Millimeter-wave emission during the 2003 low excitation phase of η Carinae

¹ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900, São Paulo, Brazil e-mail: zulema@astro.iag.usp.br
² European Southern Observatory, Casilla 19001, Santiago 19, Chile
³ Onsala Space Observatory, 439 92 Onsala, Sweden
⁴ Service d'Astrophysique (CNRS URA 2052), CEA Saclay, 91191 Gif-sur-Yvette Cedex, France

Received: 6 July 2004
Accepted: 6 April 2005

Abstract

In this paper we present observations of η Carinae in the 1.3 mm and 7 mm radio continuum, during the 2003.5 low excitation phase. The expected minimum in the light curves was confirmed at both wavelengths and was probably due to a decrease in the number of UV photons available to ionize the gas surrounding the binary system. At 7 mm a very well defined peak was superimposed on the declining flux density. It presented maximum amplitude in 29 June 2003 and lasted for about 10 days. We show that its origin can be free-free emission from the gas at the shock formed by wind-wind collision, which is also responsible for the observed X-ray emission. Even though the shock strength is strongly enhanced as the two stars in the binary system approach each other, during periastron passage the X-ray emission is strongly absorbed and the 7 mm observations represent the only direct evidence of this event.