CH in stellar atmospheres: an extensive linelist

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.

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“…The line list for the molecular bands such as NH, CH, C 2 , and CN are taken from Masseron et al (2014) and T. Masseron (priv. comm.).…”

Section: Line Listsmentioning

confidence: 99%

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

Hansen

Nordström

Hansen

et al. 2016

A&A

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“…The atomic line list (Table A.1) includes the isotopic shifts for Ba ii (with an update for isotopes 130 and 132) and the hyperfine structure for La ii from Masseron (2006). The CH molecular line list was taken from Masseron et al (2014) and CN from Sneden et al (2014).…”

Section: Abundancesmentioning

confidence: 99%

To Ba or not to Ba: Enrichment ins-process elements in binary systems with WD companions of various masses

Merle

Jorissen

Eck

et al. 2016

A&A

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Context. The enrichment in s-process elements of barium stars is known to be due to pollution by mass transfer from a companion formerly on the thermally pulsing asymptotic giant branch (AGB), now a carbon-oxygen white-dwarf (WD). Aims. We are investigating the relationship between the s-process enrichment in the barium star and the mass of its WD companion. It is expected that helium WDs, which have masses lower than about 0.5 M and never reached the AGB phase, should not pollute their giant companion with s-process elements. Therefore the companion should never turn into a barium star. Methods. Spectra with a resolution of R ∼ 86 000 were obtained with the HERMES spectrograph on the 1.2 m Mercator telescope for a sample of 11 binary systems involving WD companions of various masses. We used standard 1D local thermodynamical equilibrium MARCS model atmospheres coupled with the Turbospectrum radiative-transfer code that is embedded in the BACCHUS pipeline to derive the atmospheric parameters through equivalent widths of Fe i and Fe ii lines. Least-squares minimization between the observed and synthetic line shape was used to derive the detailed chemical abundances of CNO and s-process elements.Results. The abundances of s-process elements for the entire sample of 11 binary stars were derived homogeneously. The sample encompasses all levels of overabundances: from solar [s/Fe] = 0 to 1.5 dex in the two binary systems with S-star primaries (for which dedicated MARCS model atmospheres were used). The primary components of binary systems with a WD more massive than 0.5 M are enriched in s-process elements. We also found a trend of increasing [s/Fe] with [C/Fe] or [(C+N)/Fe]. Conclusions. Our results confirm the expectation that binary systems with WD companions less massive than 0.5 M do not host barium stars.

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“…They are listed in Table 3. The lower level excitation potentials and the transition probabilities were taken from Masseron et al (2014). The dissociation energy is D0(CH) = 3.465 eV (Huber & Herzberg 1979).…”

Section: Molecular C2 and Ch Linesmentioning

confidence: 99%

Carbon abundances of reference late-type stars from 1D analysis of atomic C i and molecular CH lines

Alexeeva

Mashonkina

2015

Mon. Not. R. Astron. Soc.

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A comprehensive model atom was constructed for C I using the most up-to-date atomic data. We evaluated non-local thermodynamical equilibrium (NLTE) line formation for neutral carbon in classical 1D models representing atmospheres of late-type stars, where carbon abundance varies from solar value down to [C/H] = −3. NLTE leads to stronger C I lines compared with their LTE strength and negative NLTE abundance corrections, ∆ NLTE . The deviations from LTE are large for the strong lines in the infrared (IR), with ∆ NLTE = −0.10 dex to −0.45 dex depending on stellar parameters, and they are minor for the weak lines in the visible spectral range, with |∆ NLTE | 0.03 dex. The NLTE abundance corrections were found to be dependent of the carbon abundance in the model. As the first application of the treated model atom, carbon NLTE abundances were determined for the Sun and eight late-type stars with well-determined stellar parameters that cover the −2.56 [Fe/H] −1.02 metallicity range. Consistent abundances from the visible and IR lines were found for the Sun and the most metal-rich star of our sample, when applying a scaling factor of S H = 0.3 to the Drawinian rates of C+H collisions. Carbon abundances were also derived from the molecular CH lines and, for each star, they agree with that from the atomic C I lines. We present the NLTE abundance corrections for lines of C I in the grid of model atmospheres applicable to the carbon-enhanced (CEMP) stars.

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CH in stellar atmospheres: an extensive linelist

Cited by 256 publications

References 63 publications

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

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

To Ba or not to Ba: Enrichment ins-process elements in binary systems with WD companions of various masses

Carbon abundances of reference late-type stars from 1D analysis of atomic C i and molecular CH lines

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