Multi-band radio study of the particle-accelerating colliding-wind binary WR 147

Abstract

Massive stars can be characterized by various properties. Two of them are their winds and their multiplicity. Since massive stars are more likely to be part of a binary system, their winds may collide and interact. These systems are called colliding-wind binaries (CWB). CWBs in which some charged particles in the wind collisions region are accelerated to rel- ativistic velocities are called particle-accelerating colliding-wind binaries (PACWB). This master thesis focuses on one of them, WR 147. The physics and properties of these objects are discussed in Chap. 1. In Chap. 2 the radiative processes are presented. Indeed, two types of radio emission can be observed in these PACWB systems. On the one hand, the thermal emission, also called Bremsstrahlung, comes from the ionized wind of the stars. On the other hand, the non- thermal emission, or synchrotron emission, is produced by relativistic electrons accelerated by the Diffusive Shock Acceleration process (DSA). The low frequency part of the spec- trum is likely to be affected by so-called turnover processes, leading to a severe drop of the synchrotron flux. Among these processes, free-free absorption is the most likely to affect the radio spectrum of PACWBs. This master thesis includes the analysis of the radio emission of WR 147 in two radio fre- quency bands. More precisely, it aims at achieving a deeper understanding of the WR 147 behavior at these frequencies through a detailed intra-band investigation. In particular, it focuses on low frequencies where the synchrotron emission is stronger but also more susceptible of being affected by turnover processes that are often difficult to constrain. Data from the Jansky Very Large Array (JVLA) are used for this purpose. After processing the radio data, including flagging, calibrating, and imaging steps, the flux density of the source is obtained in both bands but also in each sub-band. All these steps are described in Chap. 3. With these flux densities, several fits are proposed, in Chap. 4, notably based on the foreground absorption model. Several parameters are determined such as the thermal and non-thermal spectral indices. However, it is observed that this model is not adapted to the behavior of WR 147 at low frequencies. Thus a new model, the internal absorption model, is proposed. It consists in the absorption of the synchrotron radio emission by ther- mal electrons located in the emission region, and not by material surrounding the emission region. It seems that this new model better represents the behavior of WR 147, although it does not lead to a perfect fit. Then, in Chap. 5, several elements of discussion are addressed based on the parameters and fits of the different models. These include the value of the compression ratio, the justification of the turnover process at play being free-free absorption, and the possible influence of a third star in the system. The relevance of the internal absorption model is also discussed in detail. Finally, a more general model combining the foreground absorp- tion model and the internal absorption model is proposed to generalize the behavior of PACWBs. In Chap. 6 several perspectives for future observations are considered, notably based on the internal absorption model and on the general model.