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

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4. The reddening law

Cardelli et al. (1989) have shown that the law of interstellar extinction can be described by a single parameter, i.e. the ratio of total to selective extinction [FORMULA]. In fact, the shape of the extinction curve in the IR seems to be rather uniform and independent of R (Jones & Hyland 1980, Clayton & Mathis 1988, Tapia et al. 1988, Whittet 1989). As a consequence, deviations from the normal reddening law are not detectable in TCDs constructed from NIR colours alone, but require measurements at B and V. TCDs of the form [FORMULA] vs. [FORMULA], where [FORMULA] denotes any waveband longward of V are very sensitive to detect anomalous extinction without knowing exact stellar spectral types (CK1). It must only be verified that the stars under consideration are earlier than about F0 V and free of circumstellar dust emission. In the case of M 17, the first criterion is securely fulfilled, taking into account the sensitivity of the survey by CEN: At a distance of about 2.2 kpc and a visual foreground extinction of [FORMULA] mag due to the diffuse interstellar medium between the sun and M 17, a limiting magnitude of [FORMULA] corresponds to [FORMULA] mag. This is equivalent to a F2 V star. Taking into account the local extinction within M 17 ( [FORMULA] mag) the detection limit is shifted to considerably earlier spectral types.

To check the second criterion, i.e. the absence of excess emission due to circumstellar dust, we have examined all stars throughout the TCDs in Fig. 2. In total we find 18 early type stars free of IR excess which are confined to relatively narrow lines (dashed) in the TCDs of Fig. 1. These loci have the form [FORMULA] where [FORMULA] is the colour excess ratio at the waveband [FORMULA] normalised to [FORMULA]. With increasing [FORMULA] the dashed lines deviate significantly from the normal reddening vectors indicated by the solid lines. The new values for the specific colour excess ratios [FORMULA] are summarised in Table 3 and describe the reddening law within M 17; although not explicitely shown in Fig. 1 we have also calculated the corresponding value for the R -band. The ratio of total to selective extinction is extrapolated to be [FORMULA]. These results agree with our former estimates (CEN, CK1) but this time are based on almost three times more stars and include the M -band. For comparison the extinction law for the diffuse interstellar medium (ISM) is given. Values at BVRI are taken from Schultz & Wiemer (1975), those at JHKLM are averages calculated from the results published by Koornneef (1983b) and Rieke & Lebofsky (1985); the N -value also stems from the latter authors. Theoretical dust models which fit the extinction law of M 17 have demonstrated that there exists no correlation between the ratio of total-to-selective extinction and the mean dust grain radius (CK1).

In all TCDs of Fig. 1 the lines of normal and anomalous reddening intersect at [FORMULA], corresponding to [FORMULA] for early type stars. In agreement with CEN and CK1 we interpret this result as to originate from a normal foreground extinction of about 3 mag towards M 17. We want to emphasis again that the high value of [FORMULA] does not affect the slope of NIR reddening vectors in Fig. 2: if we calculate the average locus of the 18 normal early type stars used for deriving the reddening vectors in Fig. 1 we obtain NIR colour excess ratios that are consistent with the normal interstellar values and thus with the reddening vectors in Fig. 2. This is in agreement with the results from other regions (see the references at the beginning of this Section) and corroborates the interpretation by Cardelli et al. (1989) about the uniformity of the NIR extinction law.

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© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997