AAS 203rd Meeting, January 2004
Session 52 Globular Clusters
Poster, Tuesday, January 6, 2004, 9:20am-6:30pm, Grand Hall

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[52.08] On the age of M4: we are not there yet!

G. De Marchi (ESA), F. Paresce (ESO), O. Straniero (Obs. Teramo), P. Prada Moroni (Univ. Pisa)

We have reduced and analysed the deep HST observations of a field in the globular cluster M4 obtained with the WFPC2. The colour-magnitude diagrams that we derive from the deepest exposures, in the F606W and F814W bands, reveal a narrow main sequence as well as the white dwarf (WD) cooling sequence extending down to the 5 sigma detection limit at I ~q 27. The WD sequence is abruptly terminated at exactly this limit as expected by detection statistics. To separate bona fide WDs from field stars, and obtain in this way the WD luminosity function (LF), we have measured the proper motions of all objects which could be also detected in WFPC2 images of the same field taken 6 years earlier. Unfortunately, the first epoch's exposures being considerably less deep than those of the second, the magnitude range over which the WD LF can be reliably studied is reduced to I<26.5 (at the 3 sigma level, or I ~q 25.6 at 5 sigma). Below such a threshold, most stars detected in the second epoch do not have a corresponding match in the first one.

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 do not reach the peak of the WD LF, thence only allowing one to set a lower limit to the age of M4 of ~9 Gyr. We are not able to reproduce previous results claiming an age of 12.7 Gyr with an uncertainty of 0.35 Gyr (1 sigma). 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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Bulletin of the American Astronomical Society, 35#5
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