Quantitative spectroscopy of late O-type main-sequence stars with a hybrid non-LTE method

Context. Late O-type stars at luminosities log L / L ⊙ ≲ 5.2 show weak winds with mass-loss rates lower than 10 −8 M ⊙ yr −1 . This implies that, unlike their more massive and more luminous siblings, their photospheric layers are not strongly affected by the stellar wind. Aims. A hybrid non-local thermodynamic equilibrium (non-LTE) approach – line-blanketed hydrostatic model atmospheres computed under the assumption of LTE in combination with non-LTE line-formation calculations – is tested for analyses of late O-type stars with masses up to ~25 M ⊙ . A sample of 20 mostly sharp-lined Galactic O stars of spectral types O8 to O9.7 and luminosity classes V and IV, previously studied in the literature using full non-LTE model atmospheres, is investigated. Methods. Hydrostatic and plane-parallel atmospheric structures and synthetic spectra computed with Kurucz’s A TLAS 12 code together with the non-LTE line-formation codes D ETAIL and S URFACE , which account for the effects of turbulent pressure on the atmosphere, were employed. High-resolution spectra were analysed for atmospheric parameters using hydrogen lines, multiple ionisation equilibria, and elemental abundances. Fundamental stellar parameters were derived by considering stellar evolution tracks and Gaia Early Data Release 3 (EDR3) parallaxes. Interstellar reddening was characterised by fitting spectral energy distributions from the UV to the mid-IR. Results. A high precision and accuracy is achieved for all derived parameters for 16 sample stars (4 objects show composite spectra). Turbulent pressure effects turn out to be significant for the quantitative analysis. Effective temperatures are determined to 1–3% uncertainty levels, surface gravities to 0.05 to 0.10 dex, masses to better than 8%, radii to better than 10%, and luminosities to better than 20% uncertainty typically. Abundances for C, N, O, Ne, Mg, Al, and Si are derived with uncertainties of 0.05–0.10 dex and for helium within 0.03–0.05 dex (1 σ standard deviations) in general. Overall, results from previous studies using unified photosphere plus wind (full) non-LTE model atmospheres are reproduced, and with higher precision. The improvements are most pronounced for elemental abundances, and smaller microturbulent velocities are found. An overall good agreement is found between our spectroscopic distances and those from Gaia. Gaia EDR3-based distances to the Lac OB1b association and to the open clusters NGC 2244, IC 1805, NGC 457, and IC 1396 are determined as a byproduct. The derived N/C versus N/O abundance ratios tightly follow the predictions from stellar evolution models. Two ON stars show a very high degree of mixing of CNO-processed material and appear to stem from binary evolution.

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