[Paper Review] The chemical composition of the Orion star-forming region. III. C, N, Ne, Mg and Fe abundances in B-type stars revisited

María-Fernanda Nieva, S. Simón‐Díaz|arXiv (Cornell University)|Apr 15, 2011

Astrophysics and Star Formation Studies35 references47 citations

TL;DR

This study re-evaluates C, N, Ne, Mg, and Fe abundances in 13 early B-type stars within the Orion OB1 association using high-resolution spectroscopy and a self-consistent non-LTE spectral analysis. It confirms high chemical homogeneity with 1σ scatter of 0.03–0.07 dex and finds excellent agreement with the present-day cosmic abundance standard, challenging earlier reports of large abundance variations in Orion.

ABSTRACT

Early B-type stars are invaluable indicators for elemental abundances of their birth environments. In contrast to the surrounding neutral interstellar matter (ISM) and HII regions their chemical composition is unaffected by depletion onto dust grains and by the derivation of different abundances from recombination and collisional lines. In combination with ISM or nebular gas-phase abundances they facilitate the dust-phase composition to be constrained. Precise abundances of C, N, Mg, Ne, Fe in early B-type stars in the Orion star-forming region are determined in order to: a) review previous determinations using a self-consistent quantitative spectral analysis based on modern stellar atmospheres and recently updated model atoms, b) complement results found in Paper I for oxygen and silicon, c) establish an accurate and reliable set of stellar metal abundances to constrain the dust-phase composition of the Orion HII region in Paper II of the series. A detailed, self-consistent spectroscopic study of a sample of 13 narrow-lined B0V-B2V stars in Ori OB1 is performed. High-quality spectra obtained with FIES@NOT are analysed using a non-LTE method and line-profile fitting techniques, validating the approach by comparison with results obtained in Paper I using line-blanketed non-LTE model atmospheres and a curve-of-growth analysis. The two independent analysis strategies give consistent results for basic stellar parameters and abundances of oxygen and silicon. The extended analysis to C, N, Mg, Ne, and Fe finds a high degree of chemical homogeneity, with the 1sigma-scatter adopting values of 0.03--0.07 dex around the mean for the various elements. Present-day abundances from B-type stars in Ori OB1 are compatible at similar precision with cosmic abundance standard values as recently established from early-type stars in the solar neighbourhood and also with the Sun. (abridged)

Motivation & Objective

• To re-determine C, N, Ne, Mg, and Fe abundances in early B-type stars in the Orion star-forming region with improved accuracy.
• To resolve discrepancies in prior abundance studies that reported large scatters (up to 0.5 dex) in O and Si, and up to 0.45 dex in C and N.
• To establish a reliable, homogeneous set of stellar metal abundances for use in constraining dust-phase composition in the Orion H II region.
• To validate the consistency of two independent analysis methods: hybrid non-LTE with Atlas9+Detail+Surface and line-blanketed non-LTE with Fastwind.
• To compare results with solar neighborhood early-type stars and late-type stars in Orion to assess galactic chemical evolution and solar origin questions.

Proposed method

• Acquired high-quality spectra of 13 narrow-lined B0 V–B2 V stars in Ori OB1 using the FIES instrument at the Nordic Optical Telescope.
• Performed a self-consistent spectroscopic analysis using Atlas9 line-blanketed LTE model atmospheres combined with non-LTE line formation via the Detail+Surface code.
• Applied a hybrid non-LTE approach: LTE model atmospheres with non-LTE line formation for improved accuracy in atomic level populations.
• Conducted line-profile fitting techniques to validate abundance determinations and minimize systematic errors.
• Treated multiple ionization stages of each element and carefully accounted for systematic uncertainties as outlined in Nieva & Przybilla (2010a,b).
• Cross-validated results against those from Paper I (Fastwind non-LTE code) to ensure consistency across independent analysis frameworks.

Experimental results

Research questions

• RQ1 Are the C, N, Ne, Mg, and Fe abundances in early B-type stars of the Orion OB1 association chemically homogeneous, or do significant variations exist?
• RQ2 How do the abundances derived in this study compare with previous determinations that reported large scatters (e.g., 0.4–0.5 dex) in O and Si?
• RQ3 To what extent do the abundances in Orion B-type stars agree with the present-day cosmic abundance standard (PNB08) and solar values?
• RQ4 Can the hybrid non-LTE method (Atlas9+Detail+Surface) produce consistent results with the full non-LTE Fastwind code for massive star analysis?
• RQ5 What constraints do these stellar abundances place on the dust-phase composition of the Orion H II region?

Key findings

• The abundance scatter for C, N, O, Si, Ne, Mg, and Fe in the 13 B-type stars is low, with 1σ scatter ranging from 0.03 to 0.07 dex around the mean.
• The derived abundances for C, N, O, Ne, Mg, Si, and Fe in Orion B-type stars are in excellent agreement with the present-day cosmic abundance standard (PNB08), as defined by early-type stars in the solar neighborhood.
• The results confirm the high chemical homogeneity of the Orion OB1 region, contradicting earlier studies that reported large abundance variations.
• The hybrid non-LTE method (Atlas9+Detail+Surface) yields consistent results with the Fastwind non-LTE code, validating the approach for massive star abundance analysis.
• The iron abundance in Orion B-type stars (log ɛ(Fe) ≈ 7.48) is consistent with recent determinations from late-type stars in Ori OB1b and the Orion Nebula Cluster.
• The study provides a precise, homogeneous, and reliable set of stellar metal abundances that enables robust constraints on the dust-phase composition of the Orion H II region, as applied in Paper II of the series.