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Study of the very low mass main sequence stars in globular clusters

Technical requirements: V, I images down to a limiting magnitude of $V\sim27.5$ ( $I\sim 25.5$) in a $\geq0.5\times0.5$ square degrees field.

The problem of the contribution of low mass stars to the total mass of Galactic globular clusters (GC) is still open (Piotto et al. 1997). The study of the mass functions (MF) of GGCs can give information on the IMF and therefore on the formation processes. The shape of the GGC MFs are also important for an estimate of the contribution of low mass stars to the halo mass. Despite the new impulse given by HST to programs aimed at the study of the faintest end of the luminosity functions (LF) in GGCs (Piotto et al. 1997), it is still unclear whether the MFs are rising till the limit of the core hydrogen burning ignition mass (about $\leq0.09 m_\odot$), or if the MF is flattening or even dropping below $\sim 0.15 m_\odot$. The main problems are that: 1) even with HST, low mass stars can be found only in the outer part of the cluster (because of mass segregation and because of crowding), where the cluster density is lower; 2) due to a steepening of the mass-luminosity relation, the number of stars per magnitude bin is dramatically dropping below $\sim 0.14 m_\odot$. Given the typical diameter of a GGC (from 20 to 60 arcmin) a big field ( $\ge 30\times 30$arcmin2) is of fundamental importance, to collect a statistically significant number of low mass stars to reduce the noise in the LF and corresponding MF (note that one also needs to go well beyond the tidal radius in order to determine the field star contamination): in this respect, HST is of no help. Even in the closest cluster (NGC 6397) a $0.10 m_\odot$ star has an apparent visual magnitude V=26.0, which becomes 27.6 for a $0.095 m_\odot$ star.


next up previous contents
Next: White Dwarf Cooling Sequence Up: Stellar populations Previous: Mass segregation in globular
Guido Buscema
1999-01-29