The 85Kr s-Process Branching and the Mass of Carbon Stars

By C. Abia, M. Busso, Roberto Gallino, I. Domínguez, O. Straniero, Jordi Isern. 2001

152 citations recorded by the source corpus.

We present new spectroscopic observations for a sample of C(N)-type red giants. These objects belong to the class of asymptotic giant branch stars, experiencing thermal instabilities in the He-burning shell (thermal pulses). Mixing episodes called third dredge-up enrich the photosphere with newly synthesized 12C in the He-rich zone, and this is the source of the high observed ratio between carbon and oxygen (C/O ≥ 1 by number). Our spectroscopic abundance estimates confirm that, in agreement with the general understanding of the late evolutionary stages of low- and intermediate-mass stars, carbon enrichment is accompanied by the appearance of s-process elements in the photosphere. We discuss the details of the observations and of the derived abundances, focusing in particular on rubidium, a neutron density sensitive element, and on the s-elements Sr, Y, and Zr belonging to the first s-peak. The critical reaction branching at 85Kr, which determines the relative enrichment of the studied species, is discussed. Subsequently, we compare our data with recent models for s-processing in thermally pulsing asymptotic giant branch stars, at metallicities relevant for our sample. A remarkable agreement between model predictions and observations is found. Thanks to the different neutron density prevailing in low- and intermediate-mass stars, comparison with the models allows us to conclude that most C(N) stars are of low mass (M ≲ 3 M☉). We also analyze the 12C/13C ratios measured, showing that most of them cannot be explained by canonical stellar models. We discuss how this fact would require the operation of an ad hoc additional mixing, currently called cool bottom process, operating only in low-mass stars during the first ascent of the red giant branch and, perhaps, also during the asymptotic giant branch.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/c8edcecc0b947ef56e2d7c80427a9aa4c969a2ec

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s‐Process in low‐metallicity stars – I. Theoretical predictions (2010, background): “For oxygen, we assumed a linear increase [O/Fe] with decreasing [Fe/H] (Abia et al. 2001), ([O/Fe] = −0.4 [Fe/H]).”
s-Process Nucleosynthesis in Carbon Stars (2002, background): “In a previous work (Abia et al. 2001, hereafter Paper II) we presented high-resolution spectroscopic observations for a sample of N stars, focusing our attention on light s-elements (ls), sited at the abundance peak near the neutron magic number N ¼ 50, around the 85Kr branching point of the…”
GALACTIC CHEMICAL EVOLUTION AND SOLAR s-PROCESS ABUNDANCES: DEPENDENCE ON THE 13C-POCKET STRUCTURE (2014, background): “…evidence of the strong activation of the 22Ne(α, n)25Mg reaction (associated to IMS) is the excess of Rb with respect to Zr (see, e.g., Lambert et al. 1995; Tomkin & Lambert 1999; Abia et al. 2001, and most recently Yong et al. 2008; Garcı́a-Hernández et al. 2009, 2013; D’Orazi et al. 2013).”
NUGRID STELLAR DATA SET. I. STELLAR YIELDS FROM H TO BI FOR STARS WITH METALLICITIES Z = 0.02 and Z = 0.01 (2013, background): “22), in agreement with observations (Lambert et al. 1995; Abia et al. 2001).”
The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, background): “…Galactic disc MS, S, C(N) and Ba stars show a spread in the distribution of the sprocess elements for a given metallicity (Busso et al. 1995, 2001; Abia et al. 2001, 2002; Gallino et al. 2005; Husti et al. 2009 and references therein), which increases in the halo of CEMP-s stars (Sneden et al.…”
Super and massive AGB stars - IV. Final fates - Initial to final mass relation (2014, unknown): “Mini & 4M⊙ (Iben 1977; Abia et al. 2001; van Raai et al. 2012).”
The s process: Nuclear physics, stellar models, and observations (2010, background): “A similar conclusion was obtained for carbon stars by Abia et al. (2001), who measured the abundances of Rb and other elements for 21 N-type carbon stars and found that the [Rb/Sr, Y, Zr] abundances are better explained by AGB models for low-mass (M <∼ 3M⊙) than for intermediate-mass stars,…”
EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES (2009, background): “…telescopes, from very metal-poor C-rich stars (see Bisterzo et al. 2010 and references therein) to the Galactic and extragalactic C (N-type) stars (Abia et al. 2001, 2002, 2008, 2010; de Laverny et al. 2006; Zamora et al. 2009), Ba-stars (Allen & Barbuy 2006; Smiljanic et al. 2007; Liu et al.…”
Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars (2003, background): “486 12C/13C observed in N-type C stars (Lambert et al. 1986; Dominy & Wallerstein 1987; Smith & Lambert 1990; Abia et al. 2001), which are of Population I, range from 30 to 100; this is similar to the 12C/13C distribution measured in mainstream SiC grains (Hoppe et al. 1994, 1996).”
THE s-PROCESS IN ASYMPTOTIC GIANT BRANCH STARS OF LOW METALLICITY AND THE COMPOSITION OF CARBON-ENHANCED METAL-POOR STARS (2011, background): “The 22Ne source instead produces smaller abundances of the s-process elements but it efficiently activates branching points along the s-process path (Abia et al. 2001; van Raai et al. 2011).”
Abundances in giant stars of the globular cluster NGC 6752 (2005, unknown): “Measurements of Rb will provide further observational constraints on the AGB pollution scenario since the ratio Rb/Zr is sensitive to the neutron density and therefore mass of the AGB star (Lambert et al. 1995; Tomkin & Lambert 1999; Busso et al. 1999; Abia et al. 2001).”
CARBON-ENHANCED METAL-POOR STARS IN SDSS/SEGUE. I. CARBON ABUNDANCE ESTIMATION AND FREQUENCY OF CEMP STARS (2013, background): “As argued by Abia et al. (2001) and Lucatello et al. (2005), and most recently by Suda et al. (2013), the large fraction of CEMP stars at the lowest metallicities could indicate that the IMF in the early universe included a larger number of intermediate- to high-mass stars than the present-day IMF.”
s-Process Nucleosynthesis in Asymptotic Giant Branch Stars: A Test for Stellar Evolution (2002, methodology): “The [Rb/Sr] ratio in AGB stars is typically negative (Lambert et al. 1995; Abia et al. 2001).”
A holistic approach to carbon-enhanced metal-poor stars (2009, background): “…phase if HBB occurs at the bottom of the convective envelope or if there is “extra mixing” below the convective envelope, of a similar nature than the one occurring during the RGB, but called Cool-Bottom Processing (CBP)in the framework of the AGB evolution (Abia et al., 2001; Nollett etal., 2003).”
The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars (2014, methodology): “The second reason for the difference in the predicted distribution is that branching points on the s-process path are activated by the 22Ne source (Abia et al. 2001; van Raai et al. 2012; Karakas et al. 2012).”