Wind-Wind Collision in the eta Carinae Binary System a Shell-Like Event Near Periastron.pdf

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Wind-Wind Collision in the eta Carinae Binary System a Shell-Like Event Near Periastron.pdf

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Wind-Wind Collision in the eta Carinae Binary System a Shell-Like Event Near Periastron

a r X i v : a s t r o - p h / 0 4 0 4 3 6 3 v 2 2 2 D e c 2 0 0 4 Mon. Not. R. Astron. Soc. 000, 1–9 (2004) Printed 2 February 2008 (MN LATEX style file v2.2) Wind-wind collision in the η Carinae binary system: a shell-like event near periastron D. Falceta-Gonc?alves1?, V. Jatenco-Pereira1 and Z. Abraham1 1Instituto de Astronomia, Geof??sica e Cie?ncias Atmosfe?ricas, Universidade de Sa?o Paulo, Rua do Mata?o 1226, CEP 05508-900, Sa?o Paulo, Brazil ABSTRACT The exact nature of η Carinae is still an open issue. Strict periodicity in the light curves at several wavelengths seem to point out to a binary system, but the observed radial velocities, measured from space with high spatial resolution are in conflict with the ground based observations used to calculate the binary orbit. Also, the observed 2-10 keV X-ray flux is much larger that what is expected from a single star, and favors the wind-wind collision hypothesis, characteristic of high mass binary systems. However, to explain the duration of the dip in the light curve by wind collisions, it is necessary to postulate a very large increase in the η Carinae mass loss rate. Finally, the optical and UV light curves are better explained by periodic shell-ejection events. In this paper we conciliate the two hypothesis. We still assume a binary system to explain the strong X-ray emission, but we also take into account that, near periastron and because of the highly eccentric orbit, the wind emerging from η Carinae accumulates behind the shock and can mimic a shell-like ejection event. For this process to be effective, at periastron the secondary star should be located between η Carinae and the observer, solving also the discrepancy between the orbital parameters derived from ground and space based observations. We show that, as the secondary moves in its orbit, the shell cools down and the number of available stellar ionizing photons is not enough to maintain the shell temperature at its equilibrium value of about