The Origins of Two Classes of Carbon‐enhanced, Metal‐poor Stars

Ryan, Sean G.; Aoki, Wako; Norris, John E.; Beers, Timothy C.

doi:10.1086/497282

Cited by 77 publications

(102 citation statements)

References 37 publications

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“…However, as already noted by Aoki et al (2007), some main sequence CEMP-no stars exist with similar N enhancements and similar 12 C/ 13 C ratios as CEMP-no giants (Fig. 19), thus ruling out the hypothesis of Ryan et al (2005). Therefore, if supernovae and/or massive winds are responsible for the C, N, and isotopic ratios, the yields from these objects must already bear the signature of CN-processed material at the time it leaves the massive star.…”

Section: Absence Of Neutron-capture Signaturesupporting

confidence: 58%

“…1, there is a lack of radial-velocity measurements to constrain the binary rate of CEMP-no stars. Ryan et al (2005) observed that all CEMP-no stars in their compilation were post-main-sequence stars, leading these authors to suggest that CEMP-no stars have undergone first dredgeup and have processed some pristine C into N themselves. In their scenario, CEMP-no stars were born out of gas with high C content from pollution by (possibly) low-energy supernovae or winds from massive stars, C that was then processed to produce the high N and low 12 C/ 13 C in the CEMP-no stars in their sample.…”

Section: Absence Of Neutron-capture Signaturementioning

confidence: 99%

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A holistic approach to carbon-enhanced metal-poor stars

Masseron

Johnson²,

Plez

et al. 2010

A&A

238

462

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Context. Carbon-enhanced metal-poor (CEMP) stars are known to have properties that reflect the nucleosynthesis of the first lowand intermediate-mass stars, because most have been polluted by a now-extinct AGB star. Aims. By considering abundances in the various CEMP subclasses separately, we try to derive parameters (such as metallicity, mass, temperature, and neutron source) characterising AGB nucleosynthesis from the specific signatures imprinted on the abundances, and separate them from the impact of thermohaline mixing, first dredge-up, and dilution associated with the mass transfer from the companion. Methods. To place CEMP stars in a broader context, we collect abundances for about 180 stars of various metallicities (from solar to [Fe/H] = −4), luminosity classes (dwarfs and giants), and abundance patterns (e.g. C-rich and poor, Ba-rich and poor), from both our own sample and the literature. Results. We first show that there are CEMP stars that share the properties of CEMP-s stars and CEMP-no stars (which we refer to as CEMP-low-s stars). We also show that there is a strong correlation between Ba and C abundances in the s-only CEMP stars. This represents a strong detection of the operation of the 13 C neutron source in low-mass AGB stars. For the CEMP-rs stars (seemingly enriched with elements from both the s-and r-processes), the correlation of the N abundances with abundances of heavy elements from the 2nd and 3rd s-process peaks bears instead the signature of the 22 Ne neutron source. Since CEMP-rs stars also exhibit O and Mg enhancements, we conclude that extremely hot conditions prevailed during the thermal pulses of the contaminating AGB stars. We also note that abundances are not affected by the evolution of the CEMP-rs star itself (especially by the first dredge-up). This implies that mixing must have occurred while the star was on the main sequence, and that a large amount of matter must have been accreted so as to trigger thermohaline mixing. Finally, we argue that most CEMP-no stars (with neutron-capture element abundances comparable to non-CEMP stars) are likely the extremely metal-poor counterparts of CEMP neutron-capture-rich stars. We also show that the C enhancement in CEMP-no stars declines with metallicity at extremely low metallicity ([Fe/H] < −3.2). This trend is not predicted by any of the current AGB models.

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Section: Absence Of Neutron-capture Signaturesupporting

confidence: 58%

Section: Absence Of Neutron-capture Signaturementioning

confidence: 99%

A holistic approach to carbon-enhanced metal-poor stars

Masseron

Johnson²,

Plez

et al. 2010

A&A

238

462

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“…Combined with the observed low 12 C/ 13 C ratios (see e.g. Ryan et al 2005 andHansen et al 2015a, and references therein), this points towards a large amount of mixing that is not included in their models. Recent work by Abate et al (2015a,b), employing models of AGB nucleosynthesis production from Karakas (2010), reaches better agreement for some stars, while for others they have the same problem as Bisterzo et al (2012).…”

Section: Nucleosynthesis Challengesmentioning

confidence: 97%

The role of binaries in the enrichment of the early Galactic halo

Hansen

Andersen

Nordström

et al. 2016

A&A

142

146

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Context. Detailed spectroscopic studies of metal-poor halo stars have highlighted the important role of carbon-enhanced metal-poor (CEMP) stars in understanding the early production and ejection of carbon in the Galaxy and in identifying the progenitors of the CEMP stars among the first stars formed after the Big Bang. Recent work has also classified the CEMP stars by absolute carbon abundance, A(C), into high-and low-C bands, mostly populated by binary and single stars, respectively. Aims. Our aim is to determine the frequency and orbital parameters of binary systems among the CEMP-s stars, which exhibit strong enhancements of neutron-capture elements associated with the s-process. This allows us to test whether local mass transfer from a binary companion is necessary and sufficient to explain their dramatic carbon excesses. Methods. We have systematically monitored the radial velocities of a sample of 22 CEMP-s stars for several years with ∼monthly, high-resolution, low S/N échelle spectra obtained at the Nordic Optical Telescope (NOT) at La Palma, Spain. From these spectra, radial velocities with an accuracy of ≈100 m s −1 were determined by cross-correlation with optimised templates. Results. Eighteen of the 22 stars exhibit clear orbital motion, yielding a binary frequency of 82 ± 10%, while four stars appear to be single (18 ± 10%). We thus confirm that the binary frequency of CEMP-s stars is much higher than for normal metal-poor giants, but not 100% as previously claimed. Secure orbits are determined for eleven of the binaries and provisional orbits for six long-period systems (P > 3000 days), and orbital circularisation timescales are discussed. Conclusions. The conventional scenario of local mass transfer from a former asymptotic giant branch (AGB) binary companion does appear to account for the chemical composition of most CEMP-s stars. However, the excess of C and s-process elements in some single CEMP-s stars was apparently transferred to their natal clouds by an external (distant) source. This finding has important implications for our understanding of carbon enrichment in the early Galactic halo and some high-redshift damped lyman alpha (DLA) systems, and of the mass loss from extremely metal-poor AGB stars.

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“…The CEMP stars with [Ba/Fe] < 0 were called CEMP-no by Ryan et al (2005). However, some of the stars in the literature with [Ba/Fe] < 0 might be CEMP-r, rather than CEMP-no, if [Eu/Fe] > +1.…”

Section: Classification Of Cemp-r and Cemp-no Starsmentioning

confidence: 99%

Elemental abundances and classification of carbon-enhanced metal-poor stars

Allen

Ryan

Rossi

et al. 2012

A&A

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We present a detailed study of carbon-enhanced metal-poor (CEMP) stars, based on high-resolution spectroscopic observations of a sample of 18 stars. The stellar spectra for this sample were obtained at the 4.2 m William Herschel Telescope in 2001 and 2002, using the Utrecht Echelle Spectrograph, at a resolving power R ∼ 52 000 and S /N ∼ 40, covering the wavelength range λλ3700−5700 Å. The atmospheric parameters determined for this sample indicate temperatures ranging from 4750 K to 7100 K, log g from 1.5 to 4.3, and metallicities −3.0 ≤ [Fe/H] ≤ −1.7. Elemental abundances for C, Na, Mg, Sc, Ti, Cr, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, Eu, Gd, Dy are determined. Abundances for an additional 109 stars were taken from the literature and combined with the data of our sample. The literature sample reveals a lack of reliable abundance estimates for species that might be associated with the r-process elements for about 67% of CEMP stars, preventing a complete understanding of this class of stars, since [Ba/Eu] ratios are used to classify them. Although eight stars in our observed sample are also found in the literature sample, Eu abundances or limits are determined for four of these stars for the first time. From the observed correlations between C, Ba, and Eu, we argue that the CEMP-r/s class has the same astronomical origin as CEMP-s stars, highlighting the need for a more complete understanding of Eu production.

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The Origins of Two Classes of Carbon‐enhanced, Metal‐poor Stars

Cited by 77 publications

References 37 publications

A holistic approach to carbon-enhanced metal-poor stars

A holistic approach to carbon-enhanced metal-poor stars

The role of binaries in the enrichment of the early Galactic halo

Elemental abundances and classification of carbon-enhanced metal-poor stars

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