[Paper Review] ISO spectroscopy of compact HII regions in the Galaxy. II Ionization and elemental abundances

N. L. Martín-Hernández, E. Peeters+10 more|University of Groningen research database (University of Groningen / Centre for Information Technology)|Oct 30, 2001

Astrophysics and Star Formation Studies61 references82 citations

TL;DR

This study uses ISO infrared spectroscopy of 34 Galactic compact H II regions to derive ionization conditions and elemental abundances (N, O, Ne, S, Ar) across the Galactic disk. By analyzing hydrogen recombination and fine-structure lines, it reveals a systematic decrease in N/O, Ne/H, and Ar/H with galactocentric radius, with corrected gradients of −0.056, −0.06, and −0.07 dex kpc⁻¹ respectively, indicating a radial abundance gradient shaped by chemical evolution and ionization effects.

ABSTRACT

Based on the ISO spectral catalogue of compact HII regions by Peeters et al. (2001), we present a first analysis of the hydrogen recombination and atomic fine-structure lines originated in the ionized gas. The sample consists of 34 HII regions located at galactocentric distances between Rgal = 0 and 15 kpc. The SWS HI recombination lines between 2 and 8 mum are used to estimate the extinction law at these wavelengths for 14 HII regions. An extinction in the K band between 0 and $\sim$ 3 mag. has been derived. The fine-structure lines of N, O, Ne, S and Ar are detected in most of the sources. Most of these elements are observed in two different ionization stages probing a range in ionization potential up to 41 eV. The ISO data, by itself or combined with radio data taken from the literature, is used to derive the elemental abundances relative to hydrogen. The present data thus allow us to describe for each source its elemental abundance, its state of ionization and to constrain the properties of the ionizing star(s).

Motivation & Objective

• To determine elemental abundances (N, O, Ne, S, Ar) in compact H II regions across the Galactic disk using ISO infrared spectroscopy.
• To assess ionization conditions via ratios of fine-structure lines (e.g., Ar++/Ar+, N++/N+, Ne++/Ne+).
• To investigate the Galactic abundance gradient in key elements and its dependence on electron temperature and ionizing radiation hardness.
• To correct for temperature gradients to refine abundance gradients and improve consistency with previous studies.

Proposed method

• Analysis of ISO SWS and LWS grating spectra (2.3–196 μm) from 43 compact H II regions, focusing on hydrogen recombination and atomic fine-structure lines.
• Derivation of extinction in the K band (0–3 mag) using H recombination lines at 2–8 μm.
• Use of ionization ratio diagnostics (e.g., Ar++/Ar+, Ne++/Ne+) to infer the hardness of the ionizing radiation field.
• Calculation of elemental abundances relative to hydrogen using photoionization models and line flux ratios.
• Application of a Galactic electron temperature gradient (~330 K kpc⁻¹) to correct abundance gradients for temperature dependence.
• Comparison of derived abundances with literature data and previous IRAS and KAO studies to validate results.

Experimental results

Research questions

• RQ1 How do ionization ratios such as Ar++/Ar+, N++/N+, and Ne++/Ne+ vary with galactocentric distance in Galactic H II regions?
• RQ2 What is the radial gradient in the N/O abundance ratio, and does it reflect primary or secondary nitrogen production?
• RQ3 To what extent do temperature gradients affect the derived elemental abundance gradients for Ne and Ar?
• RQ4 Why are sulphur and oxygen abundances poorly constrained in this study, and what are the implications for abundance determinations?
• RQ5 How do the observed ionization ratios correlate, and what do they reveal about the spectral energy distribution of ionizing stars?

Key findings

• A Galactocentric gradient in N/O of Δlog N/O = −0.056 ± 0.009 dex kpc⁻¹ is observed, indicating a decreasing nitrogen-to-oxygen ratio with increasing distance from the Galactic center.
• The Ne/H and Ar/H abundance gradients are Δlog Ne/H = −0.039 ± 0.007 dex kpc⁻¹ and Δlog Ar/H = −0.045 ± 0.011 dex kpc⁻¹, respectively, with both elements showing similar nucleosynthetic histories.
• When corrected for a Galactic electron temperature gradient of ~330 K kpc⁻¹, the Ne/H and Ar/H gradients increase to −0.06 and −0.07 dex kpc⁻¹, respectively, improving agreement with prior IRAS and KAO studies.
• The Ar/Ne abundance ratio is approximately solar and constant with galactocentric distance, indicating that both elements are produced and ejected with similar yields.
• Sulphur and oxygen abundances are underestimated due to missing diagnostic lines (e.g., O II), leading to only lower limits being derived for these elements.
• Supersolar nitrogen abundances are found in H II regions within the inner 5 kpc, consistent with enhanced secondary nitrogen production in high-metallicity environments.