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First stars IX - Mixing in extremely metal-poor giants. Variation of the C-12/C-13, [Na/Mg] and [Al/Mg] ratios

Spite, M.; Cayrel, R.; Hill, V.; Spite, F.; Francois, P.; Plez, B.; Bonifacio, P.; Molaro, P.; Depagne, E. and Andersen, J., et al. (2006) In Astronomy & Astrophysics 455(1). p.291-301
Abstract
Context. Extremely metal-poor (EMP) stars preserve a fossil record of the composition of the ISM when the Galaxy formed. It is crucial, however, to verify whether internal mixing has modified their surface composition, especially in the giants where most elements can be studied. Aims. We aim to understand the CNO abundance variations found in some, but not all EMP field giants analysed earlier. Mixing beyond the first dredge-up of standard models is required, and its origin needs clarification. Methods. The C-12/C-13 ratio is the most robust diagnostic of deep mixing, because it is insensitive to the adopted stellar parameters and should be uniformly high in near-primordial gas. We have measured C-12 and C-13 abundances in 35 EMP giants... (More)
Context. Extremely metal-poor (EMP) stars preserve a fossil record of the composition of the ISM when the Galaxy formed. It is crucial, however, to verify whether internal mixing has modified their surface composition, especially in the giants where most elements can be studied. Aims. We aim to understand the CNO abundance variations found in some, but not all EMP field giants analysed earlier. Mixing beyond the first dredge-up of standard models is required, and its origin needs clarification. Methods. The C-12/C-13 ratio is the most robust diagnostic of deep mixing, because it is insensitive to the adopted stellar parameters and should be uniformly high in near-primordial gas. We have measured C-12 and C-13 abundances in 35 EMP giants (including 22 with [Fe/H] < -3.0) from high-quality VLT/UVES spectra analysed with LTE model atmospheres. Correlations with other abundance data are used to study the depth of mixing. Results. The C-12/C-13 ratio is found to correlate with [C/Fe] (and Li/H), and clearly anti-correlate with [N/Fe], as expected if the surface abundances are modified by CNO processed material from the interior. Evidence for such deep mixing is observed in giants above log L/L-circle dot = 2.6, brighter than in less metal-poor stars, but matching the bump in the luminosity function in both cases. Three of the mixed stars are also Na- and Al-rich, another signature of deep mixing, but signatures of the ON cycle are not clearly seen in these stars. Conclusions. Extra mixing processes clearly occur in luminous RGB stars. They cannot be explained by standard convection, nor in a simple way by rotating models. The Na- and Al-rich giants could be AGB stars themselves, but an inhomogeneous early ISM or pollution from a binary companion remain possible alternatives. (Less)
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keywords
interiors, stars :, stars : abundances, Galaxy : abundances, Galaxy : evolution, stars : supernovae : general, stars : evolution
in
Astronomy & Astrophysics
volume
455
issue
1
pages
291 - 301
publisher
EDP Sciences
external identifiers
  • wos:000239455100030
  • scopus:33747469496
ISSN
0004-6361
DOI
10.1051/0004-6361:20065209
language
English
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yes
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05326c6b-09ec-475c-b77f-d22d3b3c3829 (old id 399246)
date added to LUP
2007-10-19 12:54:29
date last changed
2019-06-04 02:29:57
@article{05326c6b-09ec-475c-b77f-d22d3b3c3829,
  abstract     = {Context. Extremely metal-poor (EMP) stars preserve a fossil record of the composition of the ISM when the Galaxy formed. It is crucial, however, to verify whether internal mixing has modified their surface composition, especially in the giants where most elements can be studied. Aims. We aim to understand the CNO abundance variations found in some, but not all EMP field giants analysed earlier. Mixing beyond the first dredge-up of standard models is required, and its origin needs clarification. Methods. The C-12/C-13 ratio is the most robust diagnostic of deep mixing, because it is insensitive to the adopted stellar parameters and should be uniformly high in near-primordial gas. We have measured C-12 and C-13 abundances in 35 EMP giants (including 22 with [Fe/H] &lt; -3.0) from high-quality VLT/UVES spectra analysed with LTE model atmospheres. Correlations with other abundance data are used to study the depth of mixing. Results. The C-12/C-13 ratio is found to correlate with [C/Fe] (and Li/H), and clearly anti-correlate with [N/Fe], as expected if the surface abundances are modified by CNO processed material from the interior. Evidence for such deep mixing is observed in giants above log L/L-circle dot = 2.6, brighter than in less metal-poor stars, but matching the bump in the luminosity function in both cases. Three of the mixed stars are also Na- and Al-rich, another signature of deep mixing, but signatures of the ON cycle are not clearly seen in these stars. Conclusions. Extra mixing processes clearly occur in luminous RGB stars. They cannot be explained by standard convection, nor in a simple way by rotating models. The Na- and Al-rich giants could be AGB stars themselves, but an inhomogeneous early ISM or pollution from a binary companion remain possible alternatives.},
  author       = {Spite, M. and Cayrel, R. and Hill, V. and Spite, F. and Francois, P. and Plez, B. and Bonifacio, P. and Molaro, P. and Depagne, E. and Andersen, J. and Barbuy, B. and Beers, T. C. and Nordström, Birgitta and Primas, F.},
  issn         = {0004-6361},
  keyword      = {interiors,stars :,stars : abundances,Galaxy : abundances,Galaxy : evolution,stars : supernovae : general,stars : evolution},
  language     = {eng},
  number       = {1},
  pages        = {291--301},
  publisher    = {EDP Sciences},
  series       = {Astronomy & Astrophysics},
  title        = {First stars IX - Mixing in extremely metal-poor giants. Variation of the C-12/C-13, [Na/Mg] and [Al/Mg] ratios},
  url          = {http://dx.doi.org/10.1051/0004-6361:20065209},
  volume       = {455},
  year         = {2006},
}