A Stromgren view of the multiple populations in globular clusters
  We discuss a variety of photometric indices assembled from the uvby Stromgren
system. Our aim is to examine the pros and cons of the various indices to find
the most suitable one(s) to study the properties of multiple populations in
globular clusters (GCs) discovered by spectroscopy. We explore in particular
the capabilities of indices like m_1 and c_y at different metallicities. We
define a new index delta_4=(u-v)-(b-y) to separate first and second stellar
generations in GCs of any metal abundance, since it keeps the sensitivity to
multiple stellar populations over all the metallicity range and at the same
time minimizes the sensitivity to photometric errors. We detecte clear
differences in the red giant branches of the GCs examined, like skewness or
bi/multi-modality in color distribution. We connect the photometric information
with the spectroscopic results on O, Na abundances we obtained in our survey of
GCs. Finally, we compute the effects of different chemical composition on the
Stromgren filters and indices using synthetic spectra.
