Purveyors of fine halos: Re-assessing globular cluster contributions to
  the Milky Way halo build-up with SDSS-IV
  There is ample evidence in the Milky Way for globular cluster (GC)
disruption. Hence one may expect that also part of the Galactic halo field
stars may once have formed in GCs. We quantify the fraction of halo stars
donated by GCs by searching for stars that bear the unique chemical
fingerprints typical for a subset of GC stars often dubbed `second-generation
stars'. These are stars showing light element abundance anomalies such as a
pronounced CN-band strength accompanied by weak CH-bands. Based on this
indicator, past studies have placed the fraction of halo stars with a GC origin
between a few to up to 50%. Using low-resolution spectra from the most recent
data release of the latest extension of the Sloan Digital Sky Survey (SDSS-IV),
we were able to identify 118 metal-poor ($-1.8\le$[Fe/H]$\le -1.3$) CN-strong
stars in a sample of 4470 halo giant stars out to 50 kpc. This results in an
observed fraction of these stars of 2.6$\pm$0.2%. Using an updated formalism to
account for the fraction of stars lost early on in the GCs' evolution we
estimate the fraction of the halo that stems from disrupted clusters to be
11$\pm$1%. This number represents the case that stars lost from GCs were
entirely from the first generation and is thus merely an upper limit. Our
conclusions are sensitive to our assumptions of the mass lost early on from the
first generation formed in the GCs, the ratio of first-to-second generation
stars, and other GC parameters. We carefully test the influence of varying
these parameters on the final result and find that, under realistic scenarios,
the above fraction depends on the main assumptions at less than 10%. We further
recover a flat trend in this fraction with Galactocentric radius, with a
marginal indication of a rise beyond 30 kpc that could reflect the ex-situ
origin of the outer halo. (abridged)
