Chemical Composition of a Palomar 12 Blue Straggler
  With the equivalent area of a 16m telescope, ESPRESSO in 4UT mode allows to
inaugurate high resolution spectroscopy for solar-type stars belonging to
extragalactic globular clusters. We determine the chemical composition of an
extragalactic blue straggler. The star has a G magnitude of 19.01 and belongs
to the globular cluster Pal12, that is associated to the Sagittarius dwarf
galaxy. Abundances are computed by using high resolution spectroscopy and LTE
analysis. Two 50 minutes ESPRESSO spectra, co-added, provide a Signal to Noise
Ratio of 25 with a resolving power R=70000. This allows us to measure with good
precision abundance of several (13) elements. Li could help to distinguish
between formation models of Blue Stragglers; we are able to set a 3 sigma upper
limit of Li=3.1, which is still too high to discriminate between competing
models. The abundances we retrieve for the BS are compatible with those of
giant stars of Pal 12 published in literature, re-analyzed by us using the same
procedure and line list. Small differences are present, that can be ascribed to
NLTE effects, but for Mg the BS shows a large under-abundance. The most likely
explanation is that the BS atmosphere is dominated by gas processed through the
Mg-Al cycle, but we have no suitable Al or Na lines to confirm this hypothesis.
We show that ESPRESSO with 4UT can be used to derive precise abundances for
solar-type stars fainter than magnitude 19. At these magnitudes a proper sky
subtraction is needed and in crowded field the targets must be chosen with
outmost care, to avoid contamination of the sky fibre from nearby stars.
