Talk Title: The white dwarf cooling sequence and age of the globular cluster M4
Abstract: Very deep images of the Galactic globular cluster M4 were taken in 2001 with the WFPC2 in the V and I bands. A first published analysis of this data set (Richer et al. 2002) produced the result that the age of M4 is 12.7 +/- 0.7 Gyr, thus setting a robust lower limit to the age of the universe. We have subjected the same data set to the simplest possible photometric analysis that completely avoids uncertain assumptions about the origin of the detected sources. This analysis clearly reveals both a thin main sequence and a white dwarf (WD) sequence extending all the way down to the 5 sigma detection limit at I ~ 27. The WD sequence is abruptly terminated at exactly this limit as expected by detection statistics. Using our most recent theoretical WD models to obtain the expected WD sequence for different ages in the observed bandpasses, we find that the data so far obtained do not reach the peak of the WD luminosity function, thence only allowing one to set a lower limit to the age of M4 of ~9 Gyr. Thus, the problem of determining the absolute age of a globular cluster and, therefore, the onset of GC formation with cosmologically significant accuracy remains completely open. Only observations several magnitudes deeper than the limit obtained so far would allow one to approach this objective.
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