Mid-infrared evolution of eta Car from 1968 to 2018

Abstract

Eta Car is one of the most luminous and massive stars in our Galaxy and is the brightest mid-infrared (mid-IR) source in the sky, outside our solar system. Since the late 1990s the central source has dramatically brightened at ultraviolet 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 ultraviolet and optical brightening. Mid-IR images from $8-20\mu$m were obtained in 2018 with VISIR at the Very Large Telescope. Archival data from 2003 and 2005 are retrieved from the ESO Science Archive Facility and historical records are collected from publications. We present the highest angular resolution mid-IR images of $\eta$ Car to date at the corresponding wavelengths ($\ge 0.22^{\prime \prime }$). 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. Eta Car's bolometric luminosity 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 ultraviolet and optical wavelengths, but circumstellar dust must be declining in our line-of-sight only. Short-term flux variations within about 25% of the mean levels could be present.

arXiv comment5 pages, 4 Figures, accepted for publication in A&A

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• The X-ray properties of Eta Carinae during its 2020 X-ray minimum
• Changes in the Na D${}_1$ Absorption Components of $\eta$ Carinae Provide Clues on the Location of the Dissipating Central Occulter
• One of Everything: The Breakthrough Listen Exotica Catalog
• The Altered State of $\eta$Carinae: HST's Photometric Record 1998--2021
• Telluric absorption lines in the ALMA spectra of $\eta$ Car