The effects of different Type Ia SN yields on Milky Way chemical evolution

ABSTRACT We study the effect of different Type Ia SN nucleosynthesis prescriptions on the Milky Way chemical evolution. To this aim, we run detailed one-infall and two-infall chemical evolution models, adopting a large compilation of yield sets corresponding to different white dwarf progenitors (near-Chandrasekar and sub-Chandrasekar) taken from the literature. We adopt a fixed delay time distribution function for Type Ia SNe, in order to avoid degeneracies in the analysis of the different nucleosynthesis channels. We also combine yields for different Type Ia SN progenitors in order to test the contribution to chemical evolution of different Type Ia SN channels. The results of the models are compared with recent LTE and NLTE observational data. We find that ‘classical’ W7 and WDD2 models produce Fe masses and [α/Fe] abundance patterns similar to more recent and physical near-Chandrasekar and sub-Chandrasekar models. For Fe-peak elements, we find that the results strongly depend either on the white dwarf explosion mechanism (deflagration-to-detonation, pure deflagration, double detonation) or on the initial white dwarf conditions (central density, explosion pattern). The comparison of chemical evolution model results with observations suggests that a combination of near-Chandrasekar and sub-Chandrasekar yields is necessary to reproduce the data of V, Cr, Mn and Ni, with different fractions depending on the adopted massive stars stellar yields. This comparison also suggests that NLTE and singly ionized abundances should be definitely preferred when dealing with most of Fe-peak elements at low metallicity.

Neutron stars mergers in a stochastic chemical evolution model: impact of time delay distributions

We study the evolution of the [Eu/Fe] ratio in the Galactic halo by means of a stochastic chemical evolution model considering merging neutron stars as polluters of europium. We improved our previous stochastic chemical evolution model by adding a time delay distribution for the coalescence of the neutron stars, instead of constant delays. The stochastic chemical evolution model can reproduce the trend and the observed spread in the [Eu/Fe] data with neutron star mergers as unique producers if we assume: i) a delay time distribution ∝t−1.5, ii) a MEu = 3e − 6M⊙ per event, iii) progenitors of neutron stars in the range 9 − 50M⊙ and iv) a constant fraction of massive stars in the initial mass function (0.02) that produce neutron star mergers. Our best model is obtained by relaxing point iv) and assuming a fraction that varies with metallicity. We confirm that the mixed scenario with both merging neutron stars and supernovae as europium producers can provide a good agreement with the data relaxing the constraints on the distribution time delays for the coalescence of neutron stars. Adopting our best model, we also reproduce the dispersion of [Eu/Fe] at a given metallicity, which depends on the fraction of massive stars that produce neutron star mergers. Future high-resolution spectroscopic surveys, such as 4MOST and WEAVE, will produce the necessary statistics to constrain at best this parameter.

Breaking the degeneracy between gas inflow and outflows with stellar metallicity: insights on M 101

ABSTRACT An analytical chemical evolution model is constructed to investigate the radial distribution of gas-phase and stellar metallicity for star-forming galaxies. By means of the model, the gas-phase and stellar metallicity can be obtained from the stellar-to-gas mass ratio. Both the gas inflow and outflow processes play an important role in building the final gas-phase metallicity, and there exists degeneracy effect between the gas inflow and outflow rates for star-forming galaxies. On the other hand, stellar metallicity is more sensitive to the gas outflow rate than to the gas inflow rate, and this helps to break the parameter degeneracy for star-forming galaxies. We apply this analysis method to the nearby disc galaxy M 101 and adopting the classical χ2 methodology to explore the influence of model parameters on the resulted metallicity. It can be found that the combination of gas-phase and stellar metallicity is indeed more effective for constraining the gas inflow and outflow rates. Our results also show that the model with relatively strong gas outflows but weak gas inflow describes the evolution of M 101 reasonably well.

Heavy element evolution in the inner regions of the Milky Way

ABSTRACT We present results for the evolution of the abundances of heavy elements (O, Mg, Al, Si, K, Ca, Cr, Mn, Ni, and Fe) in the inner Galactic regions (RGC ≤ 4 kpc). We adopt a detailed chemical evolution model already tested for the Galactic bulge and compare the results with Apache Point Observatory Galactic Evolution Experiment data. We start with a set of yields from the literature that are considered the best to reproduce the abundance patterns in the solar vicinity. We find that, in general, the predicted trends nicely reproduce the data but in some cases either the trend or the absolute values of the predicted abundances need to be corrected, even by large factors, in order to reach the best agreement. We suggest how the current stellar yields should be modified to reproduce the data and we discuss whether such corrections are reasonable in the light of the current knowledge of stellar nucleosynthesis. However, we also critically discuss the observations. Our results suggest that Si, Ca, Cr, and Ni are the elements for which the required corrections are the smallest, while for Mg and Al moderate modifications are necessary. On the other hand, O and K need the largest corrections to reproduce the observed patterns, a conclusion already reached for solar vicinity abundance patterns, with the exception of oxygen. For Mn, we apply corrections already suggested in previous works.

Stellar population synthesis of galaxies with chemical evolution model

The derivation of accurate stellar populations of galaxies is a non-trivial task because of the well-known age-metallicity degeneracy. We aim to break this degeneracy by invoking a chemical evolution model (CEM) for isolated disk galaxy, where its metallicity enrichment history (MEH) is modelled to be tightly linked to its star formation history (SFH). Our CEM has been successfully tested on several local group dwarf galaxies whose SFHs and MEHs have been both independently measured from deep colour-magnitude diagrams of individual stars. By introducing the CEM into the stellar population fitting algorithm as a prior, we expect that the SFH of galaxies could be better constrained.

A comparison of star formation history between NGC 300 and M33

NGC 300 is a near-optical twin of the Local Group galaxy M33, which are benchmarks for understanding late-type spiral galaxies. They are two bulgeless and low-mass spiral galaxies in different environments. In order to explore the common properties and differences between the two nearby low-mass systems, we first use the simple chemical evolution model to explore the star formation history (SFH) of NGC300 and M33, and then compare the feasible model predicted SFH of NGC 300 with that of M33. Through comparing the SFHs between them, it can be found that the mean stellar age of NGC 300 is older than that of M33, there is a recent lack of primordial gas infall onto the disk of NGC 300, recent star formation along the disk of NGC 300 is less active than that of M33, and the local environment may play a key role in the secular evolution of a galaxy.

The contribution from rotating massive stars to the enrichment in Sr and Ba of the Milky Way

Most neutron capture elements have a double production by r- and s-processes, but the question of production sites is complex and still open. Recent studies show that including stellar rotation can have a deep impact on nucleosynthesis. We studied the evolution of Sr and Ba in the Milky Way. A chemical evolution model was employed to reproduce the Galactic enrichment. We tested two different nucleosynthesis prescriptions for s-process in massive stars, adopted from the Geneva group and the Rome group. Rotation was taken into account, studying the effects of stars without rotation or rotating with different velocities. We also tested different production sites for the r-process: magneto rotational driven supernovae and neutron star mergers. The evolution of the abundances of Sr and Ba is well reproduced. The comparison with the the most recent observations shows that stellar rotation is a good assumption, but excessive velocities result in overproduction of these elements. In particular, the predicted evolution of the [Sr/Ba] ratio at low metallicity does not explain the data at best if rotation is not included. Adopting different rotational velocities for different stellar mass and metallicity better explains the observed trends. Despite the differences between the two sets of adopted stellar models, both show a better agreement with the data assuming an increase of rotational velocity toward low metallicity. Assuming different r-process sources does not alter this conclusion.

2D chemical evolution model: The impact of Galactic disc asymmetries on azimuthal chemical abundance variations

Context. Galactic disc chemical evolution models generally ignore azimuthal surface density variation that can introduce chemical abundance azimuthal gradients. Recent observations, however, have revealed chemical abundance changes with azimuth in the gas and stellar components of both the Milky Way and external galaxies. Aims. Our aim is to quantify the effects of spiral arm density fluctuations on the azimuthal variations of the oxygen and iron abundances in disc galaxies. Methods. We developed a new 2D Galactic disc chemical evolution model that is capable of following not just radial but also azimuthal inhomogeneities. Results. The density fluctuations resulting from a Milky Way-like N-body disc formation simulation produce azimuthal variations in the oxygen abundance gradients of the order of 0.1 dex. Moreover, the azimuthal variations are more evident in the outer Galactic regions, which is in agreement with the most recent observations in external galaxies. Using a simple analytical model, we show that the largest fluctuations with azimuth result near the spiral structure co-rotation resonance where the relative speed between the spiral and gaseous disc is the slowest. Conclusion. We provide a new 2D chemical evolution model capable of following azimuthal density variations. Density fluctuations extracted from a Milky Way-like dynamical model lead to a scatter in the azimuthal variations of the oxygen abundance gradient, which is in agreement with observations in external galaxies. We interpret the presence of azimuthal scatter at all radii by the presence of multiple spiral modes moving at different pattern speeds, as found in both observations and numerical simulations.

A unified construction of stellar evolution and chemical evolution models for the multiple populations in globular clusters

Recent investigations of multiple stellar populations in globular clusters (GCs) suggest that the horizontal-branch (HB) morphology and mean period of type ab RR Lyrae variables are mostly sensitive to helium abundance, while the star formation timescale has the greatest effect on our chemical evolution model constructed to reproduce the Na-O anti-correlation of GCs. Therefore, by combining the results from synthetic HB model with those from chemical evolution model, we could put better constraints on star formation history and chemical evolution in GCs with multiple populations. From such efforts made for four GCs, M4, M5, M15, and M80, we find that consistent results can be obtained from these two independent models.