CEMP-s and CEMP-s/r stars: last update

Bisterzo, S.; Gallino, R.; Straniero, O.; Cristallo, S.; Käppeler, F.; Wiescher, M.

doi:10.1051/eas/1363003

Cited by 100 publications

(185 citation statements)

References 59 publications

Supporting

Mentioning

169

Contrasting

Order By: Relevance

“…The [Pb/Ba] ratio varies by almost an order of magnitude, with the highest value (1.16) achieved for the −2m profile and the lowest (0.37) with the m 1/3 profile. This makes variations in the mixing profile at low metallicity one of the possible factors that could contribute to explaining the variations in [Pb/Ba] (within a similar range as found here) observed in carbonenhanced metal-poor (CEMP) stars with enhancements in the s-process elements (Van Eck et al 2003;Bisterzo et al 2010Bisterzo et al , 2011Bisterzo et al , 2012Lugaro et al 2012).…”

Section: Discussionsupporting

confidence: 80%

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

Buntain

Doherty

Lugaro

et al. 2017

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

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.

show abstract

Section: Discussionsupporting

confidence: 80%

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

Buntain

Doherty

Lugaro

et al. 2017

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

show abstract

“…This is detectable in the isotopic abundance ratios of C and N, where low 12 C/ 13 C and [C/N] ratios indicate strong internal mixing with CN-cycled material (Spite et al 2005). For 13 of our programme CEMP stars we find ratios that cover 3 < 12 C/ 13 C < 50, where the higher end of this range is in good agreement with Bisterzo et al (2011Bisterzo et al ( , 2012, who find that their AGB models without strong mixing do not result in the low, observed values (4 < 12 C/ 13 C < 10).…”

Section: Comparison To Galactic Chemical Evolution Modelssupporting

confidence: 87%

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

Hansen

Nordström

Hansen

et al. 2016

A&A

View full text Add to dashboard Cite

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.

show abstract

“…This follows from the results of Lucatello et al (2005) and Starkenburg et al (2014) on binarity and of Bisterzo et al (2011) on nucleosynthesis.…”

Section: High-carbon Bandmentioning

confidence: 74%

“…This last requirement is supported by the study of Bisterzo et al (2011), who have analysed a sample of 100 CEMP-s stars for which detailed chemical abundances are available (almost all the stars that appear in Fig. 6 are in their sample) and showed that the abundances of light elements (C, N, O, Na, Mg) and neutron capture elements can be reproduced assuming transfer from an AGB star in the mass range 1.3−2 M and logarithmic dilution factors in the range 0.0−2.5.…”

Section: High-carbon Bandmentioning

confidence: 82%

TOPoS

Bonifacio

Caffau

Spite

et al. 2015

A&A

174

232

View full text Add to dashboard Cite

Context. In the course of the Turn Off Primordial Stars (TOPoS) survey, aimed at discovering the lowest metallicity stars, we have found several carbon-enhanced metal-poor (CEMP) stars. These stars are very common among the stars of extremely low metallicity and provide important clues to the star formation processes. We here present our analysis of six CEMP stars. Aims. We want to provide the most complete chemical inventory for these six stars in order to constrain the nucleosynthesis processes responsible for the abundance patterns. Methods. We analyse both X-Shooter and UVES spectra acquired at the VLT. We used a traditional abundance analysis based on OSMARCS 1D local thermodynamic equilibrium (LTE) model atmospheres and the turbospectrum line formation code. were not able to detect any iron lines, yet we could place a robust (3σ) upper limit of [Fe/H] < −5.0 and measure the Ca abundance, with [Ca/H] = −5.0, and carbon, A(C) = 6.90, suggesting that this star could be even more metal-poor than SDSS J1742+2531. This makes these two stars the seventh and eighth stars known so far with [Fe/H] < −4.5, usually termed ultra-iron-poor (UIP) stars. No lithium is detected in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which implies a robust upper limit of A(Li) < 1.8 for both stars. Conclusions. Our measured carbon abundances confirm the bimodal distribution of carbon in CEMP stars, identifying a high-carbon band and a low-carbon band. We propose an interpretation of this bimodality according to which the stars on the high-carbon band are the result of mass transfer from an AGB companion, while the stars on the low-carbon band are genuine fossil records of a gas Article published by EDP Sciences A28, page 1 of 20 A&A 579, A28 (2015) cloud that has also been enriched by a faint supernova (SN) providing carbon and the lighter elements. The abundance pattern of the UIP stars shows a large star-to-star scatter in the [X/Ca] ratios for all elements up to aluminium (up to 1 dex), but this scatter drops for heavier elements and is at most of the order of a factor of two. We propose that this can be explained if these stars are formed from gas that has been chemically enriched by several SNe, that produce the roughly constant [X/Ca] ratios for the heavier elements, and in some cases the gas has also been polluted by the ejecta of a faint SN that contributes the lighter elements in variable amounts. The absence of lithium in four of the five known unevolved UIP stars can be explained by a dominant role of fragmentation in the formation of these stars. This would result either in a destruction of lithium in the pre-main-sequence phase, through rotational mixing or to a lack of late accretion from a reservoir of fresh gas. The phenomenon should have varying degrees of efficiency.

show abstract

CEMP-s and CEMP-s/r stars: last update

Cited by 100 publications

References 59 publications

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

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

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

TOPoS

Contact Info

Product

Resources

About