The Ages and Distances of Globular Clusters with the Luminosity Function Method: The Case of M5 and M55

Abstract

We present new age and distance determinations for the Galactic globular clusters M55 and M5, using the luminosity function (LF) method of Padoan & Jimenez. Using the set of stellar tracks computed by Jimenez & MacDonald and taking into account a value of [α/Fe] = 0.4, we find an age of 12.5 ± 1.0 Gyr for M55 and 10.6 ± 0.8 Gyr for M5. We also find m - M = 14.21 ± 0.08 mag for M55, and m - M = 14.55 ± 0.10 mag for M5, where the errors refer to the internal accuracy obtained from the method applied to the observed LFs of M5 and M55. These values of m - M agree with the ones obtained using the tip of the red giant branch and with the ones given by the subdwarf fitting method with Reid's new Hipparcos results. If the accuracy of the LF method is further confirmed, it will mean that the period of the formation of the Galactic halo took place over a few Gyr and that halo globular clusters are not older than 14 Gyr.