Abstract
ABSTRACT Recent studies show that the chemical evolution of Sr and Ba in the Galaxy can be explained if different production sites, hosting r- and s-processes, are taken into account. However, the question of unambiguously identifying these sites is still unsolved. Massive stars are shown to play an important role in the production of s-material if rotation is considered. In this work, we study in detail the contribution of rotating massive stars to the production of Sr and Ba, in order to explain their chemical evolution, but also to constrain the rotational behaviour of massive stars. A stochastic chemical evolution model was employed to reproduce the enrichment of the Galactic halo. We developed new methods for model-data comparison which help to objectively compare the stochastic results to the observations. We employed these methods to estimate the value of free parameters which describe the rotation of massive stars, assumed to be dependent on the stellar metallicity. We constrain the parameters using the observations for Sr and Ba. Employing these parameters for rotating massive stars in our stochastic model, we are able to correctly reproduce the chemical evolution of Sr and Ba, but also Y, Zr, and La. The data supports a decrease of both the mean rotational velocities and their dispersion with increasing metallicity. Our results show that a metallicity-dependent rotation is a necessary assumption to explain the s-process in massive stars. Our novel methods of model-data comparison represent a promising tool for future galactic chemical evolution studies.
Highlights
R 10INAF/Osservatorio Astronomico di Roma, Via di Frascati 33, I-00040 Monte Porzio Catone, Italy SC Accepted XXX
A Heavy elements beyond the iron peak are formed through neutron captures (Burbidge et al 1957), which are generally divided into two classes: a slow process (s-process) if IN the timescale for neutron capture is longer than the β-decay of the freshly synthesized unstable nucleus, and a rapid process (r-process) if it is shorter
We can conclude that rotational velocity in massive stars is well described by a Gaussian curve whose centre and width depend on the stellar metallicity according to the functions: Omore times for stochasticity, we find that L varies from the minimum we found (L = 13860) up to 15824, on average
Summary
A Heavy elements beyond the iron peak are formed through neutron captures (Burbidge et al 1957), which are generally divided into two classes: a slow process (s-process) if IN the timescale for neutron capture is longer than the β-decay of the freshly synthesized unstable nucleus, and a rapid process (r-process) if it is shorter. Ofied (Sr-Y-Zr, Ba-La-Ce-Pr-Nd, and Pb-Bi), linked to the magic neutron numbers 50, 82 and 126, which give particular stability to the nucleus For this reason, it is interesting to follow the evolution of elements Sr and Ba in the Milky Way, as representative of the first and second peak of the s-process production. There are a number of reasons for which low metallicity massive stars are expected to rotate faster (Meynet & Maeder 2002; Frischknecht et al 2016; Limongi & Chieffi 2018), in this case the s-process production would be enhanced even more. Cescutti et al (2013), Cescutti & Chiappini (2014) and Cescutti et al (2015) showed that including the s-process from rotating massive stars (RMSs) in chemical evolution models is fundamental in order to explain the heavy element enrichment, in particular of Sr and Ba. On the other hand, for the r-process a large flux of free neutrons is required. D and NSMs, or totally, assuming a very short timescale for E the merging after the formation of the binary system
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