The Spectral Temperature of Optically Thick Outflows with Application to Light Echo Spectra from $\eta$~Carinae's Giant Eruption

TL;DR

This paper revises the wind outflow model for $\eta$ Carinae's eruption, showing that a low spectral temperature around 5000K is compatible with high mass loss rates, challenging previous assumptions and aiding interpretation of light echo spectra.

Contribution

It updates the wind outflow model with new opacity data and demonstrates that low temperatures are consistent with extreme mass loss, impacting the understanding of eruptive stellar phenomena.

Findings

Low spectral temperature (~5000K) is compatible with high mass loss in wind models.

Opacity drops sharply below 6500K due to electron recombination, affecting spectral predictions.

Results are relevant for various optically thick outflows like novae, LBV eruptions, and supernova precursors.

Abstract

The detection by Rest et al. (2012) of light echoes from $\eta$ Carinae has provided important new observational constraints on the nature of its 1840's era giant eruption. Spectra of the echoes suggest a relatively cool spectral temperature of about 5500K, lower than the lower limit of about 7000K suggested in the optically thick wind outflow analysis of Davidson (1987). This has lead to a debate about the viability of this steady wind model relative to alternative, explosive scenarios. Here we present an updated analysis of the wind outflow model using newer low-temperature opacity tabulations and accounting for the stronger mass loss implied by the $>$10 Msun mass now inferred for the Homunculus. A major conclusion is that, because of the sharp drop in opacity due to free electron recombination for $T<$6500K, a low temperature of about 5000K is compatible with, and indeed expected from, a wind with the extreme mass loss inferred for the eruption. Within a spherical gray model in radiative equilibrium, we derive spectral energy distributions for various assumptions for the opacity variation of the wind, providing a basis for comparisons with observed light echo spectra. The scaling results here are also potentially relevant for other highly optically thick outflows, including those from classical novae, giant eruptions of LBVs and SN Type IIn precursors. A broader issue therefore remains whether the complex, variable features observed from such eruptions are better understood in terms of a steady or explosive paradigm, or perhaps a balance of these idealizations.