OBSERVATIONAL ESTIMATION OF NITROGEN PRODUCTION

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OBSERVATIONAL ESTIMATION OF NITROGEN
PRODUCTION IN UPPER AGB STARS BY HOT-BOTTOM
BURNING Peter Wood Research School of Astronomy
Astrophysics Australian National
University In collaboration with Jenny
McSaveney, Michael Scholz, John Lattanzio Ken
Hinkle (MNRAS, 378, 1089)
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• SITES OF NITROGEN PRODUCTION
• Interiors of massive rotating stars
  circulation currents bring CNO-cycles matter to
  the surface, where it can be lost via stellar
  winds and supernova explosion
• Hot-Bottom Burning (HBB) in AGB stars with M gt
  3-4 Msun

CN cycle
N up C down
Third dredge-up of primary 12C 4H -gt 4He 34He
-gt 12C
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Theoretical models are uncertain (convective
overshoot). Observational calibration is required.
N produced by HBB
Iben Truran (1978)
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Indications that HBB is occurring
Smith Lambert (1990) luminous LMC AGB stars
show enhanced Li line strengths
Brett (1991) CN bandstrengths stronger than
expected for 3rd dredge-up alone
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N abundance measurements Smith et al (2002) LMC
red giants show evidence for CN cycling only
(first and second dredge-up) Galactic M and S
stars are mostly similar. However, a few S stars
show evidence for enhanced N, but their masses
and luminosities are not known.
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• The stars where HBB occurs are near the end of
  their AGB lives.
• They are large-amplitude, long-period variables.
• Modelling their atmospheres for spectral analyis
  is difficult.

Magellanic Cloud Long Period Variables
Wood et al. (1983)
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Gemini South Phoenix high-resolution infrared
spectrograph Spectra taken in service mode in
2002-2003.
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The bottom 3 (coolest) spectra in the figure are
impossibly difficult didn't attempt to model
them. OH -gt O abundance CN ...
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CO O abundance -gt C abundance CN C abundance
-gt N abundance
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THE MODELS

• Pulsation models to reproduce light curves,
  colours (V-K)
• Model atmospheres to refine the temperature
  structure of the outer layers, given the density
  and velocity structure of the pulsation model
• Spectral synthesis given the model atmosphere

L from JK photometry LMC or SMC
distance Teff from V-K colour M to get
correct pulsation period
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Pulsation and atmosphere models V light curve
from MACHO K light curve from 2.3m
Structure from pulsation models (solid lines)
and atmosphere model (dashed line).
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A star with a strong shock wave (deep) in the
atmosphere
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Line emission

• Two stars could not be modelled because of
  atmospheric shocks.
• The models show the phases of the light cycle at
  which stars should be observed for successful
  modelling.
• Only two stars left for abundance analysis!!!!

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Individual lines were synthesized to get
abundances
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Carbon down by a factor 5
Nitrogen up by a factor 10
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The N abundances are larger than can be
produced by CN cycling of pre-existing C N
nuclei require Third Dredge-Up HBB.
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HV2576 AGB models (6 Msun)
Initial C abund Initial N abund
Domain where N up by 10, C down by 5. Mid-HBB
phase. Optically visible stars.
Karakas (2003)
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5 Msun AGB models
Initial C abund Initial N abund
Domain where N up by 10, C down by 5. Early HBB
phase. Optically visible stars.
Karakas (2003)
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NGC18664 AGB models (4 Msun)
Initial C abund Initial N abund
No hot-bottom burning! Need deeper convection
overshoot (Ventura et al. (2002)
Karakas (2003)
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HV2576 AGB models (6 Msun)
Li abundance Smith et al (1995) -gt A(Li) 3.8
(HV2576) Higher than the models suggest.
Karakas (2003)
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NGC18664 AGB models (4 Msun)
Initial C abund Initial N abund
Li abundance Maceroni et al (2002) -gt A(Li)
1.5 (NGC1866 4). Supports the need for deeper
convection or overshoot.
Karakas (2003)
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SUMMARY

• First determination of N production by HBB in
  massive AGB stars
• Should take spectra at light phases 0.4 to 0.6
• Standard AGB models underestimate the amount of
  hot-bottom burning overshoot at convective
  boundaries is required
• Only two stars analysed needs confirmation
  (CRIRES)