The Evolution of the C/O Ratio in Metal-poor Halo Stars

By C. Akerman, L. Carigi, P. Nissen, M. Pettini, Martin Asplund. 2003

183 citations recorded by the source corpus.

We report new measurements of carbon and oxygen abundances in 34 F and G dwarf and subgiant stars belonging to the halo population and spanning a range of metallicity from (Fe/H) = −0. 7t o−3.2 . The survey is based on observations of four permitted lines of C  near 9100 A and the O  λ7774 triplet, all recorded at high signal-to-noise ratios with the UVES echelle spectrograph on the ESO VLT. The line equivalent widths were analysed with the 1D, LTE, MARCS model atmosphere code to deduce C and O abundances; corrections due to non-LTE and 3D effects are discussed. When combined with similar published data for disk stars, our results confirm the metallicity dependence of the C/O ratio known from previous stellar and interstellar studies: C/O drops by a factor of ∼3-4 as O/H decreases from solar to ∼1/10 solar. Analysed within the context of standard models for the chemical evolution of the solar vicinity, this drop results from the metallicity dependence of the C yields from massive stars with mass loss, augmented by the delayed release of C from stars of low and intermediate mass. The former is, however, always the dominant factor. Our survey has also uncovered tentative evidence to suggest that, as the oxygen abundance decreases below (O/H) = −1, (C/O) may not remain constant at (C/O) = −0.5, as previously thought, but increase again, possibly approaching near-solar values at the lowest metallicities ((O/H) < ∼ − 3). With the current dataset this is no more than a 3σ effect and it may be due to metallicity-dependent non-LTE corrections to the (C/O) ratio which have not been taken into account. However, its potential importance as a window on the nucleosynthesis by Population III stars is a strong incentive for future work, both observational and theoretical, to verify its reality.

Subfields: Astronomy and Astrophysics, Electrical and Electronic Engineering, Instrumentation

View Paper: https://www.semanticscholar.org/paper/90dbcd551d498363789073034439f0bd001b448d

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In Astronomy and Astrophysics

1. M. Pettini
2. C. Steidel
3. A. Shapley
4. Naveen A. Reddy
5. D. Erb
6. C. Conroy
7. Timothy C. Beers
8. P. Hopkins

In Electrical and Electronic Engineering

1. S. Charlot
2. Julia Gutkin
3. D. Stark
4. C. Steidel
5. Naveen A. Reddy
6. M. Pettini
7. D. Erb
8. Brant Robertson

In Instrumentation

1. C. Steidel
2. A. Shapley
3. M. Pettini
4. Naveen A. Reddy
5. D. Erb
6. C. Conroy
7. S. Charlot
8. Brant Robertson

Citations in the corpus

• Probing the Interstellar Medium near Star-forming Regions with Gamma-Ray Burst Afterglow Spectroscopy: Gas, Metals, and Dust (2007, methodology): “Therefore, we infer N(C+) from the [α/H] abundance and NHI value, assuming [α/C]=+0.3 (i.e. a modest α-enhancement; Akerman et al. 2004): logN(C+) = [α/H] + logNHI − 12 + 8.3 .”
• First stars VI - Abundances of C, N, O, Li, and mixing in extremely metal-poor giants. Galactic evolution of the light elements (2004, background): “Akerman et al. (2004) recently studied the variation of the [C/O] ratio vs. [O/H] in metal-poor halo stars.”
• Evolution of interstellar dust and stardust in the solar neighbourhood (2007, methodology): “The observed stellar element abundances for F and G stars from the solar neighbourhood are shown with di ff erent symbols for each of the sources (Akerman et al. 2004; Reddy et al. 2003; Soubiran et al. 2005; Melendez et al. 2002; Jonsell et al. 2005; Venn et al. 2004; Chen et al. 2000; Gratton et…”
• C, N, O abundances in the most metal-poor damped Lyman alpha systems★ (2007, methodology): “One of the aims of the present work is to investigate the behaviour of the C/O ratio at low metallicities, motivated by the results of Akerman et al. (2004) and Spite et al. (2005) which we now summarise.”
• Modelling the nebular emission from primeval to present-day star-forming galaxies (2016, background): “Intriguingly, this trend appears to turn over at the lowest metallicities, where the C/O ratio increases again (e.g., Akerman et al. 2004).”
• A High-Resolution Survey of Low-Redshift QSO Absorption Lines: Statistics and Physical Conditions of O VI Absorbers (2007, background): “Some studies (e.g., Akerman et al. 2004) have indicated that C/O is not constant as a function of metallicity and may even have a somewhat complex dependence on metallicity.”
• THE BIMODAL METALLICITY DISTRIBUTION OF THE COOL CIRCUMGALACTIC MEDIUM AT z ≲ 1 (2013, unknown): “8, we show [C/α]-[α/H] dependency determined from stellar (Akerman et al. 2004; Fabbian et al. 2009) and z > 2 SLLS and DLAs (Pettini et al. 2008; Penprase et al. 2010) observations.”
• The most metal-poor damped Lyα systems: insights into chemical evolution in the very metal-poor regime★ (2011, methodology): “The third of these possibilities has recently been ruled out by Fabbian et al. (2009a), who conducted a more detailed analysis of the lines used by Akerman et al. (2004), thus confirming the stellar C/O trend at low metallicity.”
• PHYSICAL CONDITIONS IN A YOUNG, UNREDDENED, LOW-METALLICITY GALAXY AT HIGH REDSHIFT (2010, background): “Carigi et al. (2005) find that massive and low to intermediate mass stars contribute roughly equal amounts of carbon in the solar vicinity, while Akerman et al. (2004) find that the metallicitydependent mass loss of high mass stars is the primary factor in producing the C/O vs.”
• Quantifying the uncertainties of chemical evolution studies II. Stellar yields (2010, methodology): “Akerman et al. (2004) corrected the oxygen abundances for non-LTE effects, but did not apply any non-LTE correction to the [C/O] ratios, under the assumption that non-LTE corrections for the permitted C I and O I lines are of similar size.”
• A reappraisal of the chemical composition of the Orion nebula based on Very Large Telescope echelle spectrophotometry (2004, result): “This difference is in agreement with the estimations of the chemical evolution models by Carigi (2003) and Akerman et al. (2004) who found that the O/H ratio at the solar galactocentric distance has increased by 0.12 dex since the Sun was formed.”
• CARBON-ENHANCED METAL-POOR STAR FREQUENCIES IN THE GALAXY: CORRECTIONS FOR THE EFFECT OF EVOLUTIONARY STATUS ON CARBON ABUNDANCES (2014, unknown): “Individual references include: Allen et al. (2012), Akerman et al. (2004), Aoki et al. (2002, 2005, 2006, 2007, 2008, 2013), Barklem et al. (2005), Cohen et al. (2008, 2013), Cui et al. (2013), Goswami et al. (2006), Gratton et al. (2000), Hansen et al. (2011, 2014), Hollek et al. (2011, 2014), Ito…”
• KECK HIRES SPECTROSCOPY OF EXTRAGALACTIC H ii REGIONS: C AND O ABUNDANCES FROM RECOMBINATION LINES (2009, background): “This behavior has also been observed in samples of nearby F and G dwarf stars of the Galactic disk (Bensby & Feltzing 2006) and dwarf and subgiant stars of the same spectral types belonging to the Galactic halo population (Akerman et al. 2004).”