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Figure 1: The Spectral Energy Distributions (SED) of $\eta$Car now (blue solid line, Humphreys & Davidson1994), ObjectX (black solid line, Khan et al.2011), and M33VarA (black dashed line, Humphreys et al.2006). The black triangles mark luminosity at the IRAC band centers. Although $\eta$Car and ObjectX have similar luminosities up to 3.6$\micron$, the SED of $\eta$Car is steeply rising in the IRAC bands ($a$$\simeq 2.6$; Eqn.1) while ObjectX is almost flat ($a$$\simeq 0.2$; Eqn.1). ObjectX, and M33VarA (Humphreys et al.2006; Hubble & Sandage1953) are both dust obscured stars with comparable bolometric luminosities (Khan et al.2011), but in the IRAC bands, ObjectX is much more luminous ( $\sim 1.5\times 10^5$L$_\odot$) than M33VarA ( $\sim 0.5\times 10^5$L$_\odot$).

Figure: Integrated mid-IR luminosity $L_{mIR}$ as a function of the slope $a$ (Equation 1) for bright sources in M81. The vertical dashed lines show the slopes of blackbodies with the indicated temperatures and peak wavelengths (Equation 2). The top-right (thick red) box shows the candidate selection region ( $L_{mIR}>10^{5}\,$L$_\odot$ and $a>0$). The red triangles show the sources that also satisfy the third selection criteria, that at least 30% of the integrated mid-IR luminosity is emitted between 3.6 and 5.8$\micron$ ($f>0.3$). Of these, the open red triangles correspond to candidates that are known to be non-stellar in nature (see Sections 2.3 and 3.1), and the solid red triangles represent the surviving candidates. The green open circles show sources with $f<0.3$ and the black cross marks represent all the other sources. The narrow clump of points at $a\simeq -2.75$ correspond to normal stars with steeply falling mid-IR SEDs, while the wider clump of points to the right correspond to sources dominated by 8$\micron$ PAH emission. The top-left box shows the region $L_{mIR}>10^{5.5}\,$L$_\odot$ and $a<-1$ that was used to select normal stars in the M33 image (see Figure5). The labeled blue points represent objects not in M81 that are shown for comparison: ObjectX (``X'', solid square), the compact cluster M33-8 (``C'', open square), M33VarA (``A'', large open circle), $\eta$Car (``$\eta$", open star), the Carina nebula excluding $\eta$Car itself (``$\eta -$", solid circle; Smith & Brooks2007), and the Carina nebula including $\eta$Car (``$\eta$+", spiked open circle; see Section2.4 and Figure6).

Figure 3: Integrated mid-IR luminosity $L_{mIR}$ as a function of the fraction $f$ of $L_{mIR}$ that is emitted between 3.6 and 5.8$\micron$ for bright sources in M81. The box shows the candidate selection region ( $L_{mIR}>10^{5}\,$L$_\odot$ and $f>0.3$). The red triangles show the sources that also satisfy the third selection criteria that the mid-IR SED slope (Equations 1) is either flat or rising ($a>0$). Of these, the open red triangles correspond to candidates that are known to be non-stellar in nature (see Sections 2.3 and 3.1), and the solid red triangles represent the surviving candidates. The green open circles show sources with $a<0$ and the black cross marks represent all the other sources. The narrow clump of points at $f\simeq 0.8$ correspond to normal stars with steeply falling (negative slope) mid-IR SEDs, while the wider clump of points at $f\simeq 0.25$ correspond to sources dominated by 8$\micron$ PAH emission. The labeled blue points are same as in Figure2.

Figure 4: Extragalactic contamination for M81. Here we show all sources from a 6deg$^2$ region of the SDWFS survey transformed to the distance of M81. The symbols, lines, and axis-limits are the same as in Figure2. In this SDWFS region, 449 ($\sim 75$deg$^{-2}$) sources pass our selection criteria, indicating that we should expect $\sim 13$ background sources meeting our selection criteria given our 0.17deg$^2$ survey region around M81. Note that very few of the contaminating background sources have properties comparable to $\eta$Car.

Figure 5: Mid and far-IR SEDs of the candidates in M33 (red lines) compared to the SEDs of normal stars with $L_{mIR}>10^{5.5}$L$_\odot$, which steeply falling SEDs (mid-IR slope $a<-1$, top left box of the Figure2). The dotted portions of the SEDs correspond to the MIPS 70 and 160$\micron$ flux upper limits. The SED of ObjectX is highlighted (red-black lighter dashed line) and $\eta$Car (black heavier dashed line) is shown for comparison.

Figure 6: The SEDs of $\eta$Car (``$\eta$'', black triangles, Humphreys & Davidson1994), the Carina nebula excluding $\eta$Car itself (``$\eta$-'', blue squares, Smith & Brooks2007, and the entire dusty complex containing $\eta$Car and other massive stars including $\eta$Car (``$\eta$+'', red circles, Section2.4). The first two SEDs are spline interpolated and summed to produce the third. The SED of the compact cluster M33-8 (``C'', green dashed line, HST image in Figure10) is shown for comparison. In Figures 2, 3, and 4 we label these $\eta$, $\eta -$, $\eta +$, and ``C'' respectively.

Figure 7: SEDs of four different classes of objects that met our selection criteria: a candidate dusty star in NGC2403, a star-cluster in M33, a QSO behind M81, and a galaxy behind NGC7793. Figure10 shows IRAC and HST images of the compact cluster and the galaxy.

Figure: SEDs of sources that met our selection criteria but were rejected due to association with non-stellar sources. The dotted portions of the SEDs correspond to the MIPS 70 and 160$\micron$ flux upper limits. The SED of $\eta$Car (dashed blue line) is shown for comparison.

Figure: SEDs of sources that met our selection criteria and were not rejected due to association with non-stellar sources. The dotted portions of the SEDs correspond to the MIPS 70 and 160$\micron$ flux upper limits. The SEDs of $\eta$Car (dashed blue line) and ObjectX (dot-dashed black line) are shown for comparison.

Figure: IRAC and HST images of the compact stellar clusters M33-5, M33-8, and M81-10, and the background galaxy N7793-2. The clusters are resolved in the HST images with FWHM of $0\farcs87\simeq4.1$pc (M33-5), $0\farcs77\simeq3.6$pc (M33-8) and $0\farcs34\simeq6.1$pc (M33-8). They are very luminous (few$\times 10^7\,$L$_\odot$) and their SED shapes are very similar to $\eta$Car (Figure9).