[Paper Review] Chemical abundances and temperature structure of H II regions

G. Domínguez-Guzmán, M. Rodríguez+2 more|arXiv (Cornell University)|Jun 5, 2019

Atmospheric Ozone and Climate4 citations

TL;DR

This study analyzes high-resolution spectra of 37 H II regions, including eight new Magellanic Clouds observations, to investigate chemical abundance ratios and temperature structure. By adjusting the temperature structure model—using intermediate temperatures and combining [N ii] and [O iii] electron temperatures—it shows that S/O, Cl/O, and Ar/O ratios are constant with metallicity, resolving discrepancies from traditional two-zone models.

ABSTRACT

We use a sample of 37 H II regions with high quality spectra to study the behavior of the relative abundances of several elements as a function of metallicity. The sample includes spectra for eight H II regions of the Magellanic Clouds, obtained with UVES/VLT; the rest are gathered from the literature. We find that if we use the traditional twozone scheme of temperature for the observed ions, the S/O, Cl/O and Ar/O abundance ratios increase with metallicity. However, with slight changes in the temperature structure, which include the use of intermediate temperatures, these ratios are constant with metallicity, as expected. Therefore, high quality observations allow us to deepen our understanding of the temperature structure of H II regions.

Motivation & Objective

• To improve the accuracy of chemical abundance determinations in H II regions by using high-quality spectra from the Magellanic Clouds.
• To investigate why traditional two-zone temperature models produce increasing S/O, Cl/O, and Ar/O ratios with metallicity, contrary to theoretical expectations.
• To test whether refined temperature structure models—incorporating intermediate temperatures and combined diagnostics—yield more consistent abundance ratios across metallicities.
• To validate the use of ionization correction factors (ICFs) from photoionization models for H II regions, especially for neon and other elements with complex ionization structures.
• To determine the most appropriate extinction law for individual H II regions based on Balmer and Paschen line ratios, improving reddening corrections.

Proposed method

• Acquired deep echelle spectra of eight H II regions in the Small and Large Magellanic Clouds using UVES/VLT with high spectral resolution (λ/Δλ ≈ 11,600) and atmospheric dispersion correction to minimize airmass effects.
• Used PyNeb for a homogeneous analysis of physical conditions, including electron density (via [O ii], [S ii], [Cl iii], [Ar iv]) and electron temperature (via [N ii], [O iii]).
• Applied the traditional two-zone temperature scheme (T([N ii]) for singly ionized ions, T([O iii]) for doubly ionized ions) and compared results with modified schemes using intermediate temperatures and averaged T([N ii]) and T([O iii]) for heavy ions.
• Calculated ionic abundances using ICFs from Delgado-Inglada et al. (2014) and validated them with photoionization models from Reyes-Pérez et al. (2019, in prep.).
• Determined the reddening coefficient c(Hβ) from Balmer and Paschen line ratios, testing both the Howarth (1983) and O’Donnell (1994) extinction laws to select the best fit per source.
• Compiled 29 additional high-quality spectra from the literature to extend the metallicity baseline, enabling robust analysis of abundance trends across a wide range of metallicities.

Experimental results

Research questions

• RQ1 Why do traditional two-zone temperature models produce increasing S/O, Cl/O, and Ar/O abundance ratios with increasing metallicity, contrary to theoretical expectations?
• RQ2 Can a modified temperature structure model—using intermediate temperatures and combined diagnostics—yield metallicity-independent abundance ratios for S, Cl, and Ar?
• RQ3 How do different extinction laws (Rv = 3.1 vs. Rv = 5.5) affect reddening corrections and abundance determinations in H II regions?
• RQ4 To what extent do ionization correction factors (ICFs) for neon and other elements contribute to observed scatter in Ne/O abundance ratios?
• RQ5 How well do photoionization models reproduce the observed abundance trends when analyzed with the same methods as real data?

Key findings

• The traditional two-zone temperature model leads to increasing S/O, Cl/O, and Ar/O abundance ratios with metallicity, contradicting theoretical expectations of constancy.
• When intermediate temperatures are used and T([N ii]) is applied for Cl++ while averaging T([N ii]) and T([O iii]) for S++ and Ar++, the S/O, Cl/O, and Ar/O ratios become approximately constant across metallicities.
• The observed increase in abundance ratios with metallicity in the standard model is an artifact of incorrect temperature structure assumptions, not a physical trend.
• The ICF for neon is the primary source of scatter in Ne/O abundance ratios, as confirmed by consistent results in photoionization model predictions.
• The O’Donnell (1994) extinction law (Rv = 5.5) provides a better fit than Howarth (1983) (Rv = 3.1) for two H II regions (N88A and N90), indicating variations in dust properties.
• High-quality spectra, especially from the Magellanic Clouds, are essential for detecting subtle temperature structure effects that significantly impact abundance determinations.