Knowing the result that each star has its own extinction law, which is therefore a local characteristic, these extinction laws will not provide additional information about the distribution of the dark material in NGC 6611. To obtain such information we have plotted values of , the foreground value being up to 1 5, for each object as it is located in NGC 6611, Fig. 8 (click here). This figure contains also the MS programme stars from Thé et al. (1990). The values used are listed in Table 7 (click here) and calculated from Table 4 of Thé et al. (1990).
Although the combined sample is not high in number (88 objects), it is clear that the most obscured objects are located near the centre (= W280) with a distribution of very high values to the north and to the northwest. Note that the field of NGC 6611, e.g. Fig. 7 (click here)a, already shows that these regions are relatively less crowded. This would indicate that most dark material is distributed towards these directions and would be the region to look for young objects or even traces of star formation.
Indeed, most embedded sources as discussed by Hillenbrand et al. (1993) are distributed towards the northeast of W280. This region would then be most important to look for on-going star formation. Hillenbrand et al. (1993) mention that more than 50% of the infrared objects located south or southeast are background objects. If we then refer to their Table 7 (click here) of embedded stars with circumstellar material, we find that many of these objects are located to the northwest of the stars with the highest values as plotted in Fig. 8 (click here). This supports our finding and indicates that most visible young objects seem to be distributed north-northwest of the centre (= W280) and on-going star formation could probably be occurring north-northeast of it.
Figure 8: Field of NGC 6111 reproduced from a Hale Observatory photograph of plates taken in H and [NII]6548, 6583 with the 200 inch Hale telescope (Osterbrock 1974). The bright HII region, M 16, is clearly visible. The central area of NGC 6611 is drawn by an overlay. We have indicated the values for the PMS-programme stars and the MS-objects of Thé et al. (1990) by an intensity scale, indicated at the right