2. Why the globular cluster M 55?
M 55 is a low central concentration, (Trager et al. 1995), low metallicity, (Zinn 1980), cluster located at kpc from the Sun (Mandushev et al. 1996). Although it is a nearby object, it has received little or sporadic attention until very recently. The works of Mateo et al. (1996) and Fahlman et al. (1996) presented photometric datasets of M 55 that have been used principally to establish the age and the tidal extension of the Sagittarius dwarf galaxy. Mandushev et al. (1996) published the first deep (down to ) photometry of the cluster (other previous studies of the stellar population of M 55 are in Lee (1977), Shade et al. (1988), and Alcaino et al. (1992). From the data of a field at core radii from the center, Mandushev et al. (1996) estimated a new apparent distance modulus for M 55, , and from the luminosity function they found that the high-mass end of the mass function () is well fitted by a power law with , whereas at the low-mass end () the mass function has a slope of .
From the dynamical point of view, M 55 has been previously studied by Pryor et al. (1991) in their papers on the mass-to-light ratio of globular clusters. Their principal conclusion is that this cluster might have a power law mass function with an exponent , with a lower limit of the mass function in the range (i.e. a total absence of low mass stars): a conclusion opposite to that found recently by Mandushev et al. (1996).
An original work on M 55 is in Irwin & Trimble (1984). They studied the radial star count density profile using photographic material digitalized with the Automatic Plate Measuring System (APM) of the Cambridge University. Irwin & Trimble (1984) used a single photometric band, which did not allow them to lower the contribution of the field stars in the construction of the radial star counts. Nevertheless in this work (never repeated in other clusters), the authors reach some interesting conclusions: they claim one of the first evidences of mass segregation (even if they cannot quantify it); the central stellar luminosity function seems to be flat (with a corresponding mass function having a slope of ) with a partial deviation from the King models. Moreover, they claim the presence of 8 short period variables, at the limit of their photometry, compatible with contact binaries of W UMa type. This last point is interesting for the presence of a large population of Blue Straggler (BS) stars in M 55 to which the variables of Irwin & Trimble (1984) could belong.
Despite the potential interest of this nearby cluster for problems such as the dynamical evolution of globular clusters and interaction with the tidal field of the Galaxy, the existing data on M 55 are so far limited and have been used to address only particular problems. Now large field CCDs offer the possibility to attack this problems in a suitable way. The following section is dedicated to the presentation of the M 55 data set and our observing strategy; in Sect. 4 we show the luminosity and mass function of the cluster; in Sect. 5 we present the analysis of the radial density profile and the conclusions. The details of the techniques adopted in the reduction and analysis of the data can be found in the appendix of the paper.
© European Southern Observatory (ESO) 1997
Online publication: April 6, 1998