Very deep images of the Galactic globular cluster M4 through the F606W and F814W filters were taken in 2001 with the WFPC2 on board the HST. A first published analysis of this data set 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. 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 avoids uncertain assumptions about the origin of the detected sources. This analysis 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 theoretical WD models to obtain the expected WD sequence for different ages in the observed bandpasses we find that these data allow one only 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.
On the Age of the Globular Cluster M4
PRADA MORONI, PIER GIORGIO
2003-01-01
Abstract
Very deep images of the Galactic globular cluster M4 through the F606W and F814W filters were taken in 2001 with the WFPC2 on board the HST. A first published analysis of this data set 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. 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 avoids uncertain assumptions about the origin of the detected sources. This analysis 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 theoretical WD models to obtain the expected WD sequence for different ages in the observed bandpasses we find that these data allow one only 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
