14 N/ 15 N ratios in AGB C-stars and the origin of SiC grains Eurogenesis- Perugia Workshop, Nov 12-14, 2012 C. Abia R. Hedrosa (Granada) B. Plez (Montpellier) I. Domínguez (Granada) O. Straniero (Teramo) S. Cristallo (Teramo) S. Palmerini (Granada) Universidad de Granada
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14 N/ 15 N ratios in AGB C-stars and the origin of SiC grains Eurogenesis- Perugia Workshop, Nov 12-14, 2012 C. Abia R. Hedrosa (Granada) B. Plez (Montpellier)
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14N/15N ratios in AGB C-starsand the origin of SiC grains
• ~ 40 Galactic C-stars of spectral types N, SC, J and R-hot • 3.5 m TNG with SARG, R~170000, S/N > 300
• Red System of the CN molecule ~ 8000 Å (B. Plez)
• Atmosphere parameters (Teff, log g, [Fe/H], ) from literature
• MARCS models for C-stars, 2 M (Gustafsson et al. 2008)
• C/O ratio derived from 2.2 m spectra in a consistent way C2 lines C CO lines O CN lines N
Iteration until convergence was reached !!
Strong molecular contribution: metal lines barely seen C,N,O abundances critical: iteratively from CN, C2,CO (Carina) C-rich, metal-poor, spherical models from the Uppsala’s group
Errors are large, [X/H] ± 0.3- 0.5 dex
12C15N were carefuly selected according to:
- blending- sensitivity to the 14N/15N ratio- lines in the linear part of the curve-of-growth- uncertainty in the spectral continuum
Only eight 12C15N features selected in 7900 - 8100 Å
LQ Cyg
Total formal error in 14N/15N is large: factor 3-5
(dispersion among 12C15N, atmosphere parameters, CNO abundances, 12C/13C...)
Differential analysis line-by-line respect to TX Psc
...this would reduce systematic errors (N-LTE effects, oscillator strenghts, continuum position etc.)
For TX Psc we derived 14N/15N = 1700
....nevertheless, [14N/15N]TX Psc = 0.3 - 0.4 dex
Testing our results with the expert code DEGAS Gaia & ESO-Gaia projects
- DEGAS compares observed spectra with a huge grid of theoretical spectra, deriving stellar parameters & abundances (Kordopatis et al. 2011)
- Three free parameters: 14N/15N, continuum position,