The HST/ACS star formation history of the Tucana dwarf spheroidal
  galaxy: clues from the horizontal branch
  We report a new star formation history for the Tucana dwarf spheroidal
galaxy, obtained from a new look at a deep HST/ACS colour-magnitude diagram. We
combined information from the main sequence turn-off and the horizontal branch
to resolve the ancient star formation rates on a finer temporal scale than
previously possible. We show that Tucana experienced three major phases of star
formation, two very close together at ancient times and the last one ending
between 6 and 8 Gyr ago. We show that the three discrete clumps of stars on the
horizontal branch are linked to the distinct episodes of star formation in
Tucana. The spatial distribution of the clumps reveals that each generation of
stars presents a higher concentration than the previous one. The simultaneous
modelling of the horizontal branch and the main sequence turn-off also allows
us to measure the amount of mass lost by red giant branch stars in Tucana with
unprecedented precision, confirming dwarf spheroidals to be excellent
laboratories to study the advanced evolution of low-mass stars.
