14N/15N ratios in AGB C-starsand 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

CW Leo @ 0.6 m

10 m

AGB stars: polluters of the ISM

CW Leo 1 M

Detached shells: evidence of mass loss 10-8- 10-4 M/yr

> 50% of the mass (dust) injected into the ISM comes from AGB stars

Busso et al. (1999)

12C

12C

M MS S SC ? N C/O ~ 0.5................1... > 1

AGB chemical & spectral evolution

C/O < 1

C/O > 1

Effect on the spectrum

Li I Ca I

CN & C2

The C/O ratio also determines types of condensates

C/O < 1, oxides & silicatesC/O>1, carbides, carbon, etc

SiC grains

The giant C-stars Zoo

Type Li s-process 12C/13C Remarks

N no yes >40

SC some some 4 -100 very few

J yes no <15 15%

30% binary

R-cool no yes >40 identical to N Zamora et al. 2010

R-hot no no low No AGBs, many are CH stars Zamora et al. 2010

Thus, real AGB C-stars are N-, J- & and SC-type only

The Zoo of SiC grains & AGB C-stars

© Davis (2011)

standar AGB evolution

extramixing

allowed zone

Observations & Analysis

• ~ 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,

For 7 stars studied so far with DEGAS

(14N/15N)DEGAS- here = -29 230

Results

(14N/15N)TX Psc = 1700

Comparison with SiC grains

Open symbols are lower limits

http://presolar.wustl.edu/Laboratory_for_Space_Sciences/Home.html

Theoretical dependence on the initial 14N/15N ratio (no extramixing in RGB & AGB)

2 M, Z = 0.014, 14N/15N

Wielen & Wilson (1997)

ISM

Initial FDU 50 212

254 1007

2550 5117

254 883 (1.5 M)

254 1632 (3.0 M)

© FRANEC code

Different initial masses might help to explain the observed 14N/15N range in N-type stars...

Variation along the AGB phase: s-elements6

96Mo is s-only & 92Mo p-only

14N/15N appears not be afected by TDU episodes

Correlations with tracers of stellar interior: Li & F

18O(p,)15N(,)19F

SUMMARY

We derive for the first time 14N/15N ratios in AGB C-stars

14N/15N appear to be different for different spectral types

14N/15N in N-type C-stars agree nicely with MS SiC grains

First evidence of a contribution of J-type stars to AB grains

SC-type stars might be enhanced in 15N.... challenge for stellar evolution and nucleosynthesis !!

Future To extend the sample of C-stars with 14N/15N ratios Study the 16O/17O/18O ratios in the sample stars Theoretical interpretation.....

Structure of an AGB star (1 M/M 8)

CO

H-rich

He

H shell burning

He shell burning

Convective envelope

~ 200 R