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

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1. Introduction

One of the most luminous HII regions in the Galaxy is M 17 with [FORMULA] L [FORMULA] at a distance of 2.2 kpc. The stellar content of M 17 and in particular its ionizing sources were a puzzle for many decades and have been the subject of several investigations. Due to the large amount of visual extinction only two OB stars, associated with the optical nebula could be identified by Schulte (1956), although the number of Lyman-continuum photons of [FORMULA] s-1 (Felli et al. 1984) requires about 6 early O-type stars. Beetz et al. (1976) found an embedded star cluster by imaging the highly obscured SW region of the optical nebula at 0.92 [FORMULA] m; the location of this star cluster was coincident with the peak of IR and radio emission. Chini et al. (1980, hereafter CEN) investigated the cluster members by UBVRI photometry and found sufficient early type stars to account for the IR- and Lyman-continuum flux. Recently, the problem of ionizing sources was re-addressed by Hanson & Conti (1995) by means of K -band spectroscopy; the number of early O-type stars identified in that study also matches well the observed Lyman-continuum flux. Therefore, the excitation problem of M 17 can be regarded as solved.

The UBVRI photometry by CEN indicated an abnormal extinction law within M 17. Subsequent NIR photometry by Chini & Krügel (1983, hereafter CK1) showed that the ratio of total to selective extinction [FORMULA] was [FORMULA]. Likewise, these NIR studies revealed the presence of a number of IR excess candidates whose spectral energy distributions (SEDs) rise steeply until 20 [FORMULA] m, suggesting the presence of circumstellar dust (Chini 1982, Chini & Krügel 1985, hereafter CK2). JHK imaging of the cluster field by Lada et al. (1991) corroborated the finding that the IR colours of many cluster members are not consistent with purely reddened stellar photospheres but may be due to circumstellar emission. We have started a longterm IR study using photoelectric and CCD techniques from 1.2 to 20 [FORMULA] m to re-address the problem of abnormal reddening and to investigate the nature of IR excess objects in M 17. The present paper describes the results from the photoelectric studies at 1.2 to 4.8 [FORMULA] m. We concentrate on the derivation of the abnormal extinction law and on the identification of new IR excess sources. In a subsequent paper we will discuss the SEDs of these IR sources by means of photometry from 1.2 to 20 [FORMULA] m and CCD data at JHK from different epochs.

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© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997