The effects of the initial mass function on Galactic chemical enrichment

Abstract

Context. We have been seeing mounting evidence that the stellar initial mass function (IMF) might extend far beyond the canonical Mi ∼ 100 M⊙ limit, but the impact of such a hypothesis on the chemical enrichment of galaxies is yet to be clarified. Aims. We aim to address this question by analysing the observed abundances of thin- and thick-disc stars in the Milky Way with chemical evolution models that account for the contribution of very massive stars dying as pair instability supernovae. Methods. We built new sets of chemical yields from massive and very massive stars up to Mi ∼ 350 M⊙ by combining the wind ejecta extracted from our hydrostatic stellar evolution models with explosion ejecta from the literature. Using a simple chemical evolution code, we analysed the effects of adopting different yield tables by comparing predictions against observations of stars in the solar vicinity. Results. After several tests, we set our focus on the [O/Fe] ratio that best separates the chemical patterns of the two Milky Way components. We find that with a standard IMF, truncated at Mi ∼ 100 M⊙, we can reproduce various observational constraints for thin-disc stars; however, the same IMF fails to account for the [O/Fe] ratios of thick-disc stars. The best results are obtained by extending the IMF up to Mi = 350 M⊙, while including the chemical ejecta of very massive stars in the form of winds and pair instability supernova (PISN) explosions. Conclusions. Our study indicates that PISN may have played a significant role in shaping the chemical evolution of the thick disc of the Milky Way. Including their chemical yields makes it easier to reproduce not only the level of the α-enhancement, but also the observed slope of thick-disc stars in the [O/Fe] vs. [Fe/H] diagram. The bottom line is that the contribution of very massive stars to the chemical enrichment of galaxies is potentially quite important and should not be neglected in models of chemical evolution.

The impact of very massive stars on the chemical evolution of extremely metal-poor galaxies

“…Table 1: Input parameters of the selected chemical evolution and yield models. MTW is our set of stellar ejecta described in (Goswami et al 2021). These models are shown in Fig.…”

“…For 8 M ≤ M i ≤ 350 M , we adopt the yield compilation by Goswami et al (2021) for massive stars and very massive objects. They include stellar wind ejecta, based on the non-rotating PARSEC models (Bressan et al 2012), and explosive ejecta carefully extracted from explosive models of electron capture SN (ECSN) (Wanajo et al 2009), core collapse supernova (CCSN) (Chieffi & Limongi 2004), PPISN (Woosley et al 2002;Chen et al 2014;Yoshida et al 2016;Woosley 2017), and PISN Heger et al 2003).…”

“…To fully test the effect of PISN in both O and N production we have computed new yields with the latest PARSEC presupernova models with rotation from 100 M to 350 M up to the beginning of the PISN instability process, as described in Costa et al (2021). We have then extracted the explosive yields for Z i = 0.0001, 0.001 and 0.006 from the models by using the He core mass as the only parameter, as in Goswami et al (2021). For the massive stars that do not enter the PISN channel (including the most massive stars with Z i =0.02), we have adopted the yields from .…”

“…A peculiarity of the ejecta of PISN is the production of large amounts of Fe and O, in proportions that are strongly dependent on the He core mass at the end of the central H burning phase Kozyreva et al 2014;Takahashi et al 2018). Indeed chemical evolution models that include PISN yields may possess a very early phase with a low [O/Fe] and then rapidly evolve into the domain of the α-enhanced regime (Goswami et al 2021). Based on these findings, we will test here whether the high Fe/O together with the low N/O abundance ratios of EMPGs can be both explained using the yields of very massive stars or if additional ingredients are required.…”

“…In particular, to include the PISN yields, we need to shift the upper mass limit of the IMF in the domain of very massive objects. In this section, we also discuss the PISN yields with particular emphasis on an update of the Goswami et al (2021) tables that concern the effects of stellar rotation. In Section 3, we present the observational data that we wish to analyse.…”