[Paper Review] A NLTE analysis of the hot subdwarf O star BD+28 4211. II. The optical spectrum

M. Latour, G. Fontaine+2 more|arXiv (Cornell University)|Apr 24, 2015

Stellar, planetary, and galactic studies42 references14 citations

TL;DR

This study proposes using metal-enriched (10× solar) NLTE line-blanketed model atmospheres to accurately derive atmospheric parameters of hot subdwarf O stars from optical spectroscopy alone. By artificially boosting metallicity to saturate opacity effects, the method achieves significantly improved fits to Balmer and helium lines in BD+28◦4211, yielding effective temperatures within 1,000 K of UV-based values, resolving the long-standing Balmer line problem in hot stars.

ABSTRACT

We present the second part of our detailed analysis of the hot sdO and spectroscopic standard star BD+28 4211, in which we focus on the optical spectrum. In the first part of our study, we determined the abundances of some 11 metals detected in the atmosphere of BD+28 4211 using UV spectra of the star and corroborated the fundamental parameters estimated in past studies (Teff $\sim$ 82,000 K, log g $\sim$ 6.2, and solar N(He)/N(H)). In this work, we aim at rederiving these secured parameters on the sole basis of high-quality optical spectra. A first grid of non-LTE line-blanketed model atmospheres, including metals with the abundances derived from the UV spectrum, does not give satisfactory results when we apply a standard simultaneous fitting procedure to the observed H and He lines of our optical spectra. The line profiles are not finely reproduced and the resulting effective temperatures, in particular, are too low by $\sim$10,000 K. We next investigate the probable cause of this failure, that is, the importance of missing opacity sources on the atmospheric stratification. We compare line profiles computed from models with artificially boosted metallicities, from solar abundances to 15$ imes$ these values. We find that the structural effects saturate for a metallicity of $\sim$10x solar, and use this to compute a second full grid of models and synthetic spectra. This metal-enriched grid allows us to achieve significantly improved spectral fits with models having the expected parameters. Our test case thus reveals that there is still a need for models with enhanced metallicity for better estimating the atmospheric parameters of objects such as hot subdwarfs and hot white dwarfs if only optical spectra are available.

Motivation & Objective

• To re-derive fundamental parameters of the hot subdwarf O star BD+28◦4211 using only high-quality optical spectra.
• To investigate why standard NLTE models with solar metallicities fail to reproduce observed Balmer and helium line profiles in hot stars.
• To test whether artificially enhanced metallicity in model atmospheres can correct for missing opacity sources that distort atmospheric structure.
• To validate the improved model fits against high-resolution archival HIRES spectra.
• To establish a practical method for reliable parameter determination in hot subdwarfs when UV data are unavailable.

Proposed method

• Constructed a grid of NLTE line-blanketed model atmospheres with metals (C, N, O, Mg, Si, S, Fe, Ni) at solar abundances, based on UV-derived abundances from Paper I.
• Applied a standard simultaneous fitting procedure to match observed H and He lines in BD+28◦4211's optical spectrum, using the solar-metallicity models.
• Systematically increased model metallicity from solar to 15× solar to assess its impact on line profile reproduction and atmospheric structure.
• Identified a saturation effect in line profiles at approximately 10× solar metallicity, beyond which further increases had no significant structural effect.
• Built a new, full grid of models using 10× solar metallicity and re-ran the fitting procedure to achieve improved spectral fits.
• Validated the final model against high-resolution, high signal-to-noise archival HIRES spectra, confirming excellent agreement in line profiles.

Experimental results

Research questions

• RQ1 Why do standard NLTE models with solar metallicities fail to reproduce the Balmer and helium line profiles in the optical spectrum of BD+28◦4211?
• RQ2 To what extent does increasing metallicity in model atmospheres affect the atmospheric structure and line profile reproduction in hot subdwarf O stars?
• RQ3 Does a saturation point exist in the structural response of model atmospheres to increasing metallicity, and if so, at what level?
• RQ4 Can a metal-enriched model grid (10× solar) produce accurate and consistent atmospheric parameters (Teff, log g) when fitting only optical spectra?
• RQ5 Is the improved fit with enhanced metallicity consistent with high-resolution archival observations?

Key findings

• Standard NLTE models with solar metallicities failed to reproduce the observed line profiles of BD+28◦4211, yielding an effective temperature approximately 10,000 K too low.
• Increasing model metallicity up to 15× solar revealed a saturation effect in atmospheric structure and line profiles at approximately 10× solar metallicity.
• At 10× solar metallicity, further increases in metal abundance no longer altered the atmospheric structure or line profiles, indicating saturation.
• The use of a 10× solar metallicity model grid enabled a successful fit to the optical spectrum, yielding atmospheric parameters consistent with UV-based estimates within uncertainties.
• The final model showed excellent agreement with high-resolution HIRES spectra, confirming the reliability of the metal-enriched approach.
• The study concludes that metal-enriched models (10× solar) are essential for accurate parameter derivation from optical spectroscopy alone in hot subdwarf O stars.