Remaining Candidates

Table 5: Remaining Candidates

ID	RA	Dec	Slope	$\log_{10} { L_{mIR} \over L_\odot }$	$f$	Spectral Energy Distribution [ $\log_{10} (\lambda L_\lambda/\,$L$_\odot)$]
			($a$)			[3.6]	[4.5]	[5.8]	[8.0]	[24]	[70]	[160]
M33-1a,b	23.35015	30.42626	0.24	5.11	0.57	5.14	5.17	5.25	5.21	5.13	$<$5.78	$<$6.09
M33-2a	23.39209	30.69071	1.06	5.10	0.46	5.06	4.91	5.27	5.34	6.10	$<$6.90	$<$6.76
M33-3	23.43939	30.61357	1.98	5.10	0.39	4.87	4.69	5.31	5.42	5.51	$<$6.15	$<$6.13
M33-4	23.55650	30.56175	2.17	5.01	0.36	4.77	4.52	5.21	5.36	5.64	$<$6.44	$<$6.58
M33-5	23.31891	30.88054	2.32	5.57	0.36	5.19	5.33	5.67	5.96	6.86	$<$6.90	$<$6.54
M33-6	23.36988	30.67363	1.79	5.05	0.36	4.90	4.81	5.08	5.47	6.20	$<$6.57	$<$6.33
M33-7	23.39793	30.65805	2.33	5.08	0.36	4.76	4.63	5.28	5.43	5.73	$<$6.33	$<$5.99
M33-8a	23.50089	30.67987	2.40	5.56	0.35	5.22	5.13	5.72	5.94	6.46	$<$6.72	$<$6.50
M33-9	23.37096	30.67276	2.34	5.02	0.32	4.73	4.61	5.10	5.45	6.12	$<$6.56	$<$6.34
N300-1	13.71123	$-$37.67159	0.96	5.27	0.50	5.17	5.27	5.37	5.50	5.28	$<$6.23	$<$6.30
N2403-1a	114.32964	65.59473	1.37	5.22	0.38	5.20	5.03	5.20	5.63	5.96	$<$6.96	$<$7.37
N2403-2	114.20582	65.60922	2.00	5.11	0.36	4.93	4.68	5.25	5.49	5.31	$<$7.03	$<$7.57
N2403-3	114.09702	65.61411	2.56	5.16	0.33	4.75	4.83	5.24	5.59	5.53	$<$6.77	$<$7.33
N2403-4a	114.22210	65.59257	2.20	5.17	0.32	4.98	4.71	5.24	5.61	5.24	$<$6.71	$<$7.45
N2403-5	114.22632	65.59669	2.14	5.20	0.30	4.98	4.89	5.15	5.69	5.44	$<$6.90	$<$7.50
M81-5	148.75421	69.12405	0.61	5.01	0.52	5.01	4.99	5.13	5.20	5.22	$<$6.06	$<$7.12
M81-6	148.83128	68.95947	0.89	5.01	0.52	4.88	5.02	5.15	5.19	5.08	$<$5.50	$<$6.56
M81-7	148.72035	69.14713	0.73	5.05	0.51	5.03	5.07	5.11	5.29	5.31	$<$6.74	$<$7.07
M81-10	148.97075	68.98440	1.72	5.57	0.37	5.39	5.43	5.54	5.99	6.25	$<$7.20	$<$7.43
M81-11	149.00545	68.98351	1.71	5.10	0.37	4.95	4.89	5.12	5.51	5.20	$<$6.70	$<$7.46
M81-12	149.01479	68.98553	2.46	5.05	0.32	4.71	4.65	5.15	5.47	5.45	$<$6.70	$<$7.46
M81-14	148.66461	69.08003	2.63	5.08	0.30	4.74	4.57	5.19	5.53	5.65	$<$6.92	$<$7.28
N247-1	11.51449	$-$20.72367	1.12	5.14	0.50	4.97	5.15	5.25	5.38	5.28	...	$<$5.36
N247-3a	11.91676	$-$20.90363	1.45	5.14	0.31	5.07	5.02	4.64	5.68	5.32	$<$5.95	$<$6.33
N7793-1	359.39191	$-$32.54715	0.31	5.13	0.55	5.20	5.28	4.95	5.40	5.21	$<$5.45	$<$6.47
N7793-3	359.43268	$-$32.60958	0.77	5.06	0.47	5.13	4.94	5.13	5.35	5.21	$<$6.06	$<$7.43
N7793-4a	359.38553	$-$32.66690	0.91	5.34	0.46	5.34	5.32	5.31	5.67	5.53	$<$5.98	$<$6.36
N7793-6a	359.47568	$-$32.60091	1.58	5.14	0.38	5.05	4.85	5.22	5.51	5.43	$<$6.38	$<$7.51
N7793-8a	359.42096	$-$32.65040	1.65	5.21	0.33	5.20	4.83	5.19	5.67	5.24	$<$6.09	$<$6.68
N7793-9	359.46133	$-$32.58006	2.16	5.05	0.33	4.83	4.65	5.14	5.47	5.22	$<$6.94	$<$7.46
N7793-10	359.41776	$-$32.61156	2.23	5.19	0.32	4.99	4.68	5.30	5.61	5.54	$<$6.67	$<$7.43
N7793-11	359.41071	$-$32.60335	2.48	5.19	0.31	4.89	4.72	5.30	5.62	5.79	$<$6.96	$<$7.50
N7793-12	359.48233	$-$32.60726	2.34	5.02	0.31	4.73	4.65	5.06	5.47	5.54	$<$6.46	$<$7.65
N7793-13	359.42245	$-$32.59289	3.08	5.23	0.31	4.72	4.64	5.41	5.64	5.77	$<$6.93	$<$7.60
N7793-14	359.47440	$-$32.57985	2.73	5.16	0.30	4.78	4.61	5.29	5.59	5.48	$<$6.59	$<$7.32

aIdentified as point sources in the 2MASS catalog (Cutri et al.2003). See Table6 for near-IR magnitudes. bM33-1 is ObjectX from Khan et al. (2011).

Table 6: Optical/NIR Luminosities of Stellar Candidates with 2MASS Counterparts

ID	RA	Dec	$U$	$B$	$V$	$R$	$I$	$J$	$H$	$K$
M33-1	23.35015	30.42626	$\gtrsim24.1$	$\gtrsim24.2$	23.15	21.61	19.99	17.07	15.04	13.6
M33-2	23.39209	30.69071	20.61	21.84	21.14	20.13	20.48	15.96	14.68	14.02
M33-8	23.39026	30.69038	18.89	19.85	18.81	17.94	17.68	16.15	15.49	14.25
N2403-1	114.32964	65.59473	...	...	...	...	...	14.89	14.42	14.21
N2403-4	114.22210	65.59257	...	...	...	...	...	17.21	16.05	14.45
N247-3	11.91676	$-$20.90363	...	...	...	...	...	15.73	14.79	14.58
N7793-4	359.38553	$-$32.66690	...	...	...	...	...	16.30	15.59	15.17
N7793-6	359.47568	$-$32.60091	...	...	...	...	...	16.45	15.98	15.58
N7793-8	359.42096	$-$32.65040	...	...	...	...	...	16.07	15.56	14.14

The mid-IR luminosities, slopes, and the fractions $f$ of the 35 remaining candidates, including ObjectX, are presented in Table5. Of these, 9 also have 2MASS photometry and their near-IR magnitudes are presented in Table6 along with the $UBVRI$ magnitudes of the closest optical counterparts for the three candidates in M33 for which there is a publicly available catalog (Massey et al.2007). We select ObjectX (Khan et al.2011) as a candidate (M33-1) but not M33VarA (Humphreys et al.2006; Hubble & Sandage1953). Although both are dust obscured stars with comparable bolometric luminosities and exist in our initial source list, it is apparent from Figure1 that in the IRAC bands ObjectX is much more luminous ( $\sim1.5\times10^5 \,$L$_\odot$) than M33VarA ( $\sim0.5\times10^5 \,$L$_\odot$). In fact M33VarA recently emerged from a $\sim50$year dust obscured phase and is fading in the IRAC bands (Humphreys et al.2006), and we would probably have selected it in observations at an earlier epoch.

It is apparent from the SEDs of the candidates (Figures9) that few have mid-IR luminosities comparable to $\eta$Car. Moreover, the SEDs of many sources, such as N2403-2, N247-3 and N7793-10, appear so dissimilar from the SEDs of $\eta$Car and ObjectX that they seem unlikely to be stellar sources. The MIPS band luminosity limits are also useful here. For example, the relatively low MIPS 70 and 160$\micron$ luminosity limits indicate that N247-1 must have a very flat or falling SED at these redder wavelengths and is probably an AGN, which is also suggested by its distance from the galaxy.

On the other hand, the SEDs of M33-5, M33-8 and M81-10 are similar to $\eta$Car while N300-1, M81-5, M81-6 and M81-7 are similar to ObjectX (M33-1). HST images show that M33-5, M33-8, and M81-10 are compact star clusters (Figure10, top three panels). M81-5 is 056 from a variable X-ray source with maximum luminosity of $2\times10^{38}$ergss$^{-1}$, which is consistent with the source being an X-ray binary (Remillard & McClintock2006). M81-7 has been classified as a galaxy by the SDSS survey but with a photometric redshift $z=0.00049$ comparable to that of M81 (Abazajian et al.2009). HST images show a region of enhanced star formation consisting of at least two components. Ancillary data also shows that N7793-3 is a High-Mass X-ray Binary (Mineo et al.2012) with maximum X-ray luminosity of $3.9\times10^{37}$ergss$^{-1}$ (Liu2011). Although we do not currently have an explanation, 5 of the candidates in M33 (M33-3 through M33-7) are within $\lesssim2\farcs0$ of radio-selected supernova remnants (Gordon et al.1999).