[Paper Review] Radial gradients and metallicities in the galactic disk

Walter J. Maciel|arXiv (Cornell University)|Dec 8, 2000

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper derives an independent [O/Fe] × [Fe/H] relation for the galactic disk using radial abundance gradients from HII regions, hot stars, and open clusters. It finds that [O/Fe] does not exceed ≈0.4 dex for metallicities ≥ [Fe/H] ≈ −1.5, supporting lower oxygen abundances over high ones and favoring models with moderate [O/Fe] evolution.

ABSTRACT

Radial O/H abundance gradients derived from HII regions, hot stars and planetary nebulae are combined with [Fe/H] gradients from open cluster stars in order to derive an independent [O/Fe] x [Fe/H] relation for the galactic disk. A comparison of the obtained relation with recent observational data and theoretical models suggests that the [O/Fe] ratio is not higher than [O/Fe] ~ 0.4, at least within the metallicity range of the considered samples.

Motivation & Objective

• To derive an independent [O/Fe] × [Fe/H] relation for the galactic disk using observed radial abundance gradients.
• To resolve discrepancies in the [O/Fe] ratio at low metallicities between observational data and theoretical models.
• To assess whether high [O/Fe] ratios (~1.0) observed in some studies are consistent with disk abundance gradients.
• To constrain the maximum possible [O/Fe] ratio in the galactic disk based on observed O/H and [Fe/H] gradients.
• To evaluate the reliability of different oxygen abundance indicators (e.g., forbidden lines, OH bands) in metal-poor stars.

Proposed method

• Combines radial O/H gradients from HII regions, hot stars, and planetary nebulae (determined via spectroscopy of ionized gas and stellar atmospheres).
• Uses [Fe/H] gradients from open cluster stars, derived from high-resolution stellar spectroscopy.
• Applies the relation log(O/H) + 12 = a + bR and [Fe/H] = c + dR to model radial abundance trends, with R in kpc and R₀ = 7.6 kpc.
• Derives the [O/Fe] × [Fe/H] relation via the expression [O/Fe] = log(O/H) − log(Fe/H), using the derived gradients and intercepts.
• Compares the resulting [O/Fe]–[Fe/H] relation with observational data and theoretical models (e.g., Matteucci et al. 1999, Chiappini et al. 1997).
• Performs error propagation to assess uncertainty in the slope of the [O/Fe]–[Fe/H] relation, considering uncertainties in O/H and [Fe/H] gradients.

Experimental results

Research questions

• RQ1 What is the [O/Fe] ratio in the galactic disk as derived from observed radial abundance gradients of O/H and [Fe/H]?
• RQ2 Is the observed [O/Fe] ratio consistent with high values (~1.0) reported in some studies based on OH bands or forbidden lines?
• RQ3 Can the observed gradients constrain the maximum possible [O/Fe] ratio in the disk at metallicities ≥ [Fe/H] ≈ −1.5?
• RQ4 How do uncertainties in the O/H and [Fe/H] gradients affect the derived slope of the [O/Fe]–[Fe/H] relation?
• RQ5 Which observational indicators (e.g., [OI] lines, OH bands) are most reliable for determining [O/Fe] in metal-poor stars?

Key findings

• The derived [O/Fe] × [Fe/H] relation from radial gradients yields a maximum [O/Fe] ratio of approximately 0.4 dex for metallicities ≥ [Fe/H] ≈ −1.5.
• The observed O/H gradient is −0.070 ± 0.014 dex/kpc, and the [Fe/H] gradient is −0.085 ± 0.008 dex/kpc, both consistent with a declining metallicity with galactocentric radius.
• The average slope of the [O/Fe]–[Fe/H] relation is approximately −0.2, with steeper slopes ruled out by current data and uncertainties.
• The data and derived relation are inconsistent with high [O/Fe] ratios (~1.0) reported in some studies using OH bands or infrared lines.
• The results support models such as Matteucci et al. (1999) and Chiappini et al. (1997), which predict moderate [O/Fe] evolution, over models predicting [O/Fe] ≈ 1.0 at low metallicities.
• Recent analyses (e.g., Carretta et al. 2000) suggest that high [O/Fe] values from OH bands may be unreliable due to continuum placement issues, further supporting the lower [O/Fe] limit.