Discovery of Five Candidate Analogs for η Carinae in Nearby Galaxies

Abstract

The late-stage evolution of very massive stars such as η Carinae may be dominated by episodic mass ejections that may later lead to Type II superluminous supernova (SLSN-II; e.g., SN 2006gy). However, as long as η Car is one of a kind, it is nearly impossible to quantitatively evaluate these possibilities. Here, we announce the discovery of five objects in the nearby (∼4-8 Mpc) massive star-forming galaxies M51, M83, M101, and NGC 6946 that have optical through mid-infrared (mid-IR) photometric properties consistent with the hitherto unique η Car. The Spitzer mid-IR spectral energy distributions of these L_bol ≃ 3-8 × 10⁶ L_⊙ objects rise steeply in the 3.6-8 μm bands and then turn over between 8 and 24 μm, indicating the presence of warm (∼400-600 K) circumstellar dust. Their optical counterparts in HST images are ∼1.5-2 dex fainter than their mid-IR peaks and require the presence of ∼5-10 M_⊙ of obscuring material. Our finding implies that the rate of η Car-like events is a fraction f = 0.094 (0.040 < f < 0.21 at 90% confidence) of the core-collapse supernova (ccSN) rate. If there is only one eruption mechanism and Type II superluminous supernovae are due to ccSNe occurring inside these dense shells, then the ejection mechanism is likely associated with the onset of carbon burning (∼10³-10⁴ years), which is also consistent with the apparent ages of massive Galactic shells.