Discovery of Five Candidate Analogs for $\hat{I}\cdot$Carinae in Nearby Galaxies

Abstract

The late-stage evolution of very massive stars such as $\hat{I}\cdot$ 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 $\hat{I}\cdot$ 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 ($\sim 4-8$ Mpc) massive star-forming galaxies M51, M83, M101 and NGC6946 that have optical through mid-IR photometric properties consistent with the hitherto unique $\hat{I}\cdot$ Car. The Spitzer mid-IR spectral energy distributions of these $L_{bol}\simeq 3-8\times 10^6{L}_{\odot }$ objects rise steeply in the $3.6-8\hat{I}¼$m bands, then turn over between $8$ and $24\hat{I}¼$m, indicating the presence of warm ($\sim 400-600$ K) circumstellar dust. Their optical counterparts in HST images are $\sim 1.5-2$ dex fainter than their mid-IR peaks and require the presence of $\sim 5-10M_{\odot }$ of obscuring material. Our finding implies that the rate of $\hat{I}\cdot$ 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.

14 pages, 3 figures, 2 tables. Accepted for publication in ApJ Letters