Episodic Post-Shock Dust Formation in the Colliding Winds of Eta Carinae

TL;DR

Eta Carinae's near-infrared peaks are caused by episodic formation of hot dust in post-shock zones of colliding winds, revealing new insights into dust formation processes in massive binary systems.

Contribution

This paper proposes that transient hot dust forms episodically in Eta Carinae's colliding winds, a process analogous to dust formation in Wolf-Rayet binaries and supernovae, which was not previously established.

Findings

Near-IR peaks are due to hot dust emission, not free-free wind or photospheric emission.

Dust likely forms from corundum or similar high-temperature condensates.

Dust formation occurs near and after periastron, affecting system observations.

Abstract

Eta Carinae shows broad peaks in near-infrared (IR) JHKL photometry, roughly correlated with times of periastron passage in the eccentric binary system. After correcting for secular changes attributed to reduced extinction from the thinning Homunculus Nebula, these peaks have IR spectral energy distributions (SEDs) consistent with emission from hot dust at 1400-1700 K. The excess SEDs are clearly inconsistent, however, with the excess being entirely due to free-free wind or photospheric emission. One must conclude, therefore, that the broad near-IR peaks associated with Eta Carinae's 5.5 yr variability are due to thermal emission from hot dust. I propose that this transient hot dust results from episodic formation of grains within compressed post-shock zones of the colliding winds, analogous to the episodic dust formation in Wolf-Rayet binary systems like WR140 or the post-shock dust formation seen in some supernovae like SN2006jc. This dust formation in Eta Carinae seems to occur preferentially near and after periastron passage; near-IR excess emission then fades as the new dust disperses and cools. With the high grain temperatures and Eta Car's C-poor abundances, the grains are probably composed of corundum or similar species that condense at high temperatures, rather than silicates or graphite. Episodic dust formation in Eta Car's colliding winds significantly impacts our understanding of the system, and several observable consequences are discussed.