673 research outputs found
Non-thermal radio emission from O-type stars. V. 9 Sgr
The colliding winds in a massive binary system generate synchrotron emission
due to a fraction of electrons that have been accelerated to relativistic
speeds around the shocks in the colliding-wind region. We studied the radio
light curve of 9 Sgr = HD 164794, a massive O-type binary with a 9.1-yr period.
We investigated whether the radio emission varies consistently with orbital
phase and we determined some parameters of the colliding-wind region. We
reduced a large set of archive data from the Very Large Array (VLA) to
determine the radio light curve of 9 Sgr at 2, 3.6, 6 and 20 cm. We also
constructed a simple model that solves the radiative transfer in the
colliding-wind region and both stellar winds. The 2-cm radio flux shows clear
phase-locked variability with the orbit. The behaviour at other wavelengths is
less clear, mainly due to a lack of observations centred on 9 Sgr around
periastron passage. The high fluxes and nearly flat spectral shape of the radio
emission show that synchrotron radiation dominates the radio light curve at all
orbital phases. The model provides a good fit to the 2-cm observations,
allowing us to estimate that the brightness temperature of the synchrotron
radiation emitted in the colliding-wind region at 2 cm is at least 4 x 10^8 K.
The simple model used here already allows us to derive important information
about the colliding-wind region. We propose that 9 Sgr is a good candidate for
more detailed modelling, as the colliding-wind region remains adiabatic during
the whole orbit thus simplifying the hydrodynamics.Comment: 10 pages, 3 figures, accepted for publication in A&
Modeling Ultraviolet Wind Line Variability in Massive Hot Stars
We model the detailed time-evolution of Discrete Absorption Components (DACs)
observed in P Cygni profiles of the Si IV lam1400 resonance doublet lines of
the fast-rotating supergiant HD 64760 (B0.5 Ib). We adopt the common assumption
that the DACs are caused by Co-rotating Interaction Regions (CIRs) in the
stellar wind. We perform 3D radiative transfer calculations with hydrodynamic
models of the stellar wind that incorporate these large-scale density- and
velocity-structures. We develop the 3D transfer code Wind3D to investigate the
physical properties of CIRs with detailed fits to the DAC shape and morphology.
The CIRs are caused by irregularities on the stellar surface that change the
radiative force in the stellar wind. In our hydrodynamic model we approximate
these irregularities by circular symmetric spots on the stellar surface. We use
the Zeus3D code to model the stellar wind and the CIRs, limited to the
equatorial plane. We constrain the properties of large-scale wind structures
with detailed fits to DACs observed in HD 64760. A model with two spots of
unequal brightness and size on opposite sides of the equator, with opening
angles of 20 +/- 5 degr and 30 +/- 5 degr diameter, and that are 20 +/- 5 % and
8 +/- 5 % brighter than the stellar surface, respectively, provides the best
fit to the observed DACs. The recurrence time of the DACs compared to the
estimated rotational period corresponds to spot velocities that are 5 times
slower than the rotational velocity. The mass-loss rate of the structured wind
model for HD 64760 does not exceed the rate of the spherically symmetric smooth
wind model by more than 1 %. The fact that DACs are observed in a large number
of hot stars constrains the clumping that can be present in their winds, as
substantial amounts of clumping would tend to destroy the CIRs.Comment: 58 pages, 16 figures, 1 animation. Accepted for publication in The
Astrophysical Journal, Main Journal. More information and animations are
available at http://alobel.freeshell.org/hotstars.htm
Massive non-thermal radio emitters: new data and their modelling
During recent years some non-thermal radio emitting OB stars have been
discovered to be binary, or multiple systems. The non-thermal emission is due
to synchrotron radiation that is emitted by electrons accelerated up to high
energies. The electron acceleration occurs at the strong shocks created by the
collision of radiatively-driven winds. Here we summarize the available radio
data and more recent observations for the binary Cyg OB2 No. 9. We also show a
new emission model which is being developed to compare the theoretical total
radio flux and the spectral index with the observed radio light curves. This
comparison will be useful in order to solve fundamental questions, such as the
determination of the stellar mass loss rates, which are perturbed by clumping.Comment: 3 pages, 1 figure, poster at Four Decades of Research on Massive
Stars-A Scientific Meeting in Honour of Anthony F.J.Moffa
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