Abnormal extinction and pre-main sequence stars in M 16 (NGC 6611).

Photometric observations of the young stellar cluster NGC 6611 (M 16) over the wavelength range from 0.3 micron to 3.7 microns provide further evidence for the abnormal extinction of dense interstellar dust within this region. The ratio of total to selective extinction is confirmed to be R = 4.8. Deviations from the normal extinction law occur only at wavelengths shorter than 0.55 microns. By means of their special reddening law and on the basis of their location in various two-color diagrams, cluster members can be uniquely distinguished from foreground stars: the latter ones are only affected by the normal dust of the diffuse interstellar medium and thus follow the usual reddening path, whereas all early type stars (O5-B5) in M 16 are aligned along a separate relation, indicating that they are embedded within the abnormal dust of the H II region. Using this criterion to discriminate between members and nonmembers of the cluster, all 'premain sequence' stars identified as such in the classical investigation by Walker turn out to be only reddened due to normal dust, suggesting that they are foreground objects.