The evolution of isotope ratios in the Milky Way Galaxy

Abstract

Isotope ratios have opened a new window into the study of the details of stellar evolution, supernovae and galactic chemical evolution. We present the evolution of the isotope ratios of elemental abundances (from C to Zn) in the solar neighbourhood, bulge, halo and thick disc, using chemical evolution models with updated yields of asymptotic giant branch (AGB) stars and core-collapse supernovae. The evolutionary history of each element is different owing to the effects of the initial progenitor mass and metallicity on element production. In the bulge and thick disc the star formation time-scale is shorter than in the solar neighbourhood, leading to higher [alpha/Fe] ratios. Likewise, the smaller contribution from Type Ia supernovae in these regions leads to lower [Mn/Fe] ratios. Also in the bulge, the abundances of [(Na, Al, P, Cl, K, Sc, Cu, Zn)/Fe] are higher because of the effect of metallicity on element production from core-collapse supernovae. According to our predictions, it is possible to find metal-rich stars ([Fe/H] greater than or similar to -1) that formed in the early Universe as a result of rapid star formation. The chemical enrichment time-scale of the halo is longer than in the solar neighbourhood, and consequently the ratios of [(C, F)/Fe] and C-12/C-13 are higher owing to a significant contribution from low-mass AGB stars. While the [alpha/Fe] and [Mn/Fe] ratios are the same as in the solar neighbourhood, the [(Na, Al, P, Cl, K, Sc, Cu, Zn)/Fe] ratios are predicted to be lower. Furthermore, we predict that isotope ratios such as Mg-24/Mg-25,Mg-26 are larger because of the contribution from low-metallicity supernovae. Using isotopic ratios, it is possible to select stars that formed in a system with a low chemical enrichment efficiency such as the satellite galaxies that were accreted on to our own Milky Way Galaxy

Planetary nebulae as observational constraints in chemical evolution models for NGC 6822

“…This is found in several works on chemical evolution models (e.g., Chiappini et al 2003;Carigi et al 2005;Mollá et al 2006;Romano et al 2010;Kobayashi et al 2011). The prediction of our model is 7.4σ higher than the value observed in H ii regions, therefore we consider that the N yields adopted for LIMS are in general overestimated the models.…”

“…The determination of this type of elements in stars is not so reliable and in previous papers (e.g., Timmes et al 1995;Romano et al 2010;Kobayashi et al 2011) the authors were not able to test Ne, Cl, and Ar yields owing to the lack of stellar abundances.…”

Detection of extragalactic argonium, ArH+, toward PKS 1830−211

“…Kobayashi et al (2006) calculated nucleosynthesis yields of core-collapse supernovae (SNe) and hypernovae (HNe, which are more energetic by an order of magnitude) to predict the evolution of isotopic ratios in the Milky Way, SNe and HNe being the dominant contributors to elements from Na to Fe with the possible exception of a few selected isotopes. They presented yields for different progenitor masses and metallicities, and subsequently incorporated the results into models for the enrichment of the Milky Way (Kobayashi et al 2011). While the nucleosynthetic yields for many elements, including 36 Ar and 38 Ar, show a non-monotonic dependence on progenitor mass, the predicted 36 Ar/ 38 Ar ratio tends to decline with increasing metallicity and is typically smaller for HNe than for SNe.…”

SYSTEMATIC NON-LTE STUDY OF THE −2.6 ≤ [Fe/H] ≤ 0.2 F AND G DWARFS IN THE SOLAR NEIGHBORHOOD. II. ABUNDANCE PATTERNS FROM Li TO Eu*

“…We will mainly discuss the models of K11 (Kobayashi et al 2011b) and its updated version, K15. Their main features include slow infall, no outflow, star formation proportional to gas fraction, the Kroupa (2001) initial mass function (IMF) at 0.01-50 M e , and the SN Ia model based on the single-degenerate scenario (Kobayashi & Nomoto 2009) with the metallicity effect (Kobayashi et al 1998).…”

“…Their main features include slow infall, no outflow, star formation proportional to gas fraction, the Kroupa (2001) initial mass function (IMF) at 0.01-50 M e , and the SN Ia model based on the single-degenerate scenario (Kobayashi & Nomoto 2009) with the metallicity effect (Kobayashi et al 1998). The metallicity-dependent nucleosynthesis yields are taken from Kobayashi et al (2006, hereafter K06) and Kobayashi et al (2011b) for SNe and hypernovae (with a 0.5 fraction of hypernovae at 20 M e ), and from Karakas (2010) for AGB stars. The yield sets are identical to those in Nomoto et al (2013).…”