[Paper Review] The chemical composition of the Orion star forming region: I. Homogeneity of O and Si abundances in B-type stars

S. Simón‐Díaz|arXiv (Cornell University)|Dec 21, 2009

Stellar, planetary, and galactic studies46 references51 citations

TL;DR

This study re-evaluates oxygen and silicon abundances in 13 B-type stars across the Orion OB1 association using high-resolution spectroscopy and self-consistent atmospheric modeling with fastwind. It identifies systematic errors in prior photometric temperature estimates that led to spurious abundance inhomogeneity, and after correction, finds high homogeneity in O and Si abundances—consistent with metal-rich, homogeneous stellar populations in the solar vicinity and challenging earlier claims of chemical inhomogeneity in Orion.

ABSTRACT

Recent accurate abundance analyses of B-type main sequence stars in the solar vicinity has shown that abundances derived from these stellar objects are more homogeneous and metal-rich than previously thought. We investigate whether the inhomogeneity of abundances previously found in B-type stars in the Ori OB1 association is real (hence a signature of enrichment of the newly formed stars in an induced star formation scenario) or a consequence of intrinsic errors induced by the use of photometric indices to establish the stellar parameters prior to the abundance analysis. We obtained a new (improved) spectroscopic data set comprising 13 B-type stars in the various Ori OB1 associations, and performed a detailed, self-consistent spectroscopic abundance analysis by means of the modern stellar atmosphere code FASTWIND. We detect systematic errors in the stellar parameters determined previously which affect the derived abundances. Once these errors are accounted for, we find a high degree of homogeneity in the O and Si abundances for stars in the four Ori OB1 subgroups. The derived abundances are in very good agreement with recent determinations in other B-type stars in the solar vicinity. We also compare our results with those obtained for the Sun during the epoch of the "solar crisis", and the Orion nebula.

Motivation & Objective

• To reassess the chemical composition of early-B stars in the Orion OB1 association with improved accuracy.
• To investigate whether previously reported abundance inhomogeneity in B-type stars is real or an artifact of flawed parameter determination.
• To test the reliability of photometric temperature estimates in abundance analysis of early-type stars.
• To compare stellar O and Si abundances with nebular values from the Orion nebula and solar values during the 'solar crisis'.
• To establish a robust, self-consistent spectroscopic method for abundance analysis in early-B stars using modern stellar atmosphere models.

Proposed method

• Acquired high-resolution spectroscopic data using the FIES instrument at the Nordic Optical Telescope.
• Employed the non-LTE, line-blanketed, spherically extended stellar atmosphere code fastwind for self-consistent modeling.
• Developed custom IDL tools to automatically measure equivalent widths (EWs) of metal lines in high-resolution spectra.
• Constructed a tailored grid of fastwind HHeSiO models optimized for early-B main sequence stars.
• Used multiple Si ii-iii-iv lines to cross-validate stellar parameters and abundance determinations.
• Applied consistent model atoms and atmospheric parameters derived exclusively from spectroscopic diagnostics to minimize systematic errors.

Experimental results

Research questions

• RQ1 Is the previously reported inhomogeneity in O and Si abundances among B-type stars in Orion OB1 real or an artifact of flawed parameter determination?
• RQ2 To what extent do photometric temperature estimates introduce systematic errors in abundance analysis of early-B stars?
• RQ3 How do the derived O and Si abundances in Orion B-type stars compare with those in the solar vicinity and during the 'solar crisis'?
• RQ4 What is the consistency between stellar abundances and nebular abundances in the Orion nebula, particularly for O and Si?
• RQ5 Can a self-consistent spectroscopic approach resolve discrepancies in abundance determinations across different stellar populations?

Key findings

• Systematic errors in previously used photometric T_eff estimates were identified and corrected, which had led to spurious abundance dispersion.
• After correction, the O and Si abundances in 13 B-type stars across four Ori OB1 subgroups show high homogeneity with a dispersion of less than 0.1 dex.
• The mean O/H abundance is log(O/H) + 12 = 8.76 dex, in excellent agreement with recent determinations in B-type stars in the solar vicinity.
• The mean Si/H abundance is log(Si/H) + 12 = 7.50 dex, consistent with other metal-rich early-B stars and inconsistent with earlier claims of inhomogeneity.
• Stellar O and Si abundances in Orion OB1 lie within the range of values reported during the 'solar crisis', suggesting consistency with solar metallicity within uncertainties.
• The discrepancy between stellar and nebular Si abundances in the Orion nebula is approximately 0.3–1.0 dex, indicating potential dust depletion effects requiring further study.