Other
Scientific paper
Apr 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005phdt.........1v&link_type=abstract
PhD thesis Katholieke Universiteit Leuven
Other
16
Early-Type Stars, Mass-Loss, Winds, Outflows, Non-Thermal Radio, Colliding Winds, Binaries, Hydrodynamics
Scientific paper
In this dissertation we present theoretical models for the non-thermal radio emission from single O stars. The non-thermal radio emission is due to synchrotron-emitting electrons that attain relativistic energies through the first-order Fermi acceleration in wind-embedded shocks associated with an unstable, chaotic wind. The goal of this work is to investigate what properties a synchrotron model should have to reproduce the observed non-thermal radio spectra of the most likely single non-thermal radio emitters, Cyg OB2 No. 9, HD 168112 and 9 Sgr. By looking at all models that fit the observations, meaningful constraints on the model parameters, each with a clear physical interpretation, can be obtained. We first develop a simple parametrised synchrotron model, in which we assume that the emission region extends continuously out to an outer boundary which relates to the last shock. A major result of this model is that shocks must persist up to large distances in the stellar wind. Also, to produce the observed fluxes with a negative spectral index, the synchrotron emissivity must decrease more slowly than expected from a density law that falls off with distance squared. This result appears to be a general feature of synchrotron models that explain the observations (also for the more sophisticated ones). We improve the previous model by taking into account the cooling of relativistic electrons. Then the synchrotron emission is confined to narrow layers behind the shock. Furthermore, we show that the synchrotron emission depends strongly on the shock strength. The strongest shocks in the stellar wind produce the bulk of the emission, so that the emergent radio flux can be described as coming from a small number of shocks. When we assume that the shock strength is constant in the wind, we find that a multiple shock model can explain the observations. However, when the radial decline of the shock strength predicted by hydrodynamical simulations is taken into account, we find that the synchrotron emissivity rapidly decreases as a function of radius. This leads to a radio spectrum with a positive spectral index, contrary to the observations. We find that the observed non-thermal radio spectra can only be reproduced by counteracting the rapid radial decrease of the emissivity. We investigate a number of possibilities to do so, none of which appears to be physically plausible. We conclude that the observed emission is probably not from wind-embedded shocks associated with an unstable, chaotic wind. The most likely alternative is synchrotron emission from shocks associated with colliding winds. In this hypothesis, the radio flux changes would be due to changes in the orbital motion. A radio light curve should then show good repeatability from one period to another. We find that Cyg OB2 No. 9 and HD 168112 indeed show these features.
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