Letter to the Editor

Mid-infrared evolution of η Carinae from 1968 to 2018^⋆

¹ ESO – European Organisation for Astronomical Research in the Southern Hemisphere, Alonso de Cordova 3107, Vitacura, Santiago de Chile, Chile
e-mail: amehner@eso.org
² Department of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ, UK
³ California Institute of Technology, IPAC, M/C 100-22, Pasadena, CA 91125, USA
⁴ ESO – European Organisation for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁵ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁶ NASA Goddard Space Flight Center, Code 667, Greenbelt, MD 20771, USA
⁷ Department of Physics & Astronomy, University of Pittsburgh, 3941 O’Hara Street, Pittsburgh, PA 15260, USA
⁸ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 9 July 2019
Accepted: 23 August 2019

Abstract

η Car is one of the most luminous and massive stars in our Galaxy and is the brightest mid-IR source in the sky outside our solar system. Since the late 1990s, the central source has dramatically brightened at UV and optical wavelengths. This might be explained by a decrease in circumstellar dust extinction. We aim to establish the mid-IR flux evolution and further our understanding of the star’s UV and optical brightening. Mid-IR images from 8−20 μm were obtained in 2018 with VISIR at the Very Large Telescope. Archival data from 2003 and 2005 were retrieved from the ESO Science Archive Facility, and historical records were collected from publications. We present mid-IR images of η Car with the highest angular resolution to date at the corresponding wavelengths (≥0.22″). We reconstruct the mid-IR evolution of the spectral energy distribution of the spatially integrated Homunculus nebula from 1968 to 2018 and find no long-term changes. The bolometric luminosity of η Car has been stable over the past five decades. We do not observe a long-term decrease in the mid-IR flux densities that could be associated with the brightening at UV and optical wavelengths, but circumstellar dust must be declining in our line of sight alone. Short-term flux variations within about 25% of the mean levels could be present.