452 research outputs found
Phase resolved X-ray spectroscopy of HDE228766: Probing the wind of an extreme Of+/WNLha star
HDE228766 is a very massive binary system hosting a secondary component,
which is probably in an intermediate evolutionary stage between an Of
supergiant and an WN star. The wind of this star collides with the wind of its
O8 II companion, leading to relatively strong X-ray emission. Measuring the
orbital variations of the line-of-sight absorption toward the X-ray emission
from the wind-wind interaction zone yields information on the wind densities of
both stars. X-ray spectra have been collected at three key orbital phases to
probe the winds of both stars. Optical photometry has been gathered to set
constraints on the orbital inclination of the system. The X-ray spectra reveal
prominent variations of the intervening column density toward the X-ray
emission zone, which are in line with the expectations for a wind-wind
collision. We use a toy model to set constraints on the stellar wind parameters
by attempting to reproduce the observed variations of the relative fluxes and
wind optical depths at 1 keV. The lack of strong optical eclipses sets an upper
limit of about 68 degrees on the orbital inclination. The analysis of the
variations of the X-ray spectra suggests an inclination in the range 54 - 61
degrees and indicates that the secondary wind momentum ratio exceeds that of
the primary by at least a factor 5. Our models further suggest that the bulk of
the X-ray emission arises from the innermost region of the wind interaction
zone, which is from a region whose outer radius, as measured from the secondary
star, lies between 0.5 and 1.5 times the orbital separation
Consumer protection in Turkey: law, informality and the role of the media. Monash University, Workplace and Corporate Law Research Group, Working Paper No. 21
This report is part of a University of Oxford John Fell funded collaborative project: Informality and the Media in Consumer Protection in Emerging Economies. This pilot project seeks to shed light upon consumer complaint behaviour through social media in emerging economies
Synthesis and characterization of [Fe(BPMEN)-ACC]SbF 6 : a structural and functional mimic of ACC-oxidase †
International audienceA mononuclear Fe(II) complex bearing 1-aminocyclopropane-1-carboxylic acid (ACCH) was synthesized and characterized. X-ray crystallography demonstrated that ACC binds to the Fe(II) ion in a bidentate mode constituting the first structural mimic of the expected binding of ACC to the Fe(II) center of the ethylene forming enzyme ACC-oxidase (ACCO). [Fe(BPMEN)ACC]SbF 6 also constitutes a functional biomimetic complex of ACCO, as it reacts with hydrogen peroxide producing ethylene
Massive open star clusters using the VVV survey II. Discovery of six clusters with Wolf-Rayet stars
Context: The ESO Public Survey "VISTA Variables in the V\'ia L\'actea" (VVV)
provides deep multi-epoch infrared observations for an unprecedented 562 sq.
degrees of the Galactic bulge, and adjacent regions of the disk. In this survey
nearly 150 new open clusters and cluster candidates have been discovered. Aims:
This is the second in a series of papers about young, massive open clusters
observed using the VVV survey. We present the first study of six recently
discovered clusters. These clusters contain at least one newly discovered
Wolf-Rayet (WR) star. Methods: Following the methodology presented in the first
paper of the series, wide-field, deep JHKs VVV observations, combined with new
infrared spectroscopy, are employed to constrain fundamental parameters for a
subset of clusters. Results: We affirm that the six studied stellar groups are
real young (2-7 Myr) and massive (between 0.8 and 2.2 10^3 Msol) clusters. They
are highly obscured (Av ~ 5-24 mag) and compact (1-2 pc). In addition to WR
stars, two of the six clusters also contain at least one red supergiant star.
We claim the discovery of 8 new WR stars, and 3 stars showing WR-like emission
lines which could be classified WR or OIf. Preliminary analysis provides
initial masses of ~30-50 Msol for the WR stars. Finally,we discuss the spiral
structure of the Galaxy using as tracers the six new clusters together with the
previously studied VVV clusters.Comment: 17 pages, 8 figurs, accepted in A&
Plaskett's Star: Analysis of the CoRoT photometric data
The SRa02 of the CoRoT space mission for Asteroseismology was partly devoted
to stars belonging to the Mon OB2 association. An intense monitoring was
performed on Plaskett's Star (HD47129) and the unprecedented quality of the
light curve allows us to shed new light on this very massive, non-eclipsing
binary system. We particularly aimed at detecting periodic variability which
might be associated with pulsations or interactions between both components. We
also searched for variations related to the orbital cycle which could help to
constrain the inclination and the morphology of the binary system. A
Fourier-based prewhitening and a multiperiodic fitting procedure were applied
to analyse the time series and extract the frequencies of variations. We
describe the noise properties to tentatively define an appropriate significance
criterion, to only point out the peaks at a certain significance level. We also
detect the variations related to the orbital motion and study them by using the
NIGHTFALL program. The periodogram exhibits a majority of peaks at low
frequencies. Among these peaks, we highlight a list of about 43 values,
including notably two different sets of harmonic frequencies whose fundamental
peaks are located at about 0.07 and 0.82d-1. The former represents the orbital
frequency of the binary system whilst the latter could probably be associated
with non-radial pulsations. The study of the 0.07d-1 variations reveals the
presence of a hot spot most probably situated on the primary star and facing
the secondary. The investigation of this unique dataset constitutes a further
step in the understanding of Plaskett's Star. These results provide a first
basis for future seismic modelling. The existence of a hot region between both
components renders the determination of the inclination ambiguous.Comment: Accepted in A&A, 13 pages, 7 figures, 2 table
Dynamically inflated wind models of classical Wolf-Rayet stars
Vigorous mass loss in the classical Wolf-Rayet (WR) phase is important for
the late evolution and final fate of massive stars. We develop spherically
symmetric time-dependent and steady-state hydrodynamical models of the
radiation-driven wind outflows and associated mass loss from classical WR
stars. The simulations are based on combining the opacities typically used in
static stellar structure and evolution models with a simple parametrised form
for the enhanced line-opacity expected within a supersonic outflow. Our
simulations reveal high mass-loss rates initiated in deep and hot optically
thick layers around T\approx 200kK. The resulting velocity structure is
non-monotonic and can be separated into three phases: i) an initial
acceleration to supersonic speeds ii) stagnation and even deceleration, and
iii) an outer region of rapid re-acceleration. The characteristic structures
seen in converged steady-state simulations agree well with the outflow
properties of our time-dependent models. By directly comparing our dynamic
simulations to corresponding hydrostatic models, we demonstrate explicitly that
the need to invoke extra energy transport in convectively inefficient regions
of stellar structure and evolution models is merely an artefact of enforcing a
hydrostatic outer boundary. Moreover, the "dynamically inflated" inner regions
of our simulations provide a natural explanation for the often-found mismatch
between predicted hydrostatic WR radii and those inferred from spectroscopy.
Finally, we contrast our simulations with alternative recent WR wind models
based on co-moving frame radiative transfer for computing the radiation force.
Since CMF transfer currently cannot handle non-monotonic velocity fields, the
characteristic deceleration regions found here are avoided in such simulations
by invoking an ad-hoc very high degree of clumping.Comment: 15 pages, 9 figure
CoRoT observations of O stars: diverse origins of variability
Six O-type stars were observed continuously by the CoRoT satellite during a
34.3-day run. The unprecedented quality of the data allows us to detect even
low-amplitude stellar pulsations in some of these stars (HD 46202 and the
binaries HD 46149 and Plaskett's star). These cover both opacity-driven modes
and solar-like stochastic oscillations, both of importance to the
asteroseismological modelling of O stars. Additional effects can be seen in the
CoRoT light curves, such as binarity and rotational modulation. Some of the
hottest O-type stars (HD 46223, HD 46150 and HD 46966) are dominated by the
presence of red-noise: we speculate that this is related to a sub-surface
convection zone.Comment: 5 pages, 3 figures, conference paper. To be published in "Four
decades of Research on Massive Stars", Astronomical Society of the Pacific.
Eds. C. Robert, N. St-Louis and L. Drisse
Is HR 6819 a triple system containing a black hole? -- An alternative explanation
HR 6819 was recently proposed to be a triple system consisting of an inner
B-type giant + black hole binary with an orbital period of 40d and an outer Be
tertiary. This interpretation is mainly based on two inferences: that the
emission attributed to the outer Be star is stationary, and that the inner
star, which is used as mass calibrator for the black hole, is a B-type giant.
We re-investigate the properties of HR 6819 by spectral disentangling and an
atmosphere analysis of the disentangled spectra to search for a possibly
simpler alternative explanation for HR 6819. Disentangling implies that the Be
component is not a static tertiary, but rather a component of the binary in the
40-d orbit. The inferred radial velocity amplitudes imply an extreme mass ratio
of M_2/M_1 = 15 +/- 3. We infer spectroscopic masses of 0.4
Msun and 6 Msun for the primary and secondary, which agree well
with the dynamical masses for an inclination of i = 32 deg. This indicates that
the primary might be a stripped star rather than a B-type giant. Evolutionary
modelling suggests that a possible progenitor system would be a tight (P_i ~
2d) B+B binary system that experienced conservative mass transfer. While the
observed nitrogen enrichment of the primary conforms with the predictions of
the evolutionary models, we find no indications for the predicted He
enrichment. We suggest that HR 6819 is a binary system consisting of a stripped
B-type primary and a rapidly-rotating Be star that formed from a previous
mass-transfer event. In the framework of this interpretation, HR 6819 does not
contain a black hole. Interferometry can distinguish between these two
scenarios by providing an independent measurement of the separation between the
visible components.Comment: Submitted to A&A, 13 pages (16 figures and 2 tables); 4 pages
supplementary material (4 figures and 4 tables). Comments are welcom
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