Near-infrared evidence for a sudden temperature increase in Eta Carinae^⋆

¹ ESO, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago de Chile, Chile
e-mail: amehner@eso.org
² Division of Elementary Particle Physics and Astrophysics, Graduate School of Science, Nagoya University, 464-8602 Nagoya, Japan
³ South African Astronomical Observatory, PO Box 9, Observatory, 7935 Cape Town, South Africa
⁴ Astronomy, Cosmology and Gravity Centre, Astronomy Department, University of Cape Town, 7701 Rondebosch, South Africa

Received: 22 September 2013
Accepted: 20 January 2014

Abstract

Aims. Eta Car’s ultra-violet, optical, and X-ray light curves and its spectrum suggest a physical change in its stellar wind over the last decade. It has been proposed that the mass-loss rate has decreased by a factor of about 2 over the last 15 years. We complement these recent results by investigating the past evolution and the current state of η Car in the near-infrared (IR).

Methods. We present JHKL photometry of η Car obtained at SAAO Sutherland from 2004–2013 with the Mk II photometer at the 0.75 m telescope and JHK_s photometry with SIRIUS at the 1.4 m IRSF telescope from 2012–2013. The near-IR light curves since 1972 are analyzed.

Results. The long-term brightening trends in η Car’s JHKL light curves were discontinuous around the 1998 periastron passage. After 1998, the star shows excess emission above the extrapolated trend from earlier dates, especially in J and H, and the blueward, cyclical progression in its near-IR colors is accelerated. The near-IR color evolution is strongly correlated with the periastron passages. After correcting for the secular trend we find that the color evolution matches an apparent increase in blackbody temperature of an optically thick near-IR emitting plasma component from about 3500 K to 6000 K over the last 20 years.

Conclusions. We suggest that the changing near-IR emission may be caused by variability in optically thick bremsstrahlung emission. Periastron passages play an important role in the observed excess near-IR emission after 1998 and the long-term color evolution. We thus propose the hypothesis that angular momentum transfer (via tidal acceleration) during periastron passages leads to sudden changes in η Car’s atmosphere resulting in a long-term decrease in the mass-loss rate.