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6. The Hertzsprung-Russell diagram of NGC 6611

As discussed in the previous sections, we finally obtained for each programme star a good SED fit including accurate estimates of the stellar astrophysical parameters, such as E(B-V), tex2html_wrap_inline6062, tex2html_wrap_inline6064 and tex2html_wrap_inline6066 values, which are collected in Table 7 (click here). With the intrinsic stellar parameters tex2html_wrap_inline6068 and tex2html_wrap_inline6070, we can construct an HRD. tex2html_wrap_inline6072 is simply estimated from the difference between the observed V, for which we used the values from the best SED fits, and tex2html_wrap_inline6076, with tex2html_wrap_inline6078.

As discussed in Sect. 5, for a few objects we have multiple possibilities in the SED fit. They will be analyzed as being separate objects. With the data of the remaining 52 objects we construct an HRD in Fig. 4 (click here). In this figure we have discriminated to tex2html_wrap_inline6080 values by plotting different symbols: a cross for stars with tex2html_wrap_inline6082 = 3.1, a square for stars with tex2html_wrap_inline6084 3.1 and circles for objects with unknown extinction characteristics: the objects which are denoted by sh.

Figure 4 (click here) also contains two stellar birthlines of Palla & Stahler (1990 and 1992), which is the boundary after which stars become optically visible when evolving towards the main sequence. The upper one is for an accretion rate on the protostar of tex2html_wrap_inline6086 = 10tex2html_wrap_inline6088 tex2html_wrap_inline6090 yrtex2html_wrap_inline6092 and the lower one for tex2html_wrap_inline6094 = 10tex2html_wrap_inline6096 tex2html_wrap_inline6098 yrtex2html_wrap_inline6100. We have also drawn the Zero Age Main Sequence of Hillenbrand et al. (1993). The birthlines and ZAMS are recalculated from the published HRDs to our observational HRD by adopting a distance of 2.6 kpc and a luminosity class III. From Fig. 4 (click here) it is evident that the distance modulus corresponding to a distance of 2.6 kpc seems to be appropriate also for the PMS sample. We therefore conclude that this estimate is correct.

Before discussing the cluster membership of the sample of objects, we will first analyze one of the main criteria for this: the HRD. We start with the assumption that all the programme stars are located at the same cluster distance.

Figure 4: The tex2html_wrap_inline6102, tex2html_wrap_inline6104 diagram for our programme stars in NGC 6611. The data points plotted as tex2html_wrap_inline6106 are of stars with tex2html_wrap_inline6108 = 3.1; those as tex2html_wrap_inline6110 with tex2html_wrap_inline6112 3.1 and those as tex2html_wrap_inline6114 are for stars with an IR-excess probably due to circumstellar dust radiation. The Groups I: tex2html_wrap_inline61160tex2html_wrap6166 1; II: tex2html_wrap_inline61200tex2html_wrap6168 3tex2html_wrap_inline61240tex2html_wrap6170 7; and III: tex2html_wrap_inline61280tex2html_wrap6172 7 are clearly separated

From the location of our 52 PMS candidates in the HRD, Fig. 4 (click here), we can separate three groups: Group I are all early-type stars, tex2html_wrap_inline61320tex2html_wrap6174 1, and lie between the MS and one of the stellar birthlines. Group II contain the stars of intermediate spectral types, tex2html_wrap_inline61360tex2html_wrap6176 3 tex2html_wrap_inline61400tex2html_wrap6178 7. They are located near or just to the right of the upper stellar birthline. Group III consists of the late-type stars with tex2html_wrap_inline61440tex2html_wrap6180 7, which are located far to the right and/or far above the stellar birthline. We shall discuss now each group separately.

Group I
The early-type stars, the majority of our sample, fit nicely to the MS as derived by #Th&Thé et al. (1990) or to the ZAMS of Hillenbrand et al. (1993), or lie somewhat to the right of it. However, located to the right of the ZAMS we find the more luminous stars more frequently than the lower luminosity ones. This can be easily understood if we accept that several of these massive stars are already moving away from the ZAMS, to become post-MS stars.

The early-type stars at the lower end of the MS are remarkable. We find that 4 out of 5 of the early-type stars located in this part of the HRD have a strong near-IR-excess, probably due to dust. Although these objects are located near the ZAMS, they can still be young enough to belong to the HAeBe group. Furthermore, we see that for Group I stars there is no dependency of the tex2html_wrap_inline6148 value for their location in the HRD.
Group II
From this very interesting group none seem to have ``normal'' IR properties, either they have large tex2html_wrap_inline6150 values or they are probably surrounded by dust. Although these objects are located to the right of the birthline for large accretion rates one can not discard them as not being PMS if they are cluster members. As shown by Palla & Stahler (1993) and in Fig. 4 (click here), it could be possible that if we change the accretion rate the location of a birthline could shift (it is discussed by Lamers et al. 1996, in preparation, that this can also happen by changing the metallicity of a star). For an accretion rate (tex2html_wrap_inline6152 tex2html_wrap_inline6154 yrtex2html_wrap_inline6156) the Group II objects can be visible in their PMS phase.
Group III
All stars in this group have a normal extinction law, are located far to the right the birthline, are of very late spectral type and are relatively luminous if we place them at the cluster distance. The objects in this group are those objects for which Chini & Wargau (1990) concluded that late-type stars are foreground objects. However, we have shown here (see Group II) that we must discriminate between two types of late-type objects, depending on their IR properties and their location in the HRD.

The exceptional but interesting location of the star at tex2html_wrap_inline6158 = 0tex2html_wrap6182 20, is the star W213(1). The classification of this star used in option W213(2) is probably most appropriate one (see Table 7 (click here) and Sect. 7.2).

From Fig. 4 (click here) we encounter a major problem: where are the stars that approach the ZAMS?, or in other words: ``where are the late A-type and F-type PMS objects?'' We return to this interesting problem later.

In order to make this discussion more conclusive, we first discuss the probabilities of cluster membership for the programme stars. After which we discuss the extinction characteristics of the cluster field. Subsequently, conclusions are drawn in combination with the properties of the HRDs.

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