“ Analysis and interpretation of stellar spectra and nucleosynthesis processes in evolved stars ” D. A. García-Hernández (IAC Support Astronomer) Instituto de Astrofísica de Canarias, Seminar presentation of IAC postdocs, December

Main lines of research Chemical abundances of AGB stars and the role of AGB stars in the Early Solar System composition Physical study of transition objects between the AGB phase and the Planetary Nebula stage and the spectral analysis (e.g. by using ISO and Spitzer) of their circumstellar dust shells CNO isotopic abundances of hydrogen-deficient carbon stars (R Coronae Borealis and HdC stars)

Stellar evolution: AGB stars Asymptotic Giant Branch: late stage of evolution of low- to intermediate-mass stars (1  M  8 M  ) TP phase: strong mass loss enriches the ISM with radionuclides and circumstellar dust grains!

AGB internal structure

AGB stellar nucleosynthesis Thermal Pulsing phase  12 C production, s-element production (Rb, Zr, Sr, Nd, Ba, Tc, etc.) 3 rd dredge-up very efficient in AGB stars; C/O ratio increases Stars eventually turn C-rich and s-process rich following the M-, MS-, S-, SC-, C-type sequence unless… Hot Bottom Burning (if M > 4  5 M  ) When T bce  K  12 C  13 C, 14 N (CN-cycle) and HBB prevents the carbon enrichment (stars remain O- rich) 26 Al, 7 Li production, low 12 C/ 13 C & high 17 O/ 16 O ratios (Mazzitelli et al. 99; Karakas & Lattanzio 03 )

The s-process in AGB stars Free neutrons to form heavier elements (s-elements such as Rb, Zr, Sr, etc.) can be released by 13 C( ,n) 16 O or by 22 Ne ( ,n) 25 Mg reactions (Busso et al. 99) 13 C operates during the interpulse period. It is more efficient in 1  3 M  AGB stars. All previous observations of AGB stars are consistent with 13 C! 22 Ne is expected to be efficient in the convective thermal pulse at higher T and N n. It should become strongly activated in more massive AGB stars (M>4  5 M  ). This prediction has never been confirmed by the observations!

The 22 Ne neutron source The operation of the 22 Ne neutron source favors the production of the stable isotope 87 Rb (also of 60 Fe, 41 Ca, 96 Zr, 25 Mg, 26 Mg, etc.) because of the operation of a branching in the s-process path at 85 Kr (Beer & Macklin 89) [Rb/Zr] is a powerful neutron density (and mass) indicator in AGBs! 87 Rb is a radioactive isotope (half-life time of ~48.8 Gyr ) and it is frequently used to date moon rocks and meteorites

Massive Galactic O-rich AGBs Where are they in our Galaxy? Theoretical models predicts Rb/Zr  in massive (M>4  5 M  ) O-rich AGB stars Galactic candidates: OH/IR stars (L , C/O<1, Long Period Variables). Expected to be massive O-rich stars in the final stages of their AGB evolution  Optical observations very difficult due to strong mass-loss (~ 10  4  10  6 M  /yr)  their chemical composition (Rb, Zr, etc.) is unknown! Very red stars!Extremely variable stars!

Discovery of Rb-rich AGB stars We observed ~100 OH/IR stars in the optical range during 4 observational campaings in La Palma (Spain) and La Silla (Chile) We obtained good high-resolution optical echelle spectra for half of the sample The other ~50 stars were completely invisible! We found that these stars are Rb-rich but Zr-poor  22 Ne confirmation  as predicted theoretically 40 years ago! We confirmed for the first time that the [Rb/Zr] ratio can be used as a mass indicator in AGB stars!

Rb-rich AGB stars OH/IR stars  strong mass loss  source of dust Li-rich  HBB  26 Al, 13 C and 17 O producers Rb-rich but Zr-poor  22 Ne  important source of 87 Rb, 60 Fe, 41 Ca (but also of 96 Zr, 25 Mg, 26 Mg, etc.) The more extreme stars are not predicted by the current models which do not consider the higher mass stars neither the strong mass loss! These stars, if present at the ESS, are more important at the early stages  a highly variable Rb/Sr ratio and 87 Rb/ 87 Sr ages should be taken with caution (García-Hernández et al. 06, 07)

ESS radionuclides inventory SN scenario may explain 60 Fe but not other radionuclides such as 26 Al, 41 Ca, 107 Pd Low-mass AGBs reproduce the other radionuclides but do not explain 60 Fe. 22 Ne is needed! (Wasserburg et al.06) Both models do not completely explain many of the radionuclide ESS concentrations. In particular, they cannot explain the 87 Rb anomalies detected in CAIs We cannot discard SN and low-mass AGBs in the ESS, but massive AGBs probably also played an important role, as evidenced by important Rb/Sr variations in CAIs (Podosek et al. 91; McKeegan and Davis 03) !

CAIs evidence 87 Rb anomalies are present in CAIs (as deduced from 87 Sr/ 86 Sr variations) (e.g. Podosek et al. 91) 41 Ca and 26 Al (also 25 Mg, 26 Mg) also present CAIs display important 60 Fe concentrations and it has been found that 60 Fe excesses are correlated with 96 Zr  22 Ne! (Quitté et al. 07) It is a mere coincidence that CAIs show all the chemical anomalies expected in massive AGB stars? New AGB stellar nucleosynthesis models explain these anomalies, giving a self-consistent solution to the ESS radionuclides (Trigo-Rodríguez et al. 08, submitted to MAPS)

CS dust shells AGB-PN The dust sequence from AGB to PNe as seen by ISO - A massive O-rich star - A very low-mass O-rich star Spitzer/IRS survey of heavily obscured PN precursors - Characterization of the IR spectral properties - Study of the total obscuration phase

O-rich transition sources O-rich PN precursors where the transition from amorphous to crystalline dust structure is taking place The crystalline silicate features are detected in emission inside the amorphous silicate absorptions at 9.7 & 18  m

Young IR PNe IR PNe as indicated by the detection of nebular emission lines (e.g. [Ar II], [Ar III], [Ne II], [S III]) O-rich PNe showing amorphous and crystalline silicates  probably massive PNe

R CrB and HdC stars Hydrogen-deficient (~10 5 ) luminous stars Their origins have remained a puzzle for decades Two scenarios have survived theoretical and observational scrutiny: - DD scenario (merger of a He and C-O WD) - FF scenario (final pAGB He flash in a CSPN) CNO isotopic abundances are a powerful tool for discriminating between the DD and FF scenarios

CNO isotopic abundances Gemini/PHOENIX near-IR (R~50,000) spectra of RCB & HdC stars 12 C/ 13 C, 14 N/ 15 N 16 O/ 17 O/ 18 O ratios can be derived from CO & CN transitions in the K-band (2.3  m) 

18 O-rich stars HdC stars and some RCBs are strongly enriched in 18 O ( 16 O/ 18 O~0.2  4) but C and N are in the form of 12 C and 14 N, respectively This suggests that HdC and RCB stars are related objects and that they probably formed from a WD merger. FF scenario cannot produce 18 O-rich stars Calculations of the nucleosynthesis achieved during the merger in the DD scenario need to be developed! (merger process in a few days with acretion rates of 150 M  yr -1, Clayton et al. 07 )

A very red massive O-rich AGB Visual Red Infrared

Extremely variable stars Visually Bright! Not found!

Optical echelle spectra “Blue example”“Red example”

A wide variety of Rb abundances are needed to fit the observations! First detection of strong Rubidium overabundances in massive AGB stars! T eff =3000 K [Rb/Fe]=+0.1 [Rb/Fe]=+0.9 [Rb/Fe]=+1.6 [Rb/Fe]=+2.3

Rb abundances vs. V exp (OH) V exp (OH) can be taken as a distance- independent mass indicator in OH/IR stars (e.g. Jiménez-Esteban et al 05) The more extreme stars are not predicted by the current models which do not consider the higher mass stars neither the strong mass loss! [Rb/Zr] >[Rb/Fe]  0.5 in these stars