# APOGEE DR16: a multi-zone chemical evolution model for the Galactic disc based on MCMC methods﻿

APOGEE DR16: a multi-zone chemical evolution model for the Galactic disc based on MCMC methods
See arXiv version
Accepted for publication in Astronomy and Astrophysics (A&A), 16 pages, 19 figures

### Abstract

The analysis of the APOGEE DR16 data suggests the existence of a clear distinction between two sequences of disc stars at different Galactocentric distances in the [$$\alpha$$/Fe] vs. [Fe/H] abundance ratio space: the so-called high-$$\alpha$$ sequence, classically associated to an old population of stars in the thick disc, and the low-$$\alpha$$ sequence, which mostly comprises relatively young stars in the thin disc. We perform a Bayesian analysis based on a Markov Chain Monte Carlo method to constrain a multi-zone two-infall chemical evolution model designed for regions at different Galactocentric distances using measured chemical abundances from the APOGEE DR16 sample. An inside-out formation of the Galaxy disc naturally emerges from the best fit of our two-infall chemical-evolution model to APOGEE-DR16: inner Galactic regions are assembled on shorter time-scales compared to the external ones. In the outer disc (with radii $$R>6$$ kpc), the chemical dilution due to a late accretion event of gas with primordial chemical composition is the main driver of the [Mg/Fe] vs. [Fe/H] abundance pattern in the low-$$\alpha$$ sequence. In the inner disc, in the framework of the two-infall model, we confirm the presence of an enriched gas infall in the low-$$\alpha$$ phase as suggested by chemo-dynamical models. Our Bayesian analysis of the recent APOGEE DR16 data suggests a significant delay time, ranging from $$\sim$$3.0 to 4.7 Gyr, between the first and second gas infall events for all the analyzed Galactocentric regions. Our results propose a clear interpretation of the [Mg/Fe] vs. [Fe/H] relations along the Galactic discs. The signatures of a delayed gas-rich merger which gives rise to a hiatus in the star formation history of the Galaxy are impressed in the [Mg/Fe] vs. [Fe/H] relation, determining how the low-$$\alpha$$ stars are distributed in the abundance space at different Galactocentric distances.

K.
Verma
E. Spitoni, K. Verma, V. Silva Aguirre is in paper arXiv:2101.08803

APOGEE DR16: a multi-zone chemical evolution model for the Galactic disc based on MCMC methods
See arXiv version
Accepted for publication in Astronomy and Astrophysics (A&A), 16 pages, 19 figures

### Abstract

The analysis of the APOGEE DR16 data suggests the existence of a clear distinction between two sequences of disc stars at different Galactocentric distances in the [$$\alpha$$/Fe] vs. [Fe/H] abundance ratio space: the so-called high-$$\alpha$$ sequence, classically associated to an old population of stars in the thick disc, and the low-$$\alpha$$ sequence, which mostly comprises relatively young stars in the thin disc. We perform a Bayesian analysis based on a Markov Chain Monte Carlo method to constrain a multi-zone two-infall chemical evolution model designed for regions at different Galactocentric distances using measured chemical abundances from the APOGEE DR16 sample. An inside-out formation of the Galaxy disc naturally emerges from the best fit of our two-infall chemical-evolution model to APOGEE-DR16: inner Galactic regions are assembled on shorter time-scales compared to the external ones. In the outer disc (with radii $$R>6$$ kpc), the chemical dilution due to a late accretion event of gas with primordial chemical composition is the main driver of the [Mg/Fe] vs. [Fe/H] abundance pattern in the low-$$\alpha$$ sequence. In the inner disc, in the framework of the two-infall model, we confirm the presence of an enriched gas infall in the low-$$\alpha$$ phase as suggested by chemo-dynamical models. Our Bayesian analysis of the recent APOGEE DR16 data suggests a significant delay time, ranging from $$\sim$$3.0 to 4.7 Gyr, between the first and second gas infall events for all the analyzed Galactocentric regions. Our results propose a clear interpretation of the [Mg/Fe] vs. [Fe/H] relations along the Galactic discs. The signatures of a delayed gas-rich merger which gives rise to a hiatus in the star formation history of the Galaxy are impressed in the [Mg/Fe] vs. [Fe/H] relation, determining how the low-$$\alpha$$ stars are distributed in the abundance space at different Galactocentric distances.

V.
Silva
Aguirre
E. Spitoni, K. Verma, V. Silva Aguirre is in paper arXiv:2101.08803

APOGEE DR16: a multi-zone chemical evolution model for the Galactic disc based on MCMC methods
See arXiv version
Accepted for publication in Astronomy and Astrophysics (A&A), 16 pages, 19 figures

### Abstract

The analysis of the APOGEE DR16 data suggests the existence of a clear distinction between two sequences of disc stars at different Galactocentric distances in the [$$\alpha$$/Fe] vs. [Fe/H] abundance ratio space: the so-called high-$$\alpha$$ sequence, classically associated to an old population of stars in the thick disc, and the low-$$\alpha$$ sequence, which mostly comprises relatively young stars in the thin disc. We perform a Bayesian analysis based on a Markov Chain Monte Carlo method to constrain a multi-zone two-infall chemical evolution model designed for regions at different Galactocentric distances using measured chemical abundances from the APOGEE DR16 sample. An inside-out formation of the Galaxy disc naturally emerges from the best fit of our two-infall chemical-evolution model to APOGEE-DR16: inner Galactic regions are assembled on shorter time-scales compared to the external ones. In the outer disc (with radii $$R>6$$ kpc), the chemical dilution due to a late accretion event of gas with primordial chemical composition is the main driver of the [Mg/Fe] vs. [Fe/H] abundance pattern in the low-$$\alpha$$ sequence. In the inner disc, in the framework of the two-infall model, we confirm the presence of an enriched gas infall in the low-$$\alpha$$ phase as suggested by chemo-dynamical models. Our Bayesian analysis of the recent APOGEE DR16 data suggests a significant delay time, ranging from $$\sim$$3.0 to 4.7 Gyr, between the first and second gas infall events for all the analyzed Galactocentric regions. Our results propose a clear interpretation of the [Mg/Fe] vs. [Fe/H] relations along the Galactic discs. The signatures of a delayed gas-rich merger which gives rise to a hiatus in the star formation history of the Galaxy are impressed in the [Mg/Fe] vs. [Fe/H] relation, determining how the low-$$\alpha$$ stars are distributed in the abundance space at different Galactocentric distances.