Introduction

Despite being very rare, massive stars such as luminous blue variable (LBVs), red super giants (RSGs), and Wolf-Rayet stars (WRs) play a pivotal role in enriching the interstellar medium (ISM) through mass loss (e.g., Maeder1981). Understanding the evolution of these massive (M$\gtrsim30$M$_\odot$) stars is challenging even when mass loss is restricted to continuous winds (e.g., Fullerton et al.2006), but poorly understood impulsive mass ejections are probably an equally important, if not dominant mass loss mechanism (Kochanek2011b; Humphreys & Davidson1984; Smith & Owocki2006). Mass loss also determines the structure of the star at death and hence the observed properties of the final core-collapse supernova (ccSN). In addition, there is also evidence that some supernova (SN) progenitors undergo major mass ejection events shortly before exploding (e.g., Smith et al.2008; Gal-Yam et al.2007), further altering the properties of the explosion and implying a connection between some eruptive mass-loss events and death (Chevalier & Irwin2012; Gal-Yam et al.2007; Kochanek2011a; Smith & McCray2007). In two cases, eruptions were observed shortly before the ccSN: TypeIb SN2006jc was spatially coincident with a bright optical transient that occurred in 2004 (Pastorello et al.2007), and SN2009ip underwent a series of outbursts in 2009, 2010, and 2011 before probably exploding as a TypeIIn SN (see Pastorello et al.2012; Mauerhan et al.2012; Prieto et al.2012). These processes are likely metallicity dependent (Heger et al.2003; Meynet et al.1994) and there is good evidence that the SNe requiring a dense circumstellar medium (the hyperluminous TypeIIn) predominantly occur in lower metallicity galaxies (e.g., Neill et al.2011; Stoll et al.2011).

Traditional studies of these massive stars search for them optically and then characterize them spectroscopically (e.g., Bonanos et al.2010; Clark et al.2012; Bonanos et al.2009). This approach is not ideal for probing the episodes of major mass-loss because of dust formation in the ejecta. Dense winds tend to form dust, although for hot stars the wind must be dense enough to form a pseudo-photosphere in the wind (Davidson1987) that shields the dust formation region from the UV emission of the star (Kochanek2011b). The star will then be heavily obscured by dust for an extended period after the eruption (see, e.g., Humphreys & Davidson1994). The great eruption of $\eta$Car between 1840 and 1860 is the most famous case of a stellar outburst, ejecting $\sim10 M_{\odot}$ material before reappearing as a hot star in the 1950s (see, e.g., Humphreys et al.1997). The ejecta are now seen as a dusty nebula around the star absorbing and then reradiating $\sim90\%$ of the light in the mid-IR. This means that dusty ejecta are a powerful and long-lived signature of eruption. The emission from these dusty envelopes peaks in the mid-IR with a characteristic red color and a rising or flat spectral energy distribution (SED) in the Spitzer IRAC (Fazio et al.2004) bands.

In the Galaxy, stars with resolved shells of dust emission primarily at 24$\micron$ are easily found (Gvaramadze et al.2010; Wachter et al.2010). The advantage of the 24$\micron$ band is that it can be used to identify dusty ejecta up to $10^3 - 10^4$years after its formation. A minority of these objects are very luminous stars (L $\gtrsim10^{5.5}\,$L$_\odot$) with massive ($\sim0.1-10\,$M$_\odot$) shells (see summaries by Humphreys & Davidson1994; Smith2009; Smith & Owocki2006; Humphreys et al.1999; Vink2012). These include AGCar (Voors et al.2000), the Pistol Star (Figer et al.1999), G79.29$+$0.46 (Higgs et al.1994), Wray17$-$96 (Egan et al.2002), and IRAS18576$+$0341 (Ueta et al.2001). These systems are far older ($\gtrsim10^3$years) than $\eta$Car, which makes it difficult to use the ejecta to probe the rate or mechanism of mass-loss. Still, the abundance of Galactic shells implies that the rate of $\eta$Car-like eruptions is on the order of a modest fraction of the ccSN rate (Kochanek2011b). Their emission peaks in the shorter IRAC bands when they are relatively young ($\sim10-100$years) because, as the ejected material expands, the dust becomes cooler and the emission shifts to longer wavelengths (Kochanek et al.2012a). It is difficult to quantify searches for such objects in our Galaxy as it is difficult to determine the distances to the sources and the survey volume because we have to look through the crowded and dusty disk of the Galaxy. Surveys of nearby galaxies are both better defined and build larger samples of younger systems whose evolution can be studied to better understand the mechanism.

With Spitzer it is difficult to use the 24$\micron$ observations that have proved so successful in the Galaxy because of the poor angular resolution. However, we have shown that such surveys can be done with IRAC (3.6-8.0$\micron$). In Thompson et al. (2009) and Khan et al. (2010), we characterized the extreme AGB star populations that appear to be the progenitors of the SN2008S-like transients (Prieto et al.2008; Prieto2008) using archival IRAC images of nearby galaxies. These studies empirically confirmed that these $\sim10^{4.5}$L$_\odot$ dusty stars are rare but are also relatively easy to identify in IRAC images despite the modest angular resolution. Next, we examined all the other bright, red mid-IR sources in M33, and in Khan et al. (2011) we discovered ObjectX, the brightest mid-IR star in M33. ObjectX is a L $_{bol}\sim5\times10^5$L$_\odot$, M $\gtrsim 30 M_{\odot}$ evolved star obscured by dust formed during mass loss events over the last $\sim1$century. Its properties are similar to those of the Galactic OH/IR star IRC+10420 (Blöcker et al.1999; Humphreys et al.2002; Humphreys et al.1997), which has a complex dusty circumstellar structure resulting from episodic, low-velocity mass ejections. We proposed that ObjectX may emerge from its current ultra-short evolutionary phase as a hotter post-RSG star analogous to M33VarA (Humphreys et al.2006; Hubble & Sandage1953).

While ObjectX is intriguing, it likely underwent a period of enhanced, but relatively steady, mass loss from the parent star rather than the short transient episode of mass loss usually associated with so called ``supernova impostors'' (Van Dyk et al.2000). It is also an order of magnitude less luminous and several times less massive than $\eta$Car, one of the most luminous ( $L_{bol}\sim5\times10^6$L$_\odot$) and massive ( $M\simeq 100-150 M_{\odot}$) stars known (Humphreys & Davidson1994). No true analog of $\eta$Car in mass, luminosity, energetics, mass lost and age has been found (see Smith et al.2011; Kochanek et al.2012a). Quantifying the population of $\eta$Car analogs, or their rarity, in the local universe can allow us to investigate the rate of giant eruptions of the most massive stars. It may also help us answer open questions about the evolution of massive stars such as: (1) the frequency of major mass ejection events, (2) the number of events per star, (3) whether the frequency depends on the metallicity or other stellar properties, and (4) whether there is really any relation between mass ejections and the so called ``supernova impostors'' (see Smith et al.2011; Kochanek et al.2012a).

Here we carry out a pilot study of 7 nearby galaxies within ($\lesssim4$Mpc) to search for analogs of $\eta$Car. We concentrate on galaxies with recent star formation, as only these will have large numbers of the short-lived, very massive stars that we want to study, but we also include one small, low-mass galaxy (NGC6822) as a test case (see Table1). Section2 describes our methodology for identifying potential $\eta$Car analogs in nearby galaxies using archival Spitzer data and sources of contamination. Section3 discusses the nature of the candidates, although a detailed study is deferred to PaperII. Section4 shows how our search method allows us to quantify the selection criteria and to set an interesting limit on the rate of events similar to the Great Eruption of $\eta$Car in the local universe even before we have completed PaperII. Finally, in Section5 we outline the future of our approach.