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White Dwarf Cooling Sequence

Technical requirements: U,B V images down to a limiting magnitude of $U\sim26.5$ is a $\geq0.5\times0.5$ square degrees field.

The white dwarf (WD) cooling sequence in globular clusters has been recently identified with HST in a number of clusters (Cool, Piotto, King 1996, ApJ, 468, 655, ... add ref). For example, in a relatively low crowding field at 4.6 arcmin from the center of NGC 6397, Cool et al (1996) have been able to identify >30 WDs which distribute in a cooling sequence in the interval 22.6<V<25.2. The distance modulus of NGC 6397 is $(m-M)_V\sim 12.3$. Clearly a WD cooling sequence is well within the reach of LBT for all the clusters with distance modulus less than 15. The LBT wide field imager offer also the advantage of good performances in U. Imaging in the U-band allow to better distinguish the WDs from the other cluster/field stars. For the closest clusters, the WD cooling sequence can be followed for many magnitudes. The exceptional further benefit with respect to HST is that, thanks to the large field it would be possible not only to locate the WD sequence in the cm diagram. The LBT wide field imager would enable us to extract a luminosity function, which would allow to empirically measure the cooling time, and would give an indication of the mass function for the (already evolved) stars more massive than $0.8 m_\odot$.

The capibility of U imaging, would give a unique opportunity to distinguish between DA and non-DA WDs (this is feasible only using U-band. The ratio of non-DA/DA WDs would give information on the late phases of evolution of low mass stars (Chiosi, Bertelli, Bressan (1992, ARAA, 30, 235).

Note that this kind of study is useful not only for a better comprehension of the WD structure and evolution. As shown by Renzini et al. (1996, ApJ, 465, L23), the WD cooling sequence can be also used as an independent distance indicator.


next up previous contents
Next: Discovery of new globular Up: Stellar populations Previous: Study of the very
Guido Buscema
1999-01-29