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&

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

Long-Wavelength, Free-Free Spectral Energy Distributions from Porous Stellar Winds

The influence of macroclumps for free-free spectral energy distributions (SEDs) of ionized winds is considered. The goal is to emphasize distinctions between microclumping and macroclumping effects. Microclumping can alter SED slopes and flux levels if the volume filling factor of the clumps varies with radius; however, the modifications are independent of the clump geometry. To what extent does macroclumping alter SED slopes and flux levels? In addressing the question, two specific types of macroclump geometries are explored: shell fragments ("pancake"-shaped) and spherical clumps. Analytic and semi-analytic results are derived in the limiting case that clumps never obscure one another. Numerical calculations based on a porosity formalism is used when clumps do overlap. Under the assumptions of a constant expansion, isothermal, and fixed ionization wind, the fragment model leads to results that are essentially identical to the microclumping result. Mass-loss rate determinations are not affected by porosity effects for shell fragments. By contrast, spherical clumps can lead to a reduction in long-wavelength fluxes, but the reductions are only significant for extreme volume filling factors.Comment: to appear in MNRA