On the nature of VX Sagitarii: Is it the first galactic super-AGB star found?

Stars with initial masses above ~10 M_⊙ have an internal evolution completely different from those in the range between 4 and ~10 M_⊙. The former are able to ignite all the elements up to Fe in their cores without degeneration (becoming into red supergiants; RSGs). The latter are not, and they become into red giants first, and later into asymptotic branch giant (AGB) stars. Despite their internal differences, externally, both AGB stars and RSGs are evolved cool stars with similarly cool. The main difference between both groups is their luminosity, as typical RSGs have bolometric magnitudes in the range -9<M_bol< -7.8 mag and most luminous confirmed AGB stars have M_bol ~ -8 mag. However, according to theoretical predictions, AGB stars with masses above ~8 M_⊙ (which can be even higher than ~10 M_⊙ under the right conditions) can reach extremely high luminosities. In this context, we studied one of the most luminous (M_bol ~ -8.6 mag) galactic RSGs, VX Sgr, and we found clear proofs that it is not a RSGs. We examined the alternatives, and we found that its characteristic fit better with a AGB with unprecedentedly high mass (in the range between 10 and ~12 M_⊙), making it a strong candidate (and the first identified in the Galaxy) to be a super-AGB star (an AGB star which has ignited its Carbon core). These findings are not only important for the understanding of the most massive AGB stars, because we provided one observational case, but also for the study of the most extreme RSGs, because one of them is in fact an AGB star, showing that there may be other high-mass AGB stars concealed among RSGs.