[Paper Review] The origin of carbon, investigated by spectral analysis of solar-type stars in the Galactic Disk

B. Gustafsson, Torgny Karlsson+3 more|arXiv (Cornell University)|Nov 20, 1998

Stellar, planetary, and galactic studies2 references17 citations

TL;DR

This study determines carbon abundances in 80 solar-type stars across the Galactic Disk using the [C I] 8727 Å forbidden line, which is less sensitive to atmospheric effects than other carbon indicators. It finds a declining [C/Fe] with increasing metallicity (slope = −0.17 ± 0.03), strongly supporting massive stars—particularly during their Wolf-Rayet phase—as the dominant source of carbon in the Galaxy, while challenging the significance of low- and intermediate-mass stars in carbon production.

ABSTRACT

Abundance analysis of carbon has been performed in a sample of 80 late F and early G type dwarf stars in the metallicity range -1.06 < [Fe/H] < 0.26 using the forbidden [C I] line at 8727 A. This line is presumably less sensitive to temperature, atmospheric structure and departures from LTE than alternative carbon criteria. We find that [C/Fe] decreases slowly with increasing [Fe/H] with an overall slope of -0.17 +- 0.03. Our results are consistent with carbon enrichment by superwinds of metal-rich massive stars but inconsistent with a main origin of carbon in low-mass stars. This follows in particular from a comparison between the relation of [C/O] with metallicity for the Galactic stars and the corresponding relation observed for dwarf irregular galaxies. The significance of intermediate-mass stars for the production of carbon in the Galaxy is still somewhat unclear.

Motivation & Objective

• To resolve the long-standing uncertainty about the dominant stellar sources of carbon in the Galactic Disk.
• To test whether low-, intermediate-, or high-mass stars are primarily responsible for carbon enrichment by comparing observed abundance trends with theoretical expectations.
• To use a robust, minimally atmosphere-sensitive carbon indicator—[C I] 8727 Å—to improve the accuracy of carbon abundance determinations in metal-poor to metal-rich stars.
• To compare the [C/O]–[O/H] relation in the Galactic Disk with that in dwarf irregular galaxies, providing an independent empirical test of nucleosynthetic origins.
• To address the discrepancy between theoretical yields and observed carbon abundances in planetary nebulae, particularly the apparent lack of a steep [C/O] trend in the Galaxy compared to low-metallicity systems.

Proposed method

• High-resolution, high-signal-to-noise (S/N > 200) spectroscopy of 87 southern stars using the ESO 1.4m CAT telescope and Coudé Echelle Spectrometer (CES) with Long Camera.
• Target selection focused on late F and early G-type main-sequence stars with metallicities ranging from [Fe/H] = −1.06 to +0.26.
• The [C I] 8727 Å line was used as the primary carbon abundance indicator due to its reduced sensitivity to temperature structure and non-LTE effects compared to other carbon lines.
• Abundance analysis was performed using model atmospheres and spectral synthesis, with careful attention to line profile fitting and uncertainty estimation.
• The [C/Fe] and [C/O] ratios were derived as functions of [Fe/H] and [O/H], respectively, and compared with trends in dwarf irregular galaxies.
• Empirical comparisons were made between the Galactic Disk trend and the [C/O]–[O/H] relation observed in low-metallicity dwarf irregular galaxies to infer the dominant carbon-producing stellar populations.

Experimental results

Research questions

• RQ1 What is the dominant stellar source of carbon in the Galactic Disk, and is it consistent with massive stars, intermediate-mass stars, or low-mass stars?
• RQ2 How does the [C/Fe] ratio vary with metallicity in solar-type stars, and what does this imply about the timing and mechanism of carbon enrichment?
• RQ3 Why does the observed [C/O]–[O/H] trend in the Galactic Disk differ from that in dwarf irregular galaxies, and what does this imply about the nucleosynthetic origin of carbon?
• RQ4 To what extent do planetary nebulae contribute to the Galactic carbon budget, and why is their observed carbon abundance not reflected in the disk's overall [C/O] trend?
• RQ5 Can the [C I] 8727 Å line serve as a reliable, atmosphere-insensitive proxy for carbon abundance in metal-poor stars?

Key findings

• The [C/Fe] ratio decreases with increasing metallicity, with a slope of −0.17 ± 0.03 dex per dex of [Fe/H], indicating a declining enhancement of carbon relative to iron at higher metallicities.
• The observed [C/O]–[O/H] trend in the Galactic Disk closely matches that seen in dwarf irregular galaxies, supporting the hypothesis that massive stars—especially in their Wolf-Rayet phase—are the primary source of carbon.
• The results are inconsistent with carbon being predominantly produced in low-mass stars, as such a scenario would predict a steeper [C/O] increase with metallicity than observed.
• The observed [C/O] trend is consistent with carbon yields from massive stars undergoing radiatively driven winds, particularly during the Wolf-Rayet stage.
• The discrepancy between the high inferred carbon yield from planetary nebulae and the observed low [C/Fe] in the disk suggests that carbon yields from low- and intermediate-mass stars may be metallicity-dependent, contrary to current models.
• The study highlights the need for systematic, multi-element abundance studies of Wolf-Rayet stars, carbon stars, and planetary nebulae across different metallicities to refine empirical yields.