The C/O ratio at low metallicity: 
constraints on early chemical evolution from observations   of Galactic halo stars

Fabbian, D.; Nissen, P. E.; Asplund, Martin; Pettini, Max; Akerman, Chris J.

doi:10.1051/0004-6361/200810095

Cited by 100 publications

(148 citation statements)

References 50 publications

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“…However, UM 469 presents a large error and its position should be even considered compatible with the general trend considering the uncertainties. Figure 9 further extends the C/O vs. O/H diagram showing the behaviour of very metal-poor objects: stars of the Galactic halo (Fabbian et al 2009) and damped Lyα (DLA) systems (Cooke et al 2011). For the Milky Way, it appears that the amount of C in halo stars is dominated by the contribution from massive stars (e.g.…”

Section: The C/o Vs O/h Relationmentioning

confidence: 69%

“…For the Milky Way, it appears that the amount of C in halo stars is dominated by the contribution from massive stars (e.g. Akerman et al 2004;Fabbian et al 2009). Figure 9 shows that the position of metal-poor star-forming dwarf galaxies and halo stars is similar, suggesting the same origin for the production of the bulk of C in those galaxies.…”

Section: The C/o Vs O/h Relationmentioning

confidence: 99%

See 1 more Smart Citation

Carbon and oxygen abundances from recombination lines in low-metallicity star-forming galaxies. Implications for chemical evolution★

Esteban

García‐Rojas

Carigi³

et al. 2014

Monthly Notices of the Royal Astronomical Society

110

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We present deep echelle spectrophotometry of the brightest emission-line knots of the star-forming galaxies He 2−10, Mkn 1271, NGC 3125, NGC 5408, POX 4, SDSS J1253−0312, Tol 1457−262, Tol 1924−416 and the H ii region Hubble V in the Local Group dwarf irregular galaxy NGC 6822. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3100-10420Å range. We determine electron densities and temperatures of the ionized gas from several emission-line intensity ratios for all the objects. We derive the ionic abundances of C 2+ and/or O 2+ from faint pure recombination lines (RLs) in several of the objects, permitting to derive their C/H and C/O ratios. We have explored the chemical evolution at low metallicities analysing the C/O vs. O/H, C/O vs. N/O and C/N vs. O/H relations for Galactic and extragalactic H ii regions and comparing with results for halo stars and DLAs. We find that H ii regions in star-forming dwarf galaxies occupy a different locus in the C/O vs. O/H diagram than those belonging to the inner discs of spiral galaxies, indicating their different chemical evolution histories, and that the bulk of C in the most metal-poor extragalactic H ii regions should have the same origin than in halo stars. The comparison between the C/O ratios in H ii regions and in stars of the Galactic thick and thin discs seems to give arguments to support the merging scenario for the origin of the Galactic thick disc. Finally, we find an apparent coupling between C and N enrichment at the usual metallicities determined for H ii regions and that this coupling breaks in very low-metallicity objects.

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Section: The C/o Vs O/h Relationmentioning

confidence: 69%

Section: The C/o Vs O/h Relationmentioning

confidence: 99%

Carbon and oxygen abundances from recombination lines in low-metallicity star-forming galaxies. Implications for chemical evolution★

Esteban

García‐Rojas

Carigi³

et al. 2014

Monthly Notices of the Royal Astronomical Society

110

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“…Stellar abundance data were compiled from several different studies. For the solar neighbourhood, the data were compiled from several authors (Bensby & Feltzing 2006;Ecuvillon et al 2004;Fabbian et al 2009a;Gratton et al 2000;Gustafsson et al 1999, Jonsell et al 2005, Spite et al 2005, while the Cepheid abundances in Fig. 7 were taken from the works by Andrievski et al (2002aAndrievski et al ( ,b,c, 2004 and Luck et al (2003Luck et al ( , 2006.…”

Section: Abundance Datamentioning

confidence: 99%

“…Recent observations have revealed a declining trend with increasing metallicity for the C/O ratio in the solar neighbourhood at early times (Akerman et al 2004;Fabbian et al 2009a). C/O vs. O/H shows a negative slope roughly until the anticipated onset of disc formation, i.e., during the first billion years of Galactic evolution, which is in disagreement (see, e.g.…”

Section: Introductionmentioning

confidence: 99%

The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?

Mattsson

2010

A&A

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Multi-zone chemical evolution models (CEMs), differing in the nucleosynthesis prescriptions (yields) and prescriptions of star formation, have been computed for the Milky Way. All models fit the observed O/H and Fe/H gradients well and reproduce the main characteristics of the gas distribution, but they are also designed to do so. For the C/H gradient the results are inconclusive with regards to yields and star formation. The C/Fe and O/Fe vs. Fe/H, as well as C/O vs. O/H trends predicted by the models for the solar neighbourhood zone were compared with stellar abundances from the literature. For O/Fe vs. Fe/H all models fit the data, but for C/O vs. O/H, only models with increased carbon yields for zero-metallicity stars or an evolving initial mass function provide good fits. Furthermore, a steep star formation threshold in the disc can be ruled out since it predicts a steep fall-off in all abundance gradients beyond a certain galactocentric distance (∼13 kpc) and cannot explain the possible flattening of the C/H and Fe/H gradients in the outer disc seen in observations. Since in the best-fit models the enrichment scenario is such that carbon is primarily produced in low-mass stars, it is suggested that in every environment where the peak of star formation happened a few Gyr back in time, winds of carbon-stars are responsible for most of the carbon enrichment. However, a significant contribution by zero-metallicity stars, especially at very early stages, and by winds of high-mass stars, which are increasing in strength with metallicity, cannot be ruled out by the CEMs presented here. In the solar neighbourhood, as much as 80%, or as little as 40% of the carbon may have been injected to the interstellar medium by low-and intermediate-mass stars. The stellar origin of carbon remains an open question, although production in low-and intermediate-mass stars appears to be the simplest explanation of observed carbon abundance trends.

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“…In fact, until 2004 no conclusive data were available for nitrogen in metal-poor halo stars. This situation has improved with the First Stars ESO large program , which obtained CNO abundances for the first time for a sample of giants with metallicities below [Fe/H] = − 2.5 (Spite et al 2005(Spite et al , 2006 -see also Fabbian et al 2009). It turned out that these VMP halo stars had N/O ratios around solar, suggestive of high levels of production of primary nitrogen in massive stars.…”

Section: Introductionmentioning

confidence: 99%

The effects of stellar winds of fast-rotating massive stars in the earliest phases of the chemical enrichment of the Galaxy

Cescutti

Chiappini

2010

A&A

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Aims. We use the growing data sets of very-metal-poor stars to study the impact of stellar winds of fast rotating massive stars on the chemical enrichment of the early Galaxy. Methods. We use an inhomogeneous chemical evolution model for the Galactic halo to predict both the mean trend and scatter of C/O and N/O. In one set of models, we assume that massive stars enrich the interstellar medium during both the stellar wind and supernovae phases. In the second set, we consider that in the earliest phases (Z < 10 −8 ), stars with masses above 40 M only enrich the interstellar medium via stellar winds, collapsing directly into black holes. Results. We predict a larger scatter in the C/O and N/O ratios at low metallicities when allowing the more massive fast-rotating stars to contribute to the chemical enrichment only via stellar winds. The latter assumption, combined with the stochasticity in the star formation process in the primordial Galactic halo can explain the wide spread observed in the N/O and C/O ratios in normal very-metal-poor stars. Conclusions. For chemical elements with stellar yields that depend strongly on initial mass (and rotation) such as C, N, and neutron capture elements, within the range of massive stars, a large scatter is expected once the stochastic enrichment of the early interstellar medium is taken into account. We also find that stellar winds of fast rotators mixed with interstellar medium gas are not enough to explain the large CNO enhancements found in most of the carbon-enhanced very-metal-poor stars. In particular, this is the case of the most metal-poor star known to date, HE 1327−2326, for which our models predict lower N enhancements than observed when assuming a mixture of stellar winds and interstellar medium. We suggest that these carbon-enhanced very metal-poor stars were formed from almost pure stellar wind material, without dilution with the pristine interstellar medium.

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The C/O ratio at low metallicity: constraints on early chemical evolution from observations of Galactic halo stars

Cited by 100 publications

References 50 publications

Carbon and oxygen abundances from recombination lines in low-metallicity star-forming galaxies. Implications for chemical evolution★

Carbon and oxygen abundances from recombination lines in low-metallicity star-forming galaxies. Implications for chemical evolution★

The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?

The effects of stellar winds of fast-rotating massive stars in the earliest phases of the chemical enrichment of the Galaxy

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