Ruprecht 106: the first single population Globular Cluster?
  All old Galactic Globular Clusters studied in detail to date host at least
two generations of stars, where the second is formed from gas polluted by
processed material produced by massive stars of the first. This process can
happen if the initial mass of the cluster exceeds a threshold above which
ejecta are retained and a second generation is formed. A determination of this
mass-threshold is mandatory in order to understand how GCs form. We analyzed 9
RGB stars belonging to the cluster Ruprecht 106. Targets were observed with the
UVES@VLT2 spectrograph. Spectra cover a wide range and allowed us to measure
abundances for light (O,Na,Mg,Al), alpha (Si,Ca,Ti), iron-peak
(Sc,V,Cr,Mn,Fe,Co,Ni,Cu,Zn) and neutron-capture
(Y,Zr,Ba,La,Ce,Pr,Nd,Sm,Eu,Dy,Pb) elements. Based on these abundances we show
that Ruprecht 106 is the first convincing example of a single population GC
(i.e. a true simple stellar population), although the sample is relatively
small. This result is supported also by an independent photometric test and by
the HB morphology and the dynamical state. It is old (~12 Gyrs) and, at odds
with other GCs, has no alpha-enhancement. The material it formed from was
contaminated by both s- and r- process elements. The abundance pattern points
toward an extragalactic origin. Its present day mass (M=10^4.83 Msun) can be
assumed as a strong lower limit for the initial mass threshold below which no
second generation is formed. Clearly, its initial mass must have been
significantly greater but we have no current constraints on the amount of mass
loss during its evolution.
