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

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2. Observations

The JHKLM data were obtained at the 1.9 m telescope of the South African Astronomical Observatory (SAAO) in Sutherland during June 1992 and October 1995. We scanned an area of about 4´ x 4´ at K with a 12" aperture, thus covering the entire cluster field investigated by CEN. 69 sources brighter than [FORMULA] mag were found and subsequently observed at JHKLM ; the "cocoon stars " found by CK2 were also re-measured but their photometry is discussed elsewhere (Chini et al. in prep., hereafter CKC). We used the facility IR photometer (Mk III) and standard reduction methods; calibration stars were taken from the lists of Koornneef (1983a) and Glass (1974). The photometric results are summarized in Tables 1 and 2 together with the positions of the stars. These were determined from the I -band plates of CEN and from our K -band CCD frames (CKC); the positional accuracy is better than 1". For reasons of clarity we have devided the sample into "visible stars" (designation: CEN) and "IR sources" (designation: IRS) to distinguish between those stars visible on the B and V -plates of CEN and newly discovered IR sources, 9 of which are, however, also present on the R and I-plates of CEN.


[TABLE]

Table 1. Photometry of visible stars in M 17


[TABLE]

Table 2. Photometry of IR sources in M 17


For the photometry we used apertures of 6" and 9"; the chopper throw was selected for each star separately by checking on the I -plates of CEN that the reference position did not contain further stars. Depending on the brightness of the objects, the photometric accuracy is about 0.05 - 0.10 mag at JHK and 0.10 - 0.20 mag at L and M as estimated from repeated measurements for some of the stars. Table 1 contains 11 stars (CEN 1, 2, 3, 14, 16, 18, 26, 27, 40, 57, 61) which have been observed previously at JHKL (Chini 1982, CK1). Apart from CEN 14, 26, 27, 37 and 40 which seem to be variable stars, the new results agree fairly well with our former photometry; a few measurements where the deviations are slightly larger than 0.2 mag are probably due to different apertures and/or chopper throws in combination with the crowded field and large variations of the extended background emission. To give a complete view of the SEDs and to facilitate the access to the optical measurements we have included in both tables the UBVRI data by CEN.

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© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997
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