The Milky Way bar and bulge revealed by APOGEE DR16 and Gaia EDR3
  We investigate the inner regions of the Milky Way with a sample of
unprecedented size and coverage thanks to APOGEE DR16 and Gaia EDR3 data. Our
inner Galactic sample has more than 26,000 stars within $|X_{\rm Gal}| <5$ kpc,
$|Y_{\rm Gal}| <3.5$ kpc, $|Z_{\rm Gal}| <1$ kpc, and we also make the analysis
for a foreground-cleaned sub-sample of 8,000 stars more representative of the
bulge-bar populations. The inner Galaxy shows a clear chemical discontinuity in
key abundance ratios [$\alpha$/Fe], [C/N], and [Mn/O], probing different
enrichment timescales, which suggests a star formation gap (quenching) between
the high- and low-$\alpha$ populations. For the first time, we are able to
fully characterize the different populations co-existing in the innermost
regions of the Galaxy via joint analysis of the distributions of rotational
velocities, metallicities, orbital parameters and chemical abundances. The
chemo-kinematic analysis reveals the presence of the bar; of an inner thin
disk; of a thick disk, and of a broad metallicity population, with a large
velocity dispersion, indicative of a pressure supported component. We find and
characterize chemically and kinematically a group of counter-rotating stars,
which could be the result of a gas-rich merger event or just the result of
clumpy star formation during the earliest phases of the early disk, which
migrated into the bulge. Finally, based on the 6D information we assign stars a
probability value of being on a bar orbit and find that most of the stars with
large bar orbit probabilities come from the innermost 3 kpcs. Even stars with a
high probability of belonging to the bar show the chemical bimodality in the
[$\alpha$/Fe] vs. [Fe/H] diagram. This suggests bar trapping to be an efficient
mechanism, explaining why stars on bar orbits do not show a significant
distinct chemical abundance ratio signature.
