[Paper Review] Present-day cosmic abundances. A comprehensive study of nearby early B-type stars and implications for stellar and Galactic evolution and interstellar dust models

Mar ́ ia-Fernanda Nieva, N. Przybilla|MPG.PuRe (Max Planck Society)|Mar 26, 2012

Stellar, planetary, and galactic studiesPhysics and Astronomy112 references231 citations

TL;DR

This study establishes a present-day cosmic abundance standard (CAS) using high-resolution, non-LTE spectroscopy of 29 nearby early B-type stars, achieving 1-2% uncertainty in effective temperature and <15% in surface gravity. The CAS resolves discrepancies between interstellar and stellar abundances, reveals silicate-rich, carbon-poor interstellar dust, confirms tight CNO-cycle mixing in massive stars, and provides strong evidence that the Sun migrated outward from the inner Galaxy, explaining its near-solar metallicity despite Galactic chemical evolution.

ABSTRACT

Aims. A sample of early B-type stars in OB associations and the field within the solar neighbourhood is studied comprehensively. Present-day abundances for the astrophysically most interesting chemical elements are derived. Methods. High-resolution and high-S/N spectra of early B-type stars are analysed in NLTE. Atmospheric parameters are derived from the simultaneous establishment of independent indicators, from multiple ionization equilibria and the hydrogen Balmer lines. Results. Teff is constrained to 1-2% and logg to less than 15% uncertainty. Absolute values for metal abundances are determined to better than 25% uncertainty. The synthetic spectra match the observations reliably over almost the entire visual spectral range. Conclusions. A present-day cosmic abundance standard is established. Our results i) resolve the discrepancy between a chemical homogeneous local gas-phase ISM and a chemically inhomogeneous young stellar component, ii) facilitate the amount of heavy elements locked up in the interstellar dust to be constrained precisely: carbonaceous dust is largely destroyed inside the Orion HII region, unlike the silicates, and that graphite is only a minority species in interstellar dust -, iii) show that the mixing of CNO-burning products in the course of massive star evolution follows tightly the predicted nuclear path, iv) provide reliable present-day reference points for anchoring Galactic chemical evolution models to observation, and v) imply that the Sun has migrated outwards from the inner Galactic disk over its lifetime from a birthplace at a distance around 5-6 kpc from the Galactic Centre; a cancellation of the effects of Galactic chemical evolution and abundance gradients leads to the similarity of solar and present-day cosmic abundances in the solar neighbourhood, with a telltaling signature of the Sun's origin left in the C/O ratio. (ABRIDGED)

Motivation & Objective

• To establish a present-day cosmic abundance standard (CAS) based on a sample of nearby early B-type stars, independent of the Sun.
• To resolve the long-standing discrepancy between chemically homogeneous interstellar gas and inhomogeneous young stellar components within several hundred parsecs of the Sun.
• To precisely constrain the fraction of heavy elements locked in interstellar dust, particularly carbonaceous and silicate species.
• To test predictions of massive star evolution and CNO-cycle mixing by comparing observed abundance ratios to theoretical models.
• To investigate the implications of the CAS for Galactic chemical evolution and the Sun's origin, including radial migration.

Proposed method

• High-resolution, high signal-to-noise ratio spectroscopy of 29 sharp-lined early B-type stars in the solar neighborhood was obtained from multiple observatories, including Calar Alto and ESO.
• Non-LTE spectral analysis was applied to derive atmospheric parameters (Teff, log g, micro- and macroturbulence) using ionization equilibrium, Balmer line fits, and synthetic spectral energy distribution (SED) fitting.
• Spectroscopic parallaxes were computed and cross-validated with the new Hipparcos reduction to ensure consistency.
• Abundances of He, C, N, O, Ne, Mg, Si, and Fe were derived with uncertainties <25% using detailed model atmospheres and synthetic spectra.
• The CAS was constructed as the mean of the 29 stars' abundances, with internal scatter <10%, indicating chemical homogeneity in the local young stellar population.
• Dust phase constraints were derived by comparing observed ISM abundances with those in H II regions (e.g., Orion), assessing the depletion of carbonaceous species.

Experimental results

Research questions

• RQ1 Can a present-day cosmic abundance standard be established from a sample of early B-type stars, independent of the Sun?
• RQ2 Why do early B-type stars in the solar neighborhood show chemical homogeneity despite previous suggestions of inhomogeneity?
• RQ3 To what extent are heavy elements depleted into interstellar dust, and what is the dominant dust composition in the local ISM?
• RQ4 Do observed CNO abundance ratios in massive stars follow the predicted nuclear paths of the CNO cycle?
• RQ5 What does the similarity between solar and present-day cosmic abundances imply about the Sun's origin and Galactic migration?

Key findings

• The present-day cosmic abundance standard (CAS) is established with a mean abundance scatter of less than 10% across 29 early B-type stars, confirming chemical homogeneity in the local young stellar population.
• Carbonaceous dust is largely destroyed in the Orion H II region, indicating that graphite is a minor component of interstellar dust, while silicates and PAHs dominate.
• The C/O ratio in the CAS is approximately 50% higher than in the Sun, providing a key signature of the Sun's origin in the inner Galactic disk (5–6 kpc from center).
• The observed CNO abundance ratios in the stars follow the predicted nuclear path of the CNO cycle with high precision, confirming efficient mixing of CNO-processed material in massive stars.
• The CAS shows excellent agreement with solar abundances for most elements, but the C/O discrepancy implies that the Sun has migrated outward from its birthplace in the inner Galaxy, compensating for Galactic chemical evolution.
• The CAS provides the most precise and robust reference for anchoring Galactic chemical evolution models, particularly for the endpoint of nucleosynthesis in the solar neighborhood.