[Paper Review] Program package for the analysis of high resolution high signal-to-noise stellar spectra

N. Piskunov, T. Ryabchikova+5 more|arXiv (Cornell University)|Oct 30, 2017

Astronomical Observations and Instrumentation3 citations

TL;DR

This paper presents an updated version of the SME (Spectroscopy Made Easy) program package for high-resolution, high signal-to-noise stellar spectroscopy, enabling automatic derivation of stellar atmospheric parameters and element abundances using LTE and NLTE (non-local thermodynamic equilibrium) models. The key advancement includes integration of precomputed NLTE departure coefficients for O I, Na I, Ca I-II, and Ba II, significantly improving accuracy in abundance determinations across diverse stellar types from dwarfs to giants, with validation showing agreement within ±0.04 dex of line-by-line analysis.

ABSTRACT

The program package SME (Spectroscopy Made Easy), designed to perform an analysis of stellar spectra using spectral fitting techniques, was updated due to adding new functions (isotopic and hyperfine splittins) in VALD and including grids of NLTE calculations for energy levels of few chemical elements. SME allows to derive automatically stellar atmospheric parameters: effective temperature, surface gravity, chemical abundances, radial and rotational velocities, turbulent velocities, taking into account all the effects defining spectral line formation. SME package uses the best grids of stellar atmospheres that allows us to perform spectral analysis with the similar accuracy in wide range of stellar parameters and metallicities - from dwarfs to giants of BAFGK spectral classes.

Motivation & Objective

• To improve the accuracy of stellar atmospheric parameter and abundance determination in high-resolution, high signal-to-noise stellar spectra.
• To extend the SME spectral analysis package with NLTE corrections for key elements (O, Na, Ca, Ba) to reduce systematic errors from LTE assumptions.
• To enable robust, automated spectral fitting across a wide range of stellar parameters, including dwarfs and giants of BAFGK spectral types.
• To validate the reliability of interpolated NLTE departure coefficients by comparing them with direct NLTE calculations and observed line profiles.
• To support the Gaia mission's goal of deriving fundamental stellar parameters for millions of stars using automated, precise spectroscopic analysis.

Proposed method

• The SME package uses synthetic spectrum synthesis via external C++/Fortran libraries (SYNTH) to compute emergent spectra based on model atmospheres.
• It performs spectral fitting by minimizing the chi-squared difference between observed and synthetic spectra, adjusting free parameters such as T_eff, log g, [Fe/H], v_rot, v_turb, and elemental abundances.
• The package incorporates updated model atmospheres, including Kurucz, MARCS (plane-parallel and spherical), and LLmodels, with precomputed NLTE departure coefficients (b-factors) for selected elements.
• NLTE departure coefficients are derived using the DETAIL code and interpolated across model grids to enable efficient spectral synthesis without recalculating NLTE effects for each star.
• Input linelists are compatible with VALD’s 'Extract Stellar' output, using short format for LTE and long format for NLTE analysis.
• The fitting process includes line-by-line abundance analysis and comparison with observed spectra to validate results, particularly for the O I 7771.94 Å line.

Experimental results

Research questions

• RQ1 How accurately can SME derive stellar atmospheric parameters and element abundances when incorporating NLTE corrections for O I, Na I, Ca I-II, and Ba II?
• RQ2 To what extent does interpolation of precomputed NLTE departure coefficients affect the accuracy of synthetic line profiles compared to direct NLTE calculations?
• RQ3 How do SME-derived abundances compare with those from line-by-line analysis using carefully selected spectral lines in a cool dwarf star (HD 69830)?
• RQ4 What is the impact of NLTE effects on the equivalent width of key spectral lines, such as O I 7771.94 Å, and how does this affect abundance determinations?
• RQ5 Can SME reliably analyze both dwarf and giant stars across a wide range of metallicities and effective temperatures using consistent model grids?

Key findings

• The SME package successfully derives stellar atmospheric parameters (T_eff, log g, [Fe/H], v_rot, v_turb) with high accuracy across diverse stellar types, including dwarfs and giants.
• Interpolated NLTE departure coefficients for O I, Na I, Ca I-II, and Ba II show excellent agreement with direct NLTE calculations, with a maximum line profile difference of less than 0.2% for the O I 7771.94 Å line.
• The abundance difference between SME results and line-by-line analysis for HD 69830 is within ±0.02 dex, corresponding to a 0.02 dex uncertainty in abundance, which is consistent with the typical error of best solar abundance determinations.
• For the O I 7771.94 Å line, the equivalent width difference between NLTE models computed directly and via interpolation is less than 0.2%, confirming the reliability of the interpolation method.
• The SME package achieves consistent and reliable results across a wide range of stellar parameters, including metal-poor giants (e.g., HD 4306, [Fe/H] ≈ -2.8) and metal-rich giants (e.g., HD 74387, [Fe/H] ≈ -0.29).
• The integration of NLTE grids for key elements significantly improves the accuracy of abundance determinations, reducing systematic errors from LTE assumptions, especially for lines sensitive to non-LTE effects.