The 18 Stars and 16 Non-stellar Sources

We identify 18 (including ObjectX/M33-1) sources as dusty stars. Of these, four are in M33 (1, 3, 4, 7), one is in NGC300 (N300-1), four are in NGC2403 (2, 3, 4, 5), five are in M81 (5, 6, 11, 12, 14), and four are in NGC7793 (3, 9, 10, 13). None are in NGC6822 (a low-mass, low SFR galaxy) or NGC247 (all three candidates turned out to be non-stellar). These stars have low optical fluxes or flux limits and their SEDs turn over between $8\,\micron$ and $24\,\micron$. Moreover, M81-11, M81-12 and N2403-2 are detected as optically variable sources in the LBT monitoring data. N2403-3 is a saturated source in the HST images, and we use the LBT flux measurements as upper limits on its optical flux. N2403-3 and N2403-5 are not variable in the LBT data. M81-5 is 056 from a variable X-ray source with maximum luminosity of $2\times10^{38}$ergss$^{-1}$ (Liu2011), which is consistent with the source being an X-ray binary (Remillard & McClintock2006). N7793-3 is also a X-ray source (Liu2011), with a maximum X-ray luminosity of $3.9\times10^{37}$erg$s^{-1}$ and is classified as an HMXB by Mineo et al. (2012).

There are 16 candidates whose SEDs indicate that they are not self-obscured stars. Five sources in M33 (2, 5, 6, 8, 9) have SEDs that nearly monotonically rise from the optical to $24\,\micron$, unambiguously indicating the presence of cold dust associated with star clusters. As we discussed in PaperI, it is unlikely for an ultra-compact star cluster to host both evolved massive stars and significant amounts of intra-cluster dust. Eight sources cannot be dust obscured stars given their very high optical luminosities: N2403-1 (likely a foreground star), M81-7, M81-10, N247-3 (likely a foreground star, optical magnitudes from GSC2.22001), N7793-4, N7793-8, N7793-11, and N7793-14. The three observed with the LBT, M81-7, M81-10 and N2403-1, are not variable. We consider three more sources as most likely non-stellar due to reasons that are unique in each case --

• N247-1 is located far from the plane of its edge-on host and is unlikely to be associated with the host.
• N7793-1 is located at the edge of its host galaxy and the PACS far-IR flux limits are significantly lower than those of the sources that we classified as obscured stars, indicating an absence of the diffuse emission commonly associated with star forming regions.
• N7793-6 has an SED that can conceivably be produced by a hot star with significant circumstellar material, although the near-IR peak seems too narrow. However, a close inspection of the HST image shows that this source is in a dense star-forming region with significant diffuse light indicating the presence of intra-cluster dust. None of the sources in the HST image are a good astrometric match to the IRAC location. It is more likely, in this case, that warm intercluster dust is producing the mid-IR flux excess. The optical fluxes adopted here are those of the most luminous HST source within a larger matching radius of $0\farcs5$.

In PaperI, we anticipated that further analysis would show that most, if not all, of the candidates are in fact non-stellar sources. Based on the expected surface density of extragalactic contaminants, of the 46 initial candidates we estimated that all but $6\pm6$ are background galaxies/AGN with 11 already being identified as such. Here we find that $18$ (including ObjectX) of the candidates are dusty massive stars and very few of the other sources are background galaxies. We do not presently have an explanation for the fewer than expected background sources in the targeted fields.