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Astron. Astrophys. 329, 161-168 (1998)

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Young stellar objects and abnormal extinction within M 17

R. Chini 1 and W.F. Wargau [FORMULA] 2

1 Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstr. 150/NA 7, D-44780 Bochum, Germany
2 UNISA, Dept. of Mathematics, Applied Mathematics and Astronomy, P.O. Box 392, 0001 Pretoria, South-Africa

Received 23 December 1996 / Accepted 26 August 1997


The stellar content of M 17 is investigated by photoelectric observations from 1.2 to 4.8 [FORMULA] m 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 [FORMULA] 20.1 and 18.2 mag, respectively; the remaining 37 stars are pure IR sources with [FORMULA] mag. 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 18 mag of visual extinction. Their colours suggest a special reddening law within M 17, characterized by [FORMULA]. iii) 28 of the new IR sources can also be classified as B-type stars with up to 50 mag 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 M 17 and thus qualifies them as Class I sources. The observed luminosity of these new IR excess objects is more than 60 times larger than that found for the classical Class I 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 M 17 still surrounded by relicts of their protostellar clouds. The comparatively low visual extinction of only [FORMULA] mag 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 [FORMULA] between 2.2 and 20 [FORMULA] m, often used to determine the evolutionary stage of young stellar objects, is discussed. In the case of deeply embedded objects ( [FORMULA] mag) and wavelengths below 10 [FORMULA] m [FORMULA] may be dominated by interstellar extinction rather than circumstellar emission. A reasonable alternative in the absence of data longward of 4.8 [FORMULA] m 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 [FORMULA] and/or [FORMULA] that may trace Class I objects reliably.

Key words: interstellar medium: dust – formation of stars

Send offprint requests to: R. Chini
[FORMULA] deceased in November 1996

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© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997