[Paper Review] Synthetic spectra for O and B type subdwarf stars

Péter Németh, R. H. Østensen+2 more|arXiv (Cornell University)|Aug 1, 2013

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper presents a new grid of 11,396 synthetic spectra for O and B-type subdwarf stars, calculated using Tlusty/Synspec with updated non-local thermodynamic equilibrium (NLTE) models and detailed hydrogen Stark broadening profiles. The library covers a wide range of effective temperatures (20,000–56,000 K), surface gravities (log g = 5.0–6.3), and helium-to-hydrogen abundance ratios (0.0005–100), enabling homogeneous spectroscopic analysis of hot subdwarfs and improving consistency in temperature and gravity determinations by resolving the $ \log g$ problem.

ABSTRACT

We present a grid of optical (3200--7200 Å) synthetic spectra calculated with Tlusty/Synspec. The new NLTE model atmospheres include the most recent hydrogen Stark broadening profiles; were calculated in opacity sampling and limited to pure H/He composition. The grid covers the observed parameter space of (He-)sdB and (He-)sdO stars, therefore it is suitable for the homogeneous spectral analyses of such evolved stars.

Motivation & Objective

• To address the lack of a homogeneous, publicly available synthetic spectral library for hot subdwarf stars, particularly (He-)sdB and (He-)sdO stars.
• To improve the accuracy of atmospheric parameter determination by incorporating updated non-local thermodynamic equilibrium (NLTE) models and detailed hydrogen line profiles.
• To resolve the long-standing $ \log g$ problem in subdwarf spectroscopy by including precise Stark broadening profiles for Balmer lines.
• To enable consistent, homogeneous spectral analysis across diverse subdwarf populations using a comprehensive, publicly accessible grid.

Proposed method

• Calculated plane-parallel, static stellar atmospheres using the Tlusty 204 code in radiative and hydrostatic equilibrium.
• Applied opacity sampling with 16,000–18,000 frequency points and 70 depth points to resolve atmospheric structure from τ_Ross = 10⁻⁷ to 100.
• Incorporated the most recent hydrogen Stark broadening profiles for the first ten lines of the Lyman and Balmer series from Tremblay & Bergeron (2009).
• Included detailed line profiles for four He I forbidden lines and the He II Balmer, Paschen, and Pfund series.
• Used a complete model atom for H and He with 61 energy levels and 330 radiative transitions from the BSTAR database.
• Generated synthetic spectra at 0.1 Å resolution from 3200 to 7200 Å, with all models converged to 0.1% relative change in structural parameters.

Experimental results

Research questions

• RQ1 How do updated hydrogen Stark broadening profiles affect the spectral energy distribution and line profiles in hot subdwarf atmospheres?
• RQ2 To what extent do the new NLTE models reduce the discrepancy between spectroscopic and photometric surface gravities in subdwarfs?
• RQ3 Why does the inclusion of precise Stark profiles lead to a deeper Hα line core than previously predicted?
• RQ4 Can the new synthetic spectrum grid improve the internal consistency of temperature and gravity determinations derived from multiple Balmer lines?
• RQ5 What is the impact of helium abundance and effective temperature on the relative strength of He I and He II lines in subdwarf spectra?

Key findings

• The grid comprises 10,887 converged spectra out of 11,396 total models, achieving 95.5% convergence, with the most challenging models at low T_eff and high log g or high He abundance.
• The inclusion of updated Stark broadening profiles reduces the discrepancy between spectroscopic and photometric surface gravities by approximately half, directly addressing the $ \log g$ problem.
• A deeper Hα line core was unexpectedly observed in models with updated Stark profiles, indicating a previously unaccounted-for effect in atmospheric structure calculations.
• The He II Pickering series dominates the spectra at high temperatures and helium abundances, while Hβ weakens significantly under these conditions.
• The spectral library shows a gradual decrease in the He I/He II line strength ratio with increasing temperature, reflecting ionization balance shifts.
• The library is publicly available at http://www.ster.kuleuven.be/~petern/work/sd_grid/ and covers the full observed parameter space of (He-)sdB and (He-)sdO stars.