Discovery of Five Candidate Analogs for $eta,$Carinae in Nearby Galaxies

The late-stage evolution of very massive stars such as $eta$ Carinae may be dominated by episodic mass ejections which may later lead to Type II superluminous supernova (SLSN-II; e.g., SN 2006gy). However, as long as $eta$ 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 ($sim4-8$ Mpc) massive star-forming galaxies M51, M83, M101 and NGC6946 that have optical through mid-IR photometric properties consistent with the hitherto unique $eta$ Car. The Spitzer mid-IR spectral energy distributions of these $L_{bol}simeq3-8times10^{6} L_odot$ objects rise steeply in the $3.6-8 mu$m bands, then turn over between $8$ and $24 mu$m, indicating the presence of warm ($sim400-600$ K) circumstellar dust. Their optical counterparts in HST images are $sim1.5-2$ dex fainter than their mid-IR peaks and require the presence of $sim5-10 M_odot$ of obscuring material. Our finding implies that the rate of $eta$ 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 SLSN-II are due to ccSN occurring inside these dense shells, then the ejection mechanism is likely associated with the onset of carbon burning ($sim 10^3 - 10^4$ years) which is also consistent with the apparent ages of massive Galactic shells.