Chemical evolution with rotating massive star yields – I. The solar neighbourhood and the s-process elements

Abstract

We present a comprehensive study of the abundance evolution of the elements from H to U in the Milky Way halo and local disk. We use a consistent chemical evolution model, metallicity dependent isotopic yields from low and intermediate mass stars and yields from massive stars which include, for the first time, the combined effect of metallicity, mass loss and rotation for a large grid of stellar masses and for all stages of stellar evolution. The yields of massive stars are weighted by a metallicity dependent function of the rotational velocities, constrained by observations as to obtain a primary-like 14 N behavior at low metallicity and to avoid overproduction of s-elements at intermediate metallicities. We show that the solar system isotopic composition can be reproduced to better than a factor of two for isotopes up to the Fe-peak, and at the 10% level for most pure s-isotopes, both light ones (resulting from the weak s-process in rotating massive stars) and the heavy ones (resulting from the main s-process in low and intermediate mass stars). We conclude that the light element primary process (LEPP), invoked to explain the apparent abundance deficiency of the s-elements with A< 100, is not necessary. We also reproduce the evolution of the heavy to light s-elements abundance ratio ([hs/ls]) -recently observed in unevolved thin disk stars -as a result of the contribution of rotating massive stars at sub-solar metallicities. We find that those stars produce primary F and dominate its solar abundance and we confirm their role in the observed primary behavior of N. In contrast, we show that their action is insufficient to explain the small observed values of 12 C/ 13 C in halo red giants, which is rather due to internal processes in those stars.

Fluorine in the Solar Neighborhood: The Need for Several Cosmic Sources

“…Furthermore, together with the finding that two metal-poor, s-process element-rich stars do not show an enhanced fluorine abundance, and since both fluorine from AGB stars and Fe from SN Ia are time delayed (which for the thick disk means after [Fe/H] ∼ −0.7), we cannot reject the hypothesis that for [Fe/H] < −0.7 the fluorine originates from massive stars, most likely from processes in rapidly rotating massive stars (Prantzos et al 2018;Guerço et al 2019a;Olive & Vangioni 2019), showing a primary behavior.…”

“…It should, however, be noted that there are different theoretical predictions of the AGBstar contributions, mainly due to which nuclear reaction rates are adopted. For instance, Prantzos et al (2018) find a lower AGB stellar yield than Kobayashi et al (2011b) or Spitoni et al (2018); see also the discussion in Guerço et al (2019a).…”

“…(i) The contributions to the cosmic budget of fluorine from non-rotating massive stars and conventional Type II Supernovae are negligible (Kobayashi et al 2011b;Prantzos et al 2018). Rapidly rotating massive stars can, however, produce primary fluorine from 14 N, via proton and α captures in the presence of 13 C, which is needed for the generation of protons (Prantzos et al 2018). The 14 N comes from reactions with 12 C which is the ashes of He burning in the massive star itself, and is therefore of primary origin (Guerço et al 2019a).…”

“…Since oxygen is produced as a primary element in massive stars (see, e.g. Prantzos et al 2018), plotting the number density of fluorine as a function of the number density of oxygen will reveal whether fluorine is primary or whether it behaves like a secondary element. In Figure 8 we plot A(F) versus A(O).…”

The s-process in stellar sites

“…This has deep consequences on 22 Ne, whose abundance largely increases. The larger the initial rotation rate, the larger the 22 Ne production, with important consequences on the s-process production by massive stars at low metallicities [28].…”