Abstract

Massive non-thermal radio emitters: new data and their modelling

Delia Volpi (Royal Observatory of Belgium)

R. Blomme (1), M. De Becker (2,3), G. Rauw (2) ((1) Royal Observatory of Belgium, (2) Institut d'Astrophysique ULg, (3) Observatoire de Haute-Provence)

Several massive stars, such as Wolf-Rayet and OB-type ones, show evidence of non-thermal emission. This non-thermal radiation is produced by relativistic electrons accelerated by the shocks created by the colliding winds in a binary, through the Fermi mechanism. In the presence of the magnetic field, these electrons emit synchrotron radiation in the radio band. The synchrotron radiation is partially absorbed by the free-free absorption mechanism leading to flux variability connected to the orbital phase. We summarize the available radio data of OB systems and present some more recent observations. We then discuss our model and apply it to two representative cases: the short-period Cyg OB2 No. 8A and the long-period Cyg OB2 No. 9. The model calculates the particle acceleration at the shocks and follows the emitting electrons along the post-shock streamlines, while they are being cooled by Inverse Compton and adiabatic energy losses. The resulting total radio flux and the spectral index as a function of orbital phase are then compared with the observed radio light curves. This comparison is essential to investigate important issues related to massive stars astrophysics, such as clumping effect on the mass loss rate determination of single stars, the particle acceleration mechanism at the shocks and the binary frequency of early-type stars.