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 and
.
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
ergss
(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
erg
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 , 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 --
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 are background galaxies/AGN
with 11 already being identified as such. Here we find that
(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.