THE EFFECTS OF ROTATION ON s -PROCESS NUCLEOSYNTHESIS IN ASYMPTOTIC GIANT BRANCH STARS

Nordström

et al. 2016

A&A

Context. An increasing fraction of carbon-enhanced metal-poor (CEMP) stars is found as their iron abundance, [Fe/H], decreases below [Fe/H] = −2.0. The CEMP-s stars have the highest absolute carbon abundances, [C/H], and are thought to owe their enrichment in carbon and the slow neutron-capture (s-process) elements to mass transfer from a former asymptotic giant branch (AGB) binary companion. The most Fe-poor CEMP stars are normally single, exhibit somewhat lower [C/H] than CEMP-s stars, but show no s-process element enhancement (CEMP-no stars). Abundance determinations of CNO offer clues to their formation sites. Aims. Our aim is to use the medium-resolution spectrograph X-Shooter/VLT to determine stellar parameters and abundances for C, N, Sr, and Ba in several classes of CEMP stars in order to further classify and constrain the astrophysical formation sites of these stars. Methods. Atmospheric parameters for our programme stars were estimated from a combination of V − K photometry, model isochrone fits, and estimates from a modified version of the SDSS/SEGUE spectroscopic pipeline. We then used X-Shooter spectra in conjunction with the 1D local thermodynamic equilibrium spectrum synthesis code MOOG, 1D ATLAS9 atmosphere models to derive stellar abundances, and, where possible, isotopic 12 C/ 13 C ratios. Results. Abundances (or limits) of C, N, Sr, and Ba are derived for a sample of 27 faint metal-poor stars for which the X-Shooter spectra have sufficient signal-to-noise ratios (S/N). These moderate resolution, low S/N (∼10−40) spectra prove sufficient to perform limited chemical tagging and enable assignment of these stars into the CEMP subclasses (CEMP-s and CEMP-no). According to the derived abundances, 17 of our sample stars are CEMP-s and 3 are CEMP-no, while the remaining 7 are carbon-normal. For four CEMP stars, the subclassification remains uncertain, and two of them may be pulsating AGB stars. Conclusions. The derived stellar abundances trace the formation processes and sites of our sample stars. The [C/N] abundance ratio is useful for identifying stars with chemical compositions unaffected by internal mixing, and the [Sr/Ba] abundance ratio allows us to distinguish between CEMP-s stars with AGB progenitors and the CEMP-no stars. Suggested formation sites for the latter include faint supernovae with mixing and fallback and/or primordial, rapidly-rotating, massive stars (spinstars). X-Shooter spectra have thus proved to be valuable tools in the continued search for their origin.

“…The ratio of Sr and Ba can be used to trace the formation site -AGB stars vs. (Frischknecht et al 2012;Piersanti et al 2013;Cristallo et al 2015).…”

Section: Discussionmentioning

confidence: 99%

Abundances of carbon-enhanced metal-poor stars as constraints on their formation

Nordström

et al. 2016

A&A

Monthly Notices of the Royal Astronomical Society

“…Furthermore, rotational mixing can be present while the neutrons are released inside 13 C and 14 N pockets produced, e.g., by overshoot. This mixing can strongly inhibit the s process by carrying 14 N, which is a neutron poison via the 14 N(n,p) 14 C reaction (Wallner et al 2016), from the 14 N pocket (and/or the H-burning ashes, if the 14 N pocket is absent) into the 13 C pocket (Herwig et al 2003;Siess et al 2004;Piersanti et al 2013).…”

Section: Current Models For the Formation Of 13 C Pocketsmentioning

confidence: 99%

Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements

Buntain

Doherty

Lugaro

et al. 2017

The production of the elements heavier than iron via slow neutron captures (the s process) is a main feature of the contribution of asymptotic giant branch (AGB) stars of low mass (< 5 Msun) to the chemistry of the cosmos. However, our understanding of the main neutron source, the 13 C(α,n) 16 O reaction, is still incomplete. It is commonly assumed that in AGB stars mixing beyond convective borders drives the formation of 13 C pockets. However, there is no agreement on the nature of such mixing and free parameters are present. By means of a parametric model we investigate the impact of different mixing functions on the final s-process abundances in low-mass AGB models. Typically, changing the shape of the mixing function or the mass extent of the region affected by the mixing produce the same results. Variations in the relative abundance distribution of the three s-process peaks (Sr, Ba, and Pb) are generally within +/-0.2 dex, similar to the observational error bars. We conclude that other stellar uncertainties -the effect of rotation and of overshoot into the C-O core -play a more important role than the details of the mixing function. The exception is at low metallicity, where the Pb abundance is significantly affected. In relation to the composition observed in stardust SiC grains from AGB stars, the models are relatively close to the data only when assuming the most extreme variation in the mixing profile.

“…Physically, this may correspond to efficient mixing inside the 13 C pocket during the interpulse period induced by the difference in the angular momentum between the core and the envelope in a rotating AGB star. This mixing carries the neutron poison 14 N into the 13 C-rich layers A77, page 2 of 6 (e.g., Piersanti et al 2013), lowering the s-process efficiency. However, the actual effect is uncertain because magnetic fields can slow down the core by coupling it to the envelope and inhibit mixing.…”

Section: Stellar Modelsmentioning

confidence: 99%

Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars

Lugaro

Campbell

Winckel

et al. 2015

A&A

Aims. We explore modifications to the current scenario for the slow neutron-capture process (the s-process) in asymptotic giant branch (AGB) stars to account for the Pb deficiency observed in post-AGB stars of low metallicity ([Fe/H] −1.2) and low initial mass ( 1−1.5 M ) in the Large and Small Magellanic Clouds. Methods. We calculated the stellar evolution and nucleosynthesis for a 1.3 M star with [Fe/H] = −1.3 and tested different amounts and distributions of protons leading to the production of the main neutron source within the 13 C-pocket and proton ingestion scenarios. Results. No s-process models can fully reproduce the abundance patterns observed in the post-AGB stars. When the Pb production is lowered, the abundances of the elements between Eu and Pb, such as Er, Yb, W, and Hf, are also lowered to below those observed. Conclusions. Neutron-capture processes with neutron densities intermediate between the s and the rapid neutron-capture processes may provide a solution to this problem and be a common occurrence in low-mass, low-metallicity AGB stars.