The bimodal [Mg/Fe] versus [Fe/H] bulge sequence as revealed by APOGEE
  DR14
  The Galactic bulge has a bimodal metallicity distribution function: different
kinematic, spatial, and, potentially, age distributions characterize the
metal-poor and metal-rich components. Despite this observed dichotomy, which
argues for different formation channels for those stars, the distribution of
bulge stars in the $\alpha$-abundance versus metallicity plane has been found
so far to be a rather smooth single sequence. We use data from the fourteenth
data release of the APOGEE spectroscopic survey (DR14) to investigate the
distribution in the Mg abundance (as tracer of the
$\alpha$-elements)-versus-metallicity plane of a sample of stars selected to be
in the inner region of the bulge. A clean sample has been selected from the
DR14 using a set of data- and pipeline-flags to ensure the quality of their
fundamental parameters and elemental abundances. An additional selection made
use of computed spectro-photometric distances to select a sample of likely
bulge stars as those with ${\rm R_{GC}\leq 3.5~kpc}$. We adopt magnesium
abundance as an $\alpha$-abundance proxy for our clean sample as it has been
proven to be the most accurate $\alpha$-element as determined by ASPCAP, the
pipeline for data products from APOGEE spectra. From the distribution of our
bulge sample in the [Mg/Fe]-versus-[Fe/H] plane, we found that the sequence is
bimodal. This bimodality is given by the presence of a low-Mg sequence of stars
parallel to the main high-Mg sequence over a range of $\sim 0.5$~dex around
solar metallicity. The two sequences merge above ${\rm [Fe/H]\sim0.15~ dex}$
into a single sequence whose dispersion in [Mg/Fe] is larger than either of the
two sequences visible at lower metallicity. This result is confirmed when we
consider stars in our sample that are inside the bulge region according to
trustworthy Gaia DR2 distances.
