On the missing second generation AGB stars in NGC 6752

Abstract

In recent years the view of Galactic globular clusters as simple stellar populations has changed dramatically, it is now thought that basically all globular clusters host multiple stellar populations, each with its own chemical abundance pattern and colour-magnitude diagram sequence. Recent spectroscopic observations of asymptotic giant branch stars in the globular cluster NGC 6752 have disclosed a low [Na/Fe] abundance for the whole sample, suggesting that they are all first generation stars, and that all second generation stars fail to reach the AGB in this cluster. A scenario proposed to explain these observations invokes strong mass loss in second generation horizontal branch stars -all located at the hot side of the blue and extended horizontal branch of this cluster -possibly induced by the metal enhancement associated to radiative levitation. This enhanced mass loss would prevent second generation stars from reaching the asymptotic giant branch phase, thus explaining at the same time the low value of the ratio between horizontal branch and asymptotic giant branch stars (the R 2 parameter) observed in NGC 6752. We have critically discussed this mass-loss scenario, finding that the required mass-loss rates are of the order of 10 −9 M yr −1 , significantly higher than current theoretical and empirical constraints. By making use of synthetic horizontal branch simulations, we demonstrate that our modelling correctly predicts the R 2 parameter for NGC 6752, without the need to invoke very efficient mass loss during the core He-burning stage. As a test of our stellar models we show that we can reproduce the observed value of R 2 for both M 3, a cluster of approximately the same metallicity and with a redder horizontal branch morphology, and M 13, a cluster with a horizontal branch very similar to NGC 6752. However, our simulations for the NGC 6752 horizontal branch predict however the presence of a significant fraction of second generation stars (about 50%) along the cluster asymptotic giant branch. We conclude that there is no simple explanation for the lack of second generation stars in the spectroscopically surveyed sample, although the interplay between mass loss (with low rates) and radiative levitation may play a role in explaining this puzzle.

Agb Sodium Abundances in the Globular Cluster 47 Tucanae (Ngc 104)

“…Campbell et al (2013) proposed that, in addition to He enhancement for the Na-rich stars, a substantial ad hoc increase in the mass-loss rate (20 times the RGB value) for NGC 6752 stars on the blue horizontal branch may be required to prevent further evolution up the AGB. However, Cassisi et al (2014) note that the required mass loss rate of ∼10 −9 M e yr −1 is several orders of magnitude higher than those allowed by current analyses of hot horizontal branch and B subdwarf stars. Additionally, horizontal branch simulations by Cassisi et al (2014) that accurately reproduce the R 2 parameter (N AGB /N HB ) for NGC 6752, M3, and M13, but do not invoke enhanced mass loss, also predict that ∼50% of NGC 6752ʼs AGB stars should be Na-rich (albeit with lower Y than the most extreme values).…”

“…However, Cassisi et al (2014) note that the required mass loss rate of ∼10 −9 M e yr −1 is several orders of magnitude higher than those allowed by current analyses of hot horizontal branch and B subdwarf stars. Additionally, horizontal branch simulations by Cassisi et al (2014) that accurately reproduce the R 2 parameter (N AGB /N HB ) for NGC 6752, M3, and M13, but do not invoke enhanced mass loss, also predict that ∼50% of NGC 6752ʼs AGB stars should be Na-rich (albeit with lower Y than the most extreme values). These results indicate that enhanced mass loss for a particular subset of horizontal branch stars is unlikely to be the key link between RGB composition and post-RGB evolution.…”

Evolution of long-lived globular cluster stars

“…However, in contrast to Campbell and collaborators, who concluded that in this GC all 2P stars fail to climb the AGB, we obtain that ∼74% of the AGB stars at 13 Gyr belong to the 2P with the current model assumptions. Cassisi et al (2014) also found a non-negligible proportion of 2P stars on the AGB (∼50%) thanks to synthetic horizontal branch simulations for NGC 6752. Theoretical works also emphasize the fact that the number of AGB stars in a given GC strongly depends on the assumptions made on the mass loss in the previous evolution phases Charbonnel & Chantereau 2016;Cassisi et al 2014).…”

“…Cassisi et al (2014) also found a non-negligible proportion of 2P stars on the AGB (∼50%) thanks to synthetic horizontal branch simulations for NGC 6752. Theoretical works also emphasize the fact that the number of AGB stars in a given GC strongly depends on the assumptions made on the mass loss in the previous evolution phases Charbonnel & Chantereau 2016;Cassisi et al 2014). Moreover, in the framework of the FRMS scenario, the theoretical Na spread on the AGB phase is a function of both age and metallicity (Charbonnel & Chantereau 2016), and the expected trends can explain the presence of 2P stars found in most of the GCs studied so far, which cover a range in age and iron content (47 Tuc, NGC 2808, M 3, M 5, M 13, M 55, M 2, see Johnson & Pilachowski 2012;García-Hernández et al 2015;Johnson et al 2015;Wang et al II, in prep.…”

Evolution of long-lived globular cluster stars

“…Indeed, a change from 0.5 to 0.65 for η drastically 4 Campbell et al (2013) proposed fitting the sodium distribution along the AGB in NGC 6752 by assuming an increased mass loss rate by a factor of 20 on the HB for all 2P stars. This proposal, which would require mass loss rates of the order 10 −9 M yr −1 (i.e., significantly higher than current observational and empirical constraints) was carefully and critically discussed by Cassisi et al (2014).…”