellar astrophysics (Hertzsprung-
Russell diagram, evolution, etc...).
The next chapter covers one of the most important aspects of massive stars : mass loss
through stellar winds. The chapter opens with a chronology of the discovery of stellar
winds, followed by the foundations of stellar wind theory. Finally, different processes
involved in wind ejection are presented.
The third chapter reviews chronologically the main studies carried out on CV Ser and
helps introduce this peculiar system. Its main characteristics are given in this chapter.
Finally, chapter 4 is the central part of this work. MOST light curves taken in 2009
and 2010 show what appears to be a 62% increase of the mass-loss rate over one or-
bital period (29.701d). There also seems to be evidence for the presence of corotating
interaction regions (CIR) in the WR wind. Indeed, the analysis of the residuals yields a
CIR-like signature. A new orbit is derived, as well as the wind-collision zone parame-
ters, while the spectral types of both stars are confirmed
The effects of surface fossil magnetic fields on massive star evolution. III:The case of Ď„ 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 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
- …