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Astron. Astrophys. 362, 310-324 (2000)

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Appendix A: building the 160 nm ISRF map of N 66

We first determined the emission of the stars of N 66 at 160 nm from their magnitudes and spectral types, using the (160 nm - U) colors tabulated in Nandy et al. (1976). We considered all the 88 OB stars catalogued by Massey et al. (1989) except a few for which a spectral type cannot be assessed. 42 of these stars have spectral types given in Massey et al. (1989). For the remaining stars we evaluated the spectral type from their (B-V) and (U-B) colors and from the catalog of Azzopardi & Vigneau (1975). The colors have been corrected for reddening assuming E(B-V)=0.14 for the stars in the N 66 cluster (Massey et al. 1989) and E(B-V)=0.09 for stars catalogued in Azzopardi and Vigneau (1975), the latter value being the mean one for the field stars in the SMC (Garmany et al. 1987). Once a 160 nm flux was assigned to each star, we calculated the radiation density at this wavelength in the region. In order to derive the geometry of the OB association, we had to eliminate the contribution of the diffuse emission from the optical image. We have thus evaluated in the DSS image the emission contribution at five different spatial frequencies. The image corresponding to the smallest spatial frequency (2") represents the stars themselves. We fitted the stellar density distribution by an elliptical gaussian profile for the stars belonging to the OB association. We then assumed that the depth of the cluster is equal to the minor axis of this profile, and assigned to each star a random depth coordinate so that the result fits the chosen radial distribution. For the rest of the stars we did not take into account their depth distribution which anyway is unknown, and assumed that they are all at the same distance. This has little consequence as they are isolated or in small compact groups. We then calculated the 160 nm 3-D radiation density smoothed in cubes whose projection on the plane of the sky correspond to the DSS pixel size (1.7").

Appendix B: stars detected in the N 66 field

A number of stars are visible and identified in the LW2 (5.0-8.0 µm) image displayed Fig. 5. The fluxes we measure are given in Table B.1 together with optical photometry from Massey et al. (1989). For the two red supergiants N 346-283 and 811, the flux ratio LW2/V (6.75/0.55 µm) is about 0.4, compared to the ratio of 0.2 obtained for the red supergiant WOH 53 in the LMC (Contursi et al. 1998). The difference is not very significant and we cannot say if we observe the photospheric emission of a M supergiant at an approximate effective temperature of 3000 K, or a hotter supergiant with some circumstellar emission. A deeper optical and near-IR study is necessary to solve this ambiguity.


Table B1. Stars visible in the LW2 image and in Massey et al. (1989)

The mid-IR emission of the hotter stars is much too strong to be photospheric, and must be either circumstellar or due to some interstellar material heated by the star. All these emissions are unresolved by our observations. In the case of star N 346-593 which is the brightest of the 7 (or more) exciting stars of N 66A, the mid-IR emission is clearly due to interstellar dust heated by the star. In the other cases the emission might be circumstellar since the stars are apparently isolated and in relatively gas-free regions. The most interesting case (the only one for which we have CVF observations) is that of HD 5980. In this direction, the AIBs at 8.6, 11.3 and 13.5 µm are visible while they are absent in the surrounding area. This indicates carbon-rich dust around the star. It is dubious that this dust is circumstellar, since the star is classified OB?+WN. It is however a very peculiar object which deserves more observations before definitive conclusions can be reached.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100