Analysis and interpretation of stellar spectra and nucleosynthesis processes in evolved stars

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Analysis and interpretation of stellar spectra
andnucleosynthesis processes in evolved stars

• D. A. García-Hernández (IAC Support Astronomer)

Instituto de Astrofísica de Canarias, Seminar
presentation of IAC postdocs, December 13 2007
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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)

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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!
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AGB internal structure
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AGB stellar nucleosynthesis

• Thermal Pulsing phase ? 12C production,
  s-element production (Rb, Zr, Sr, Nd, Ba, Tc,
  etc.)
• 3rd 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 gt 4?5 M?)
• When Tbce? 2.107 K ? 12C ? 13C, 14N (CN-cycle)
  and HBB prevents the carbon enrichment (stars
  remain O-rich)
• 26Al, 7Li production, low 12C/13C high 17O/16O
  ratios (Mazzitelli et al. 99 Karakas Lattanzio
  03 )

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The s-process in AGB stars

• Free neutrons to form heavier elements
  (s-elements such as Rb, Zr, Sr, etc.) can be
  released by 13C(?,n) 16O or by 22Ne (?,n)25Mg
  reactions (Busso et al. 99)
• 13C operates during the interpulse period. It is
  more efficient in 1?3 M? AGB stars. All previous
  observations of AGB stars are consistent with
  13C!
• 22Ne is expected to be efficient in the
  convective thermal pulse at higher T and Nn. It
  should become strongly activated in more massive
  AGB stars (Mgt4?5 M?). This prediction has never
  been confirmed by the observations!

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The 22Ne neutron source

• The operation of the 22Ne neutron source favors
  the production of the stable isotope 87Rb (also
  of 60Fe, 41Ca, 96Zr, 25Mg, 26Mg, etc.) because of
  the operation of a branching in the s-process
  path at 85Kr (Beer Macklin 89)

Rb/Zr is a powerful neutron density (and mass)
indicator in AGBs! 87Rb is a radioactive isotope
(half-life time of 48.8 Gyr ) and it is
frequently used to date moon rocks and meteorites
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Massive Galactic O-rich AGBs

• Where are they in our Galaxy?
• Theoretical models predicts Rb/Zr? in massive
  (Mgt4?5 M?) O-rich AGB stars
• Galactic candidates OH/IR stars (L?, C/Olt1,
  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!
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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
  ? 22Ne 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!

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Rb-rich AGB stars

• OH/IR stars ? strong mass loss ? source of dust
• Li-rich ? HBB ? 26Al, 13C and 17O producers
• Rb-rich but Zr-poor ? 22Ne ? important source of
  87Rb, 60Fe, 41Ca (but also of 96Zr, 25Mg, 26Mg,
  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 87Rb/87Sr ages should be taken
  with caution
• (García-Hernández et al. 06, 07)

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ESS radionuclides inventory

• SN scenario may explain 60Fe but not other
  radionuclides such as 26Al, 41Ca, 107Pd
• Low-mass AGBs reproduce the other radionuclides
  but do not explain 60Fe. 22Ne is needed!
  (Wasserburg et al.06)
• Both models do not completely explain many of the
  radionuclide ESS concentrations. In particular,
  they cannot explain the 87Rb 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) !

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CAIs evidence

• 87Rb anomalies are present in CAIs (as deduced
  from 87Sr/86Sr variations) (e.g. Podosek et al.
  91)
• 41Ca and 26Al (also 25Mg, 26Mg) also present
• CAIs display important 60Fe concentrations and it
  has been found that 60Fe excesses are correlated
  with 96Zr ? 22Ne! (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)
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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

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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
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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
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R CrB and HdC stars

• Hydrogen-deficient (105) 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

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CNO isotopic abundances
Gemini/PHOENIX near-IR (R50,000) spectra of
RCB HdC stars 12C/13C, 14N/15N 16O/17O/18O
ratios can be derived from CO CN
transitions in the K-band (2.3 ?m)?
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18O-rich stars

• HdC stars and some RCBs are strongly enriched in
  18O (16O/18O0.2?4) but C and N are in the form
  of 12C and 14N, respectively
• This suggests that HdC and RCB stars are related
  objects and that they probably formed from a WD
  merger. FF scenario cannot produce 18O-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)

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A very red massive O-rich AGB
Visual Red
Infrared
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Extremely variable stars
Visually Bright!
Not found!
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Optical echelle spectra
Blue example

Red example
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Teff3000 K Rb/Fe0.1 Rb/Fe0.9
Rb/Fe1.6 Rb/Fe2.3
A wide variety of Rb abundances are needed to
fit the observations! First detection of strong
Rubidium overabundances in massive AGB stars!
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Rb abundances vs. Vexp(OH)
Vexp(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 gtRb/Fe ? 0.5 in these stars