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 
. The intersection is at about log 
= 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, 2
1 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 
versus 
diagram to the
log 
versus log 
diagram is done as explained
in Sect. 6 of Thé et al. (1990), adopting a distance modulus of
. Results of this transformation, such as 
and
log 
, 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
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 
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
yr
. In such a
circumstance the objects with 
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 
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 
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 
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 
,
which are located to the very right of the
MS, see Fig. 9 (click here)b, are then evolved massive objects with ages of
.
These ages also confirm the location of objects with 5 
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 
.
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.