The multiple eruptions of Eta Carinae

Eta Carinae is one of the most extraordinary stars known in our galaxy, having many peculiar properties. One of the most prominent features is that it is surrounded by bipolar lobes called the Homunculus nebula. This nebula originated from the Great Eruption in 1844 that was visible to the naked eye. It is also known that Eta Car had other small eruptions centuries before the Great Eruption, where smaller ejectiles were emitted with ~300 year intervals. Eta Car itself has extremely strong winds reaching mass-loss rates of Mdot~10^{-3}Msun/yr, which interacts with the wind from a companion star that orbits in a 5.5 yr period. Many attempts have been made to understand the formation of Eta Car and its many peculiar properties, but most models have failed to explain all features self-consistently.Here we propose a new scenario starting with a hierarchical triple system. The system becomes unstable after mass-transfer in the inner binary and reaches large eccentricities. Each periastron interaction peels off material off the primary star and ejects them as outer ejecta. Then the instability eventually triggers a merger, leading to the Great Eruption and formation of a bipolar nebula. In this talk I will show results of 2.5D hydrodynamical simulations of a stellar merger and show that it can reproduce the observed Homunculus nebula well. I will also discuss how close periastron encounters in triple systems can reproduce the Outer ejecta via n-body simulations.Understanding the formation of Eta Carinae helps us understand the formation and evolution of LBVs in general, and possibly many other peculiar objects.

Ryosuke Hirai (Oxford Univ)
KIAA 1st meeting room
Fri, 2019-09-20 12:00 to 13:00