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

3. Two-colour-diagrams

Two-colour-diagrams (TCDs) are a potential tool to investigate photometric data in terms of reddening and excess emission. The classical UBV diagram even allows simultaneously the determination of spectral type and reddening for stars earlier than B3 with spectroscopic accuracy. TCDs including only wavelengths m cannot clearly distinguish between intrinsic stellar colours and extinction. Vice versa, the coincidence of the stellar sequence and the reddening path allows to detect deviations from the expected loci and to examine the law of interstellar extinction in TCDs containing B and V (CK1).

TCDs constructed exclusively from NIR colours have been studied by Chini et al. (1992). The comparison of the loci of reddened stars, warm dust of 750 to 2000 K and black bodies of 1000 to 30000 K showed that the most commonly used JHK diagram has several disadvantages: i) The H -band may be contaminated by emission associated with the formation of molecular hydrogen and the absorption by in cool stellar atmospheres. ii) Emission lines as seen e.g. in TTauri stars contribute significantly at 1.6 m. iii) The reddening path and the locus of dust emission are very similar, making a discrimination between the two effects rather difficult. As shown by Aspin & Barsony (1994) from observational data in the literature, the locus of many TTauri stars and protostellar objects fall into the region of normal JHK colours. On the other hand, JKL and JKM TCDs provide a situation where the loci of main sequence stars earlier than F0 and the reddening vector are identical, thus allowing to predict the locus of reddened stars very accurately while simultaneously the locus of dust emission separates very clearly. Therefore, such diagrams are ideal for detecting anomalies in the SEDs, i.e. IR excesses due to circumstellar emission.

In order to investigate the extinction law in M 17 and to identify sources with IR excess we have plotted the photometric data in form of several TCDs. Fig. 1 shows six TCDs for 32 "visible" stars with UBVRI data from CEN and NIR data from the present work (c.f. Table 1). Only CEN 44 has been omitted from the diagrams because the UBVRI data which were taken in 1977 and the new NIR data do not fit smoothly together, indicating variability of the star between the two epochs. Furthermore we have constructed three NIR TCDs (Fig. 2) which contain both the 32 "visible" stars and the 37 "IR sources" from Table 2. As explained below in more detail, the symbols in both Figures correspond to the following classification: () 20 "visible" stars without any indication for an IR excess throughout all TCDs (including the two foreground giants CEN 0 and 10), () 11 "visible" stars with IR excesses in Figs. 1 and 2, (|) 28 pure "IR sources" without peculiar colours, () 9 sources with IR excess from the sample of "IR sources". For comparison, the "cocoon stars" () found by CK2 are included in all TCDs.

Fig. 1. Two-color-diagrams for the sample of 32 visible stars; normal stars without IR excess are marked as dots ( ), excess objects are marked as circles ( ). The cocoon stars found by CK2 are included for comparison and are shown as stars ( ). The solid lines denote the normal law of interstellar extinction, the dashed lines indicate the special reddening vectors within M 17.

Fig. 2. Two-colour diagrams for the sample of 32 visible and 37 IR stars; same notation as Fig. 1. The dotted curve represents the main sequence taken from Koornneef (1983b). IR sources without excess are shown as filled squares, those with excess as open squares. The solid lines denote the reddening path; tickmarks correspond to 10, 20, 30 and 40 mag of visual extinction (). The dashed lines indicate the relations and , respectively, which separate normal stars from IR excess objects.

© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997
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