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8. The nature of the anomalous extinction inNGC 6611

For each programme star the final SED fit yielded an individual extinction law correlated to one unique tex2html_wrap_inline6500 value as listed in Steenman & Thé (1991). Although it could be well possible that other sets of extinction laws, when available, could produce other tex2html_wrap_inline6502 values (note that the extinction laws used are normalized to E(B-V)), however, the important conclusion will remain that the extinction law changes from star to star. This is in contradiction with an averaged value valid for the whole cluster or parts of it as discussed in the introduction and confirms a similar conclusion by Thé et al. (1990).

Figure 6: Histogram of tex2html_wrap_inline6506 values. Part of the first column are the sh cases, for which we adopted tex2html_wrap_inline6508 3.1, and possible foreground (PF) objects

As a next step it would be interesting to investigate whether the different extinction laws depend on the star's location in the cluster and to see whether they are connected to the spectral type.

To show the total ``amount'' of anomalous extinction, a histogram is drawn for the tex2html_wrap_inline6510 values from Table 7 (click here). The first interval was chosen to be tex2html_wrap_inline6512, based on the normal extinction law with tex2html_wrap_inline6514 = 3.1 tex2html_wrap_inline6516 0.1 (see also Savage & Mathis 1979). The following intervals all have a range of 0.3 (a value slightly larger than the error spread), with statistics up to tex2html_wrap_inline6518 = 5.0. The results are drawn in Fig. 6 (click here).

More than 50% of our objects have normal extinction. And besides two exceptions, the tex2html_wrap_inline6520 value is not extreme, in the sense tex2html_wrap_inline6522. Note that we have distinguished in the first interval of Fig. 6 (click here) the sh cases, for which we have adopted a normal extinction law, and the (probable) noncluster members from Table 8 (click here). This reduces the total number of cluster objects with a normal extinction law from 32 to 11. We also have to notice that some stars have been counted double, as we treated different options as being different objects. Because of the different extinction laws found for these cases, the statistics will not change much. In this respect, we mention that the two cases with extreme tex2html_wrap_inline6524 values (W273 and W374) are included here and not their sh options as discussed in Sect. 5.

Figure 7: Field of NGC 6111 reproduced from Fig. 9 & Table 1 of Kamp (1974), a). The central 8tex2html_wrap6554tex2html_wrap_inline65288tex2html_wrap6556 area is enlarged in figure b). The PMS programme stars (indicated by crosses), as listed in Table 6, and the objects studied in Thé et al. (1990, diamonds) are indicated by their Walker number and by their individual tex2html_wrap_inline6532 value. Objects without a Walker number, the open dots, are other field stars. The scales are given in arcmin and centered on star W280 (tex2html_wrap_inline6534)

What remains besides the 11 objects with normal extinction, are 14 objects with a slightly different extinction law and 11 with very different extinction laws. No peak around some tex2html_wrap_inline6536 value seems to be present. This would indicate a more or less equal anomalous extinction, throughout NGC 6611.

To know the anomalous extinction distribution in NGC 6611, quantitative and qualitatively, we used Figs. 1 (click here)-2 (click here) of Walker (1961) and Fig. 9 (click here) and Table 1 of Kamp (1974) of the NGC 6611 field. The latter is reproduced in Fig. 7 (click here) in which our programme stars are indicated. In Fig. 7 (click here) the tex2html_wrap_inline6538 value for each of our sample stars is also given together with those of Thé et al. (1990), the main-sequence cases.
- First we consider the objects which exhibit a normal extinction law. All of those that are indicated as (probably) noncluster members in Table 8 (click here) occupy the eastern side of the central area and the outskirts of the cluster. They are all of late spectral type (Group III). From the sh options 7 out of 9 are located close to the central part of the central area of the cluster. Note that the sh option can also include very anomalous extinction. Five cluster members are scattered throughout the central area of the cluster (W240, W267, W300, W339 and W455). Of the remaining 7 stars, 6 are scattered in the eastern outskirts of the cluster and W103 at the south-west out of the central area. All of these objects are B-type stars (Group I).
- Most of the objects having a slightly anomalous extinction law, tex2html_wrap_inline6540, occupy the central area of the cluster. But W536 lies in the south-east while W556 and W559 are located in the very west region of the cluster. All of these stars are (probably) cluster members and are of spectral type B. Exceptions are W213, A7 or F9, and W396, F9 and G2.
- The objects having a significant anomalous extinction law, tex2html_wrap_inline6542, are also located in the central area of NGC 6611, they are all (probable) cluster members and are of spectral type B. Exceptions are the B-type star W290 that is located down south of NGC 6611, but is a confirmed cluster member, W396 that is of spectral type F9 or G2 and W232 (F8), for which the membership is uncertain.

Besides a clear distribution of the later type stars with normal extinction laws to the East of the central area of NGC 6611, which are thought to be foreground stars, there seems no dependency of the extinction law or the place in the NGC 6611 areas. For example, as most of the sample stars are located in the central area, we do have neighbouring stars with normal and anomalous extinction. The variability in the tex2html_wrap_inline6544 values (from 3.3 to 4.0, and possibly 4.4 or 4.8) suggests that the anomalous extinction law is very variable in the central area of the cluster. This is not surprising as we have already mentioned that the intracluster dust is unevenly distributed. Furthermore, hot stars (the B-types) can influence their direct circumstellar environment by evaporating and destroying larger grains, causing also large changes in the tex2html_wrap_inline6546 values of the intracluster material. The extinction law of a star located behind such a region will be altered as well. In cases of very young objects it is more likely that post-natal circumstellar material is the cause of a different than normal extinction. It is then interesting to note that Group II objects indeed does not have any members with a normal extinction law. This could imply that the anomalous extinction law is higher for late spectral types cluster members as they would occupy an earlier stage of their evolution than, for example, the averaged B-type members. The Group II objects could also be lying in a region where the intracluster matter is more dense, as they are all located in the central region.

Note also that most sh options, being of different spectral type, are located in the central part of the central area.

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