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Context.Central bars and spirals are known to impact significantly the evolution of their host galaxies, both in terms of dynamics and star formation. Their typically different pattern speeds cause them to regularly overlap, which induces fluctuations in bar parameters. Aims.In this paper, we analyze both numerical simulations of disk galaxies and observational data to study the effect of bar-spiral physical overlap on stellar radial migration and star formation in the bar vicinity, as a function of time and galactic azimuth. Methods.We studied three different numerical models, two of which are in a cosmological context, alongside APOGEE DR17 data and the WISE catalog of Galactic HII regions. Results.We find that periodic boosts in stellar radial migration occur when the bar and spiral structures overlap. This mechanism causes net inward migration along the bar leading side, while stars along the bar trailing side and minor axis are shifted outward. The signature of bar-spiral-induced migration is seen between the bar inner Lindbald resonance and well outside its corotation, beyond which other drivers take over. We also find that, in agreement with simulations, APOGEE DR17 stars born at the bar vicinity (which are mostly metal rich) can migrate out to the solar radius while remaining on cold orbits. For the Milky Way, 13% of stars in the solar vicinity with an eccentricity <0.5 were born inside the bar, compared to 5–20% in the simulations. Bar-spiral reconnections also result in periodic starbursts at the bar ends with an enhancement of up to a factor of 4, depending on the strength of the spiral structure. Similarly to the migration bursts, these do not always happen simultaneously at the two sides of the bar, which hints at the importance of odd spiral modes. Data from the WISE catalog suggest this phenomenon is also relevant in our own Galaxy. 

We investigate the inner regions of the Milky Way using data from APOGEE and Gaia EDR3. Our inner Galactic sample has more than 26 500 stars within | X Gal |< 5 kpc, | Y Gal |< 3.5 kpc, | Z Gal |< 1 kpc, and we also carry out the analysis for a foreground-cleaned subsample of 8000 stars that is more representative of the bulge–bar populations. These samples allow us to build chemo-dynamical maps of the stellar populations with vastly improved detail. The inner Galaxy shows an apparent chemical bimodality in key abundance ratios [ α /Fe], [C/N], and [Mn/O], which probe different enrichment timescales, suggesting a star formation gap (quenching) between the high- and low- α populations. Using a joint analysis of the distributions of kinematics, metallicities, mean orbital radius, and chemical abundances, we can characterize the different populations coexisting in the innermost regions of the Galaxy for the first time. The chemo-kinematic data dissected on an eccentricity–| Z | max plane reveal the chemical and kinematic signatures of the bar, the thin inner disc, and an inner thick disc, and a broad metallicity population with large velocity dispersion indicative of a pressure-supported component. The interplay between these different populations is mapped onto the different metallicity distributions seen in the eccentricity–| Z | max diagram consistently with the mean orbital radius and V ϕ distributions. A clear metallicity gradient as a function of | Z | max is also found, which is consistent with the spatial overlapping of different populations. Additionally, we find and chemically and kinematically characterize a group of counter-rotating stars that 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 disc that migrated into the bulge. Finally, based on 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 kpc, with a broad dispersion of metallicity. Even stars with a high probability of belonging to the bar show chemical bimodality in the [ α /Fe] versus [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.