7 Evolutionary stage

Although Fig. 6 seems to provide a straightforward interpretation for the NGC 1866 lithium sequence, Fig. 1 shows that another hypothesis shall be tested, i.e. that the lithium poor star #1 could actually be in a phase following the lithium production, when lithium is destroyed again. In this case, however, the star should have a larger bolometric luminosity and, moreover, should be surrounded by an extended circumstellar envelope, easily detectable in the mid-infrared with ISOCAM. In the ISOCAM field the three brightest sources in all bands coincide with the location of our stars #1, #2 and #3, and their fluxes in mJy are given in Table 3. The photometric results are given in Table 3, where we have as well transformed the near-IR magnitudes, previously shown in Table 2 in Jy. The fluxes in the three bands at 4.5, 6.7 and 12 show that the most luminous source (and also reddest, if we consider the ISOCAM color [4.5]-[12]) is the star which shows high lithium abundance. It is interesting to note that the brightest star in the K-band (NGC 1866 #1) is only the third in the ISO bands, and is also the "bluest'' in the ISO colors. This suggests a less evolved stage as an AGB star. In general the mid-IR colors of these stars are not "extreme'', indicating that they are still in the initial stages of the AGB phase. This fact almost completely discards the possibility of star NGC 1866 #1 being in a later AGB phase, when the lithium is destroyed again.

  

Table 3: Infrared fluxes for NGC 1866^a.

Another point to examine is the star distribution along the AGB of NGC 1866. The model shown in Fig. 1 predicts a few more stars along the AGB: if we sample two stars in the first  yr of evolution, we should find some other five in the following evolution, which lasts $\simeq$10⁵ yr longer. Most of these stars could be invisible in the optical and near infrared, as they would be surrounded by a thick circumstellar envelope, but they should be the brightest objects in the field in the mid-IR. Actually there are no other bright mid-IR sources in the ISOCAM field, apart from the three brightest sources in the K-band under analysis in this paper. The absence of heavily obscured AGB stars needs to be interpreted with caution, because of the small statistics, but it is telling us that possibly the duration of this heavily obscured AGB phase is shorter than that predicted by the models, probably because of stronger than expected mass loss rate experienced by these massive AGB stars.

Although we do not have ISO data for the stars in NGC 2031, its luminous star without lithium is at about the same absolute bolometric magnitude than the similar star in NGC 1866, indicating that they probably represent a similar evolutionary phase.

What is much less obvious is the evolutionary stage of the lithium rich star in NGC 2031 at $\mbox{$M_{\rm bol}$ }\simeq -5.2$: this is not explained by HBB models at all. Other lithium rich giants at similar magnitudes ar known, most of which are J stars. The so called "cool bottom processing'' suggested by Wasserburg et al. (1995) could be a way to explain this anomaly. However, if the clusters NGC 1866 and NGC 2031 are indeed coeval, it is intriguing that one cluster conforms to a "standard'' mixing scheme, while the other does not. It has to be noticed that, whatever the mixing process in AGB, lithium is produced through the consumption of ³He. If this occurs at $M_{\rm bol} \simeq -5$, no helium is left for production at $M_{\rm bol} \simeq -6$, where HBB becomes important. A last point to consider is that the radial velocity of NGC 2031#2 markedly differs from those of the other stars in NGC 2031, also taking into account the large errors. As this cluster field is very crowded, it is also possible that it is a background star.