FINDING η CAR ANALOGS IN NEARBY GALAXIES USING SPITZER. I. CANDIDATE SELECTION

The late-stage evolution of the most massive stars such as eta Carinae is controlled by the effects of mass loss, which may be dominated by poorly understood eruptive mass ejections. Understanding this population is challenging because no true analogs of eta Car have been clearly identified in the Milky Way or other galaxies. We utilize Spitzer IRAC images of seven nearby (less than or similar to 4 Mpc) galaxies to search for such analogs. We find 34 candidates with a flat or rising mid-IR spectral energy distributions toward longer mid-infrared wavelengths that emit >10(5) L-circle dot in the IRAC bands (3.6 to 8.0 mu m) and are not known to be background sources. Based on our estimates for the expected number of background sources, we expect that follow-up observations will show that most of these candidates are not dust enshrouded massive stars, with an expectation of only 6 +/- 6 surviving candidates. Since we would detect true analogs of eta Car for roughly 200 years post-eruption, this implies that the rate of eruptions like eta Car is less than the core-collapse supernova rate. It is possible, however, that every M > 40 M-circle dot star undergoes such eruptions given our initial results. In Paper II we will characterize the candidates through further analysis and follow-up observations, and there is no barrier to increasing the galaxy sample by an order of magnitude. The primary limitation of the present search is that Spitzer's resolution limits us to the shorter wavelength IRAC bands. With the James Webb Space Telescope, such surveys can be carried out at the far more optimal wavelengths of 10-30 mu m, allowing identification of eta Car analogs for millennia rather than centuries post-eruption.