We present a Spitzer /IRS low-resolution mid-infrared (mid-IR) spectrum (5–14 μm) of the luminous transient discovered in the nearby galaxy NGC 300 in 2008 May. This transient had peak luminosity M V ≃ −13, showed an optical spectrum dominated by relatively narrow Balmer and Ca ii lines in emission, and its progenitor was identified in pre-explosion images as a dust-enshrouded ∼10 M ☉ star, characteristics that make it a twin of SN 2008S. The Spitzer spectrum, obtained three months after discovery, shows that the transient is very luminous in the mid-IR. Furthermore, the spectrum shows strong, broad emission features at 8 μm and 12 μm that are observed in Galactic carbon-rich proto-planetary nebulae. Combining these data with published optical and near-IR photometry obtained at the same epoch, we find that the mid-IR excess traced by the Spitzer spectrum accounts for ∼20% of the total energy output. This component can be well explained by emission from ∼3 × 10 −4 M ☉ of pre-existing progenitor dust at temperature T ∼ 400 K. The spectral energy distribution of the transient also shows a near-IR excess that can be explained by emission from newly formed dust in the ejecta. Alternatively, both the near-IR and mid-IR excesses can together be explained by a single pre-existing geometrically thick dust shell. In light of the new observations obtained with Spitzer , we revisit the analysis of the optical spectra and kinematics, which were compared to the massive yellow-hypergiant IRC+10420 in previous studies. We show that proto-planetary nebulae share many properties with the NGC 300 transient and SN 2008S. We conclude that even though the explosion of a massive star ( M ≳ 10 M ☉ ) cannot be ruled out, an explosive event on a massive ( M ∼ 6–10 M ☉ ) carbon-rich AGB/super-AGB or post-AGB star is consistent with all observations of the transients and their progenitors presented thus far.
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