|Ph.D Student||Kashi Amit|
|Subject||The Periastron Passage of the Binary Star Eta Carinae|
|Department||Department of Physics||Supervisor||Professor Noam Soker|
|Full Thesis text|
Eta Car is a unique massive binary star system in our Galaxy, presenting some basic undetermined parameters and open questions. The primary is a very massive Luminous Blue Variable, and the secondary is a massive main sequence O-star. Both stars blow stellar winds that collide and create a conical shell. The 5.54 year periodicity in a very eccentric orbit is observed in all wavelengths. I identified the important physical processes that expected to take place at periastron passages, and use them to construct a model and explain observations.
I showed that close to periastron primary material is accreted onto the secondary. The accreted mass possesses enough angular momentum to form a thick belt around the secondary, and shuts down the secondary wind for ten weeks. This explains the minimum in the X-ray luminosity and in other bands, known as the spectroscopic event.
I predicted that the X-ray minimum may have a different duration if the mass accretion rate varies. That prediction was fulfilled in January 2009, when an early exit from the 2009 X-ray minimum after four weeks, instead of ten weeks as in the two previous minima was observed.
I examined the X-ray light curve, Hydrogen Column density, many spectral lines, and a variety of other observations and found that they all support an orientation where for most of the time the secondary is further away than the primary. The secondary comes closer to the observer only for a short time near
periastron passage, in this highly eccentric orbit.
By reconstructing the evolution of Eta Car in the last two centuries, I showed that the two 19th century eruptions were most likely triggered by periastron passages of the secondary star. Consequently, the mass of the binary system is constrained to be >~250 Solar masses. This new finding suggests that Eta Car is the most massive binary system in the Galaxy. The accreting secondary star can account for the extra energy of the GE. The research results suggest that all major LBV eruptions are triggered by stellar companions, and that in extreme cases a short duration event with a huge mass transfer rate can lead to a bright transient event on time scales of weeks to months (a "supernova impostor").