Global Properties of Sources

We now compare our absolute magnitudes and colors of the NGC 1569 WR stars (S sources; Table 2) to averaged Large Magellanic Cloud (LMC) WR stellar magnitudes and colors taken from [Feitzinger & Isserstedt(1983)]. The comparison with LMC stars is a logical assumption since NGC 1569 is classified as Im and their metallicities are similar. Average S absolute visual magnitudes (M$_V$ = -7.43) best agree with average late-type WN magnitudes (M$_V$ = -6.26 & (B-V)$_o$ = -0.07; Feitzinger & Isserstedt 1983). The colors of the S sources ((B-V)$_o$ = -0.2) are comparable with early-type WN stars (M$_V$ = -4.39 & (B-V)$_o$ = -0.22; Feitzinger & Isserstedt 1983). However, the uncertainties due to high reddening and to measured magnitudes allow for the agreement of colors of the LMC WNL WR stars and S sources. Therefore, we hypothesize that these S sources are similar to LMC WNL stars. Spectroscopy of the individual sources would need to be performed in order to classify each WR star.

The He II luminosity of each S source was checked to Galactic and LMC WR stellar luminosities (2.6$\times10^{35}$ - 2.9$\times10^{36}$ ergs s$^{-1}$; Drissen et al. 1999 and references therein). For reference, the average He II luminosity for an early-type WN (WNE) WR star is 5.2$\pm$2.7$\times$10$^{35}$ ergs s$^{-1}$ and 1.6$\pm$1.5$\times$10$^{36}$for a late-type WN WR star (Schaerer & Vacca 1998 and references therein). We find all S sources to have similar luminosities to the Galactic and LMC WR stars with S3 (L(He II) = 4.76$\times$10$^{35}$ ergs s$^{-1}$) at the lower end of the luminosities and S2 (L(He II) = 2.04$\times$10$^{36}$ ergs s$^{-1}$) at the top. S3 and S4 have He II luminosities similar to the average WNE He II luminosity reported by [Schaerer & Vacca(1998)], and all other S sources are 1$\sigma$ outside of this value. [Massey & Hunter(1998a)] find that 30 Doradus has several O3If* stars which were thought to be WN stars in previous surveys. These stars also emit He II and have absolute visual magnitudes similar to our S sources. However, the He II equivalent width of Of stars is typically around 5-10 Å [Nota et al.(1996)], and [Crowther & Dessart(1998)] state a He II luminosity of 1.7$\times$10$^{35}$ ergs s$^{-1}$ for one O3If*/WN star. Schaerer & Vacca (1998 and references therein) also report a He II luminosity for Of stars of 2.5$\times$10$^{35}$ ergs s$^{-1}$. This amount is a factor of 2 smaller than the faintest S source in He II. Furthermore, the detection of O3 stars would suggest an age $\sim$ 1 Myr [Massey & Hunter(1998a)], which contradicts the most recent ages calculated for the clusters in NGC 1569 (4-6 Myr; Hunter et al. 2000). Also, our sensitivity limit of three times the standard deviation of the mean background gives a minimum He II luminosity of 2.4$\times$10$^{35}$ ergs s$^{-1}$ which places the weakest WR stars and Of stars at our detection threshold. We must conclude that the S sources are Wolf-Rayet stars (most likely WN stars), but spectroscopy will have to be performed to determine their spectral type.

From Figure 2 we see that the stellar sources are concentrated close to SSC A. This supports the idea that the most recent starbursting region is primarily concentrated around the super-star clusters A & B. The number of WR stars surrounding SSC A is greater compared to SSC B (5 versus 1). [González-Delgado et al.(1997)] did not find WR signatures in the spectra around SSC B and Kobulnicky and Skillman (1997; cf. their Figure 1) have spectra over these locations and over most of the stellar sources listed here. They find nebular He II in those spectra only. This leads us to suspect that some of the point sources and He II emission are coincidental, or that the star is photoionizing the interstellar medium to produce He II. However, Conti (1999) brings up a valid point in that some starburst galaxies with the same age do not show WR stars and this could be explained by the fact that continuum dilution of the background starlight could mask over the broad, faint WR emission lines. Since these spectra came from ground-based observatories and the slit used in both papers was wide ($\geq$ 1$\farcs$5), it is possible that continuum dilution overwhelmed the He II emission from these WR stars in one or both cases. Finally, the Wolf-Rayet star spectroscopically discovered by Drissen & Roy (1994) is our S7.

Four of the clusters are also found (see Figure 2) in the largest region of star formation. One cluster is on the Eastern outskirts of the large star formation region. The four clusters within the starburst show both red stars and WR stars (see Figure 5). The reason we know that there are red stars comes from Drissen et al. (1999). They reported that F469N - F555W would produce ``holes" where red stars were, and typically there are ``holes" at the center of each cluster. Thus, the red stars are separated from the He II emission. This was first reported in De Marchi et al. (1997) as a superposition of two clusters which make up SSC A. In Figure 1 of their paper, they label the West cluster SSC A1 and the East SSC A2 (the orientation of their figure and ours is roughly the same; see our C4 in Figure 5d). SSC A1 is slightly redder than SSC A2. In our He II contours, this splitting is also seen. We find that SSC A2 is where the strongest He II emission is while SSC A1 is centered around the ``hole" in the He II contours. This detection of red supergiants along with Wolf-Rayet stars has been well reported in the past and is consistent with the evolution of massive stars (e.g., Massey 1999).

The locations of the ``unknown sources" are plotted in Figure 2. The three U sources surround SSC A. This region is filled with He II emission which is similar in surface brightness and may be related to stellar wind-shocks, or even supernova(e). Therefore, it is likely that these He II emissions are nebular in origin. Using the Galactic and LMC He II luminosities above, we see that the He II emission of the unknown sources is comparable (see our Table 4).