ST-FOC observations of globular clusters: Simulations

Using a first approximation to the Hubble space telescope point-spread function, the problems concerning observation and reduction of globular clusters in the Andromeda nebula are analysed through a simulation procedure in order to study the feasibility of accurate photometry of individual stars in the clusters.     

The Sagittarius Dwarf Galaxy Survey (SDGS) – II. The stellar content and constraints on the star formation history

A detailed study of the star formation history of the Sagittarius dwarf spheroidal galaxy is performed through the analysis of data from the Sagittarius Dwarf Galaxy Survey (SDGS). Accurate statistical decontamination of the SDGS colour–magnitude diagrams (CMDs) allows us to obtain many useful constraints on the age and metal content of the Sgr stellar populations in three different regions of the galaxy.
A coarse metallicity distribution of Sgr stars is derived, ranging from [Fe/H]∼−2.0 to [Fe/H]∼−0.7, the upper limit being somewhat higher in the central region of the galaxy. A qualitative global fit to all the observed CMD features is attempted, and a general scheme for the star formation history of the Sgr dSph is derived. According to this scheme, star formation began at a very early time from a low metal content interstellar medium and lasted for several  Gyr, coupled with progressive chemical enrichment. The star formation rate (SFR) had a peak from 8 to 10  Gyr ago, when the mean metallicity was in the range −1.3≤[Fe/H]≤−0.7. After that maximum, the SFR rapidly decreased and a very low rate of star formation took place until ∼1–0.5  Gyr ago.     

The lower main sequence of the globular cluster M3 with the Hubble Space Telescope: luminosity and mass functions

We present HST observations of the lower main sequence of the Galactic globular cluster M3. Our data, obtained with WFPC2 in the F814W and F555W filters, sample a region centred about 4 arcmin away from the cluster centre (at ∼4.2 core radii). The photometry of the WF2, WF3 and WF4 fields extends down to V ∼26.5 ( I ∼25), much deeper than any previous ground-based study on this cluster. The resulting luminosity function (LF) for the main sequence covers the interval M_I =3–10.5, and reaches a maximum between M_I =8.5 and 9, sharing in this feature the same behaviour as other clusters studied with HST so far. Making use of four different mass–luminosity (M–L) relations (theoretical and semi-empirical) a present-day mass function (MF) has also been obtained, which spans the mass interval ≃0.8–0.18 M⊙. Independent of the adopted M–L relation, the MF clearly flattens out at the low-mass limits. However, no attempt has been made to take into account possible effects due, for instance, to mass segregation and evaporation.