The evolved-star dust budget of the Small Magellanic Cloud: the critical role of a few key players

Abstract

The lifecycle of dust in the interstellar medium (ISM) is heavily influenced by outflows from asymptotic giant branch (AGB) and red supergiant (RSG) stars, a large fraction of which is contributed by a few very dusty sources. We compute the dust input to the Small Magellanic Cloud (SMC) by fitting the multi-epoch mid-infrared spectral energy distributions (SEDs) of AGB/RSG candidates with models from the {\em G}rid of {\em R}SG and {\em A}GB {\em M}odel{\em S} (GRAMS) grid, allowing us to estimate the luminosities and dust-production rates (DPRs) of the entire population. By removing contaminants, we guarantee a high-quality dataset with reliable DPRs and a complete inventory of the dustiest sources. We find a global AGB/RSG dust-injection rate of $(1.3\pm 0.1)\times 10^{-6}$ \msunperyr, in agreement with estimates derived from mid-infrared colours and excess fluxes. As in the LMC, a majority (66\%) of the dust arises from the extreme AGB stars, which comprise only $\approx$7\% of our sample. A handful of far-infrared sources, whose 24 \mic\ fluxes exceed their 8 \mic\ fluxes, dominate the dust input. Their inclusion boosts the global DPR by $\approx$1.5$\times$, making it necessary to determine whether they are AGB stars. Model assumptions, rather than missing data, are the major sources of uncertainty; depending on the choice of dust shell expansion speed and dust optical constants, the global DPR can be up to $\approx$10 times higher. Our results suggest a non-stellar origin for the SMC dust, barring as yet undiscovered evolved stars with very high DPRs.