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G. De Marchi (ESA), F. Paresce (ESO), G. Li Causi (Obs. Rome)
Very deep images of the Galactic globular cluster M4 through the V and I filters were taken in 2001 with the WFPC2 on board the HST. 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 (Hansen et al. 2002), thus setting a robust lower limit to the age of the universe. In view of the great astronomical importance of getting this number right, 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, from which can be deduced the deepest mass function yet determined for a globular cluster, and a white dwarf (WD) sequence extending all the way down to the 5 sigma detection limit at I ~27.5. The WD sequence is abruptly terminated at exactly this limit as expected by detection statistics. Using the most recent theoretical WD models of Prada Moroni & Straniero (2002) to obtain the expected WD sequence for different ages in the observed bandpasses, we find that the data so far obtained allow one only to set a lower limit to the age of M4 of 10 Gyr. Thus, the problem of determining the absolute age of a globular cluster with cosmologically significant accuracy remains completely open. Only observations several magnitudes deeper than the limit obtained so far would allow us to approach this objective.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.