Wind-wind collision in the eta Carinae binary system - II. Constraints to the binary orbital parameters from radio emission near periastron passage

In this paper we use the 7- and 1.3-mm light curves obtained during the 2003.5 low-excitation phase of the eta Carinae system to constrain the possible parameters of the binary orbit. To do that we assumed that the millimetre-wave emission is produced in a dense disc surrounding the binary system; during the low-excitation phase, which occurs close to periastron, the number of ionizing photons decreases, producing the dip in the radio emission. On the other hand, due to the large eccentricity, the density of the shock region at periastron is very high and the plasma is optically thick for free-free radiation at 7 mm, explaining the sharp peak that was observed at this frequency and lasted for about 10 d. From the shape and duration of the peak we were able to determine the orbital parameters of the binary system, independently of the stellar parameters, such as mass-loss rates, wind velocities or temperature at the post-shock region.