The s-Process in Massive Stars at Low Metallicity: The Effect of Primary 14N from Fast Rotating Stars

By M. Pignatari, Roberto Gallino, G. Meynet, R. Hirschi, Falk Herwig, M. University, Keele, UK., Jina, U. N. Dame and 24 more. 2008

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The goal of this Letter is to analyze the impact of a primary neutron source on the s-process nucleosynthesis in massive stars at halo metallicity. Recent stellar models including rotation at very low metallicity predict a strong production of primary 14N. Part of the nitrogen produced in the H-burning shell diffuses by rotational mixing into the He core where it is converted to 22Ne providing additional neutrons for the s-process. We present nucleosynthesis calculations for a 25 M☉ star at [Fe/H] = –3, –4, where about 0.8% in mass is made of primary 22Ne in the convective He-burning core. The usual weak s-process shape is changed by the additional neutron source with a peak between Sr and Ba, where the s-process yields increase by orders of magnitude with respect to the yields obtained without rotation. Iron seeds are fully consumed and the maximum production of Sr, Y, and Zr is reached. On the other hand, the s-process efficiency beyond Sr and the ratio Sr/Ba are strongly affected by the amount of 22Ne and by nuclear uncertainties, first of all by the 22Ne(α, n)25Mg reaction. Finally, assuming that 22Ne is primary in the considered metallicity range, the s-process efficiency decreases with metallicity due to the effect of the major neutron poisons 25Mg and 22Ne. This work represents a first step toward the study of primary neutron source effect in fast rotating massive stars, and its implications are discussed in the light of spectroscopic observations of heavy elements at halo metallicity.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/f543475136296c4005bbe88c30a4b1cc49224996

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NUGRID STELLAR DATA SET. I. STELLAR YIELDS FROM H TO BI FOR STARS WITH METALLICITIES Z = 0.02 and Z = 0.01 (2013, background): “…the spectroscopic observations of stars at different metallicities (Lambert et al. 1995; Busso et al. 2001; Abia et al. 2002; Masseron et al. 2010; Bisterzo et al. 2011; Lugaro et al. 2012; Maiorca et al. 2012) and planetary nebulae (Karakas et al. 2007; Pignatari et al. 2008; Karakas et al. 2009).”
THE s-PROCESS IN ASYMPTOTIC GIANT BRANCH STARS OF LOW METALLICITY AND THE COMPOSITION OF CARBON-ENHANCED METAL-POOR STARS (2011, background): “…in the initial s-process abundances may have arisen from pollution from primordial massive stars producing s-process elements during core He and shell C burning (Pignatari et al. 2008), or via the still unidentified “light element primary process” (Travaglio et al. 2004; Montes et al. 2007).”
s-process production in rotating massive stars at solar and low metallicities (2015, background): “The large production of neutrons by 22Ne is partially compensated by the larger concentration of 25Mg and 22Ne itself, which become primary neutron poisons in rotating massive stars (Pignatari et al. 2008).”
Non-standard s process in low metallicity massive rotating stars (2011, methodology): “A first attempt to assess the impact of rotation on the sprocess nucleosynthesis in low-Z massive rotating stars was made by Pignatari et al. (2008), who investigated the impact of primary 22Ne in a parametrised way.”
CONVECTIVE–REACTIVE PROTON–12C COMBUSTION IN SAKURAI'S OBJECT (V4334 SAGITTARII) AND IMPLICATIONS FOR THE EVOLUTION AND YIELDS FROM THE FIRST GENERATIONS OF STARS (2010, background): “…or complementary explanation for a missing component in the first neutron-peak region of the abundance distribution in both the solar abundance distribution as well as the metal poor stars, (light-element primary process, or LEPP Travaglio et al. 2004; Pignatari et al. 2008; Farouqi et al. 2009).”
The elemental composition of the Sun - III. The heavy elements Cu to Th (2014, background): “More recently, it has been realised that the s-process can also be efficient in rapidly-rotating massive stars at low metallicity, through the production of primary N later converted to 22Ne (Pignatari et al. 2008).”
DETAILED CHEMICAL ABUNDANCES IN THE r-PROCESS-RICH ULTRA-FAINT DWARF GALAXY RETICULUM 2 (2016, background): “…r process (e.g., Truran et al. 2002; Arcones & Thielemann 2013) and charged particle reactions (e.g., Woosley & Hoffman 1992) operating in core-collapse supernovae or the weak component of the s process (e.g., Raiteri et al. 1991; Pignatari et al. 2008) operating in massive, rapidly-rotating stars.”
CHARGED-PARTICLE AND NEUTRON-CAPTURE PROCESSES IN THE HIGH-ENTROPY WIND OF CORE-COLLAPSE SUPERNOVAE (2010, methodology): “…contributions from the νp process (see, e.g., Fröhlich et al. 2006a, 2006b) and/or by additional contributions from a new type of rs-process (Pignatari et al. 2008); and (b) for the region between Sr and Ag by the assumption of a unique, constant electron abundance in the HEW when…”