1998A&A...329..161C

The stellar content of M17 is investigated by photoelectric observations from 1.2 to 4.8µm for 69 stars. Combining these data with previous UBVRI measurements we obtain the following results: i) 32 stars are visible objects on B and V plates reaching to ∼20.1 and 18.2mag, respectively; the remaining 37 stars are pure IR sources with K<12.2mag. ii) In various two-colour-diagrams 18 of the visible stars can uniquely be identified as early type members of the young stellar cluster, suffering between 6 and 18mag of visual extinction. Their colours suggest a special reddening law within M17, characterized by R=4.8. iii) 28 of the new IR sources can also be classified as B-type stars with up to 50mag of visual extinction. iv) The major result, however, is the discovery of a strong IR excess associated with 20 stars (11 visible + 9 IR objects). Their spectral energy distributions resemble those of five previously detected ``cocoon stars'' in M17 and thus qualifies them as ClassI sources. The observed luminosity of these new IR excess objects is more than 60 times larger than that found for the classical ClassI sources in Ophiuchus. We therefore suggest them to be the high mass counterparts of this early evolutionary stage. They represent the youngest generation of massive early type stars in M17 still surrounded by relicts of their protostellar clouds. The comparatively low visual extinction of only 6<A_V<27mag is probably a consequence of the strong radiation field from neighbouring O stars which has partly cleared the region. The influence of interstellar extinction on the spectral index α_IR between 2.2 and 20µm, often used to determine the evolutionary stage of young stellar objects, is discussed. In the case of deeply embedded objects (A_V>40mag) and wavelengths below 10µm α_ IR_ may be dominated by interstellar extinction rather than circumstellar emission. A reasonable alternative in the absence of data longward of 4.8µm are certain IR-colour relations which provide limits to detect IR-excess objects independent of the amount of extinction. We find empirical colour criteria of the form (K-L)>0.26(J-K)+0.25 and/or (K-M)>0.37(J-K)+0.80 that may trace ClassI objects reliably.