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 Car between 1840 and
1860 is the most famous case of a stellar outburst, ejecting
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
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 are easily found (Gvaramadze et al.2010; Wachter et al.2010).
The advantage of the 24
band is that it can be used to identify dusty ejecta
up to
years after its formation. A minority of these objects are very luminous stars
(L
L
) with massive (
M
) 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 (
years) than
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
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 (
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 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
).
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
L
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
L
,
M
evolved star obscured by dust formed during
mass loss events over the last
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 Car,
one of the most luminous (
L
) and massive (
)
stars known (Humphreys & Davidson1994).
No true analog of
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
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 (Mpc)
to search for analogs of
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
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
Car in the local universe even before we have completed PaperII.
Finally, in Section5 we outline the future of our approach.