Constraining cosmic scatter in the Galactic halo through a differential
  analysis of metal-poor stars
  We present the abundances of Li, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Co,
Ni, Zn, Sr, Y, Zr, and Ba in a wide metallicity range ($-2.8 \le$ [Fe/H] $\le
-1.5$). Using the differential technique allowed us to obtain an unprecedented
low level of scatter in our analysis, with standard deviations as low as 0.05
dex, and mean errors as low as 0.05 dex for [X/Fe]. By expanding our
metallicity range with precise abundances from other works, we were able to
precisely constrain Galactic chemical evolution models in a wide metallicity
range ($-3.6 \le$ [Fe/H] $\le -0.4$). The agreements and discrepancies found
are key for further improvement of both models and observations. We also show
that the LTE analysis of Cr II is a much more reliable source of abundance for
chromium, as Cr I has important NLTE effects. These effects can be clearly seen
when we compare the observed abundances of Cr I and Cr II with GCE models.
While Cr I has a clear disagreement between model and observations, Cr II is
very well modeled. We confirm tight increasing trends of Co and Zn toward lower
metallicities, and a tight flat evolution of Ni relative to Fe. Our results
strongly suggest inhomogeneous enrichment from hypernovae. Our precise stellar
parameters results in a low star-to-star scatter ($0.04$ dex) in the Li
abundances of our sample, with a mean value about $0.4$ dex lower than the
prediction from standard Big Bang Nucleosynthesis; we also study the relation
between lithium depletion and stellar mass, but it is difficult to assess a
correlation due to the limited mass range. We find two blue straggler stars,
based on their very depleted Li abundances. One of them shows intriguing
abundance anomalies, including a possible zinc enhancement, suggesting that
zinc may have been also produced by a former AGB companion.
