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11. The evolutionary status of the objects inNGC 6611

In previous studies the presence of PMS stars in NGC 6611 was disputed. Some authors (e.g. Walker 1961) believe that they can be identified from their location in the HRD. New insights, however, show that stars located in the Walker (1961) PMS-part of the HRD are probably never visible as PMS (Palla & Stahler 1992 and 1993), as they are located far above the birthline. Others believe that the ``Walker PMS'' stars are all foreground objects, e.g.\ Chini & Wargau (1990). Because of the youth of NGC 6611 (tex2html_wrap_inline6588 yr, Thé et al. 1990), one expects to detect at least some PMS objects. A comparison of this age with the evolutionary tracks of Palla & Stahler (1993) for stars in their contraction phase and which move in the HRD towards the ZAMS, learns that the intersection of such tracks with the ZAMS for the age of NGC 6611 lies at stars with an initial mass of tex2html_wrap_inline6590. The intersection is at about log tex2html_wrap_inline6592 = 4.1, corresponding to spectral type B7 and is an upper limit as Hillenbrand et al. (1993) discussed the existence of one 6 Myr old object in NGC 6611. However, most other objects in their study are suggested to be considerably younger, 2tex2html_wrap_inline65941 Myr, indicating the possibility of an age spread in NGC 6611.

In Fig. 9 (click here)a we have plotted our sample of PMS candidates in a theoretical HRD. The transformation of the tex2html_wrap_inline6596 versus tex2html_wrap_inline6598 diagram to the log tex2html_wrap_inline6600 versus log tex2html_wrap_inline6602 diagram is done as explained in Sect. 6 of Thé et al. (1990), adopting a distance modulus of tex2html_wrap_inline6604. Results of this transformation, such as tex2html_wrap_inline6606 and log tex2html_wrap_inline6608, are given in Table 7 (click here). Hillenbrand et al. (1993) argued that due to the quantization of the luminosity classes the bolometric corrections can yield a very significant error in the determination of the luminosities and finally in the masses and conclusions of the evolutionary status. To avoid this, we have determined the scaled stellar luminosity by integrating over the entire fitted Kurucz model (de Winter & van den Ancker 1996). This method results in values of tex2html_wrap_inline6610 and is less sensitive to small errors in adopted spectral types and luminosity classes. These SED luminosities are also listed in Table 7 (click here) by adopting a distance of 2.6 kpc.

In Fig. 9 (click here)a we have also plotted the theoretical results of Palla & Stahler (1993). Note that in their calculations a star with 6 tex2html_wrap_inline6612 already reaches the ZAMS within 0.1 Myr. Compared to the birthlines most objects could be well in their PMS-phase if we adopt a general accretion rate onto the very young objects of tex2html_wrap_inline6614 tex2html_wrap_inline6616 yrtex2html_wrap_inline6618. In such a circumstance the objects with tex2html_wrap_inline6620 and located right of the ZAMS must be extremely young and certainly not of the cluster age.

At a cluster age of a few million years PMS-objects must be of spectral type B7 or later. Stars of earlier spectral type have therefore ended their PMS phase and are normal main-sequence stars, assuming that there is no age-spread. Such objects will slowly move to the right on the main sequence band. An object of 40 tex2html_wrap_inline6622 will have left this band already within 5 million years (Maeder & Meynet 1988). This would mean that the spread of the early type objects right of the ZAMS in Fig. 9 (click here)b is due to this post-ZAMS evolution. In this figure we have plotted our sample stars as well as those of Thé et al. (1990), as they are cluster members probably being in their MS phase, together with evolutionary tracks of Maeder & Meynet (1988). As the MS-band is even filled up to stars with 60 tex2html_wrap_inline6624 we must conclude that these stars are either still very young, less than 3 million years, or evolved as they have ended their red-giant phase which they do in about 10 Myr.

Thé et al. (1990) concluded that the estimated age was reliable compared to other estimates. A weak point involved in the age-determination is the assumption that all stars are at the same distance, furthermore, new calculations can be used to compare with our results. According to such recent theoretical calculations, the distribution in an HRD of the studied NGC 6611 members indicate that either the objects are of different ages or that either all the objects are considerably younger, about one Myr, as many objects are located close to the birthlines, see Fig. 9 (click here)a. In that case stars with tex2html_wrap_inline6626 tex2html_wrap_inline6628 will be either extremely young or will be in their post-MS phase. In the latter case such objects, that are located close to the right border of the MS in Fig. 9 (click here)b, or close to the stellar birthlines in Fig. 9 (click here)a, are then of ages close to about 100 million years (Maeder & Meynet 1988), leaving enough time for the most massive objects to have ended their ``life''. The only way out of this paradox is to assume an age spread in NGC 6611, as already proposed by Hillenbrand et al. (1993).

The three exceptional cases, with log tex2html_wrap_inline6630, which are located to the very right of the MS, see Fig. 9 (click here)b, are then evolved massive objects with ages of tex2html_wrap_inline6632. These ages also confirm the location of objects with 5 tex2html_wrap_inline6634 and the ones close to the ZAMS; they are post-ZAMS. The objects located far to the right of the ZAMS are then in their pre-ZAMS phase having ages of a few 0.1 Myr. This would explain the situation of the Group II objects as being all just formed with tex2html_wrap_inline6636 tex2html_wrap_inline6638.

As the distribution of the studied objects in the field of NGC 6611 in Figs. 9 (click here)a-b do not show any clear turn-off point, this confirms an age spread. This is more dramatically seen in Fig. 9 (click here) of Hillenbrand et al. (1993). Only if we include the Group II objects, we have a type of turn-off point but still with some points between these objects and the ZAMS. It would be these intermediate objects that are slightly older, a few million years, than the Group II objects, and, than the more massive objects located right of the ZAMS.

The age spread of less than one million year up to about 6 million years also indicates that for finding pre-ZAMS or pre-MS objects it is best to look at characteristics of individual cases as in earlier findings of our sample.

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