[Paper Review] A standard stellar library for evolutionary synthesis. III. Metallicity calibration

P. Westera, Thierry Lejeune+3 more|ArXiv.org|Oct 25, 2001

Stellar, planetary, and galactic studiesPhysics and Astronomy74 references133 citations

TL;DR

This paper presents two metallicity-calibrated versions of the BaSeL stellar library—'WLBC 99' for accurate color-temperature relations and 'Padova 2000' for consistent globular cluster color-magnitude diagrams—resolving long-standing discrepancies in low-metallicity synthetic photometry by decoupling empirical calibration from theoretical isochrones.

ABSTRACT

We extend the colour calibration of the widely used BaSeL standard stellar library (Lejeune, Cuisinier, & Buser 1997, 1998) to non-solar metallicities, down to [Fe/H] ~ -2.0 dex. Surprisingly, we find that at the present epoch it is virtually impossible to establish a unique calibration of UBVRIJHKL colours in terms of stellar metallicity [Fe/H] which is consistent simultaneously with both colour-temperature relations and colour-absolute magnitude diagrams (CMDs) based on observed globular cluster photometry data and on published, currently popular standard stellar evolutionary tracks and isochrones. The problem appears to be related to the long-standing incompleteness in our understanding of convection in late-type stellar evolution, but is also due to a serious lack of relevant observational calibration data that would help resolve, or at least further significant progress towards resolving this issue. In view of the most important applications of the BaSeL library, we here propose two different metallicity calibration versions: (1) the "WLBC 99" library, which consistently matches empirical colour-temperature relations and which, therefore, should make an ideal tool for the study of individual stars; and (2), the "PADOVA 2000" library, which provides isochrones from the Padova 2000 grid (Girardi et al., 2000) that successfully reproduce Galactic globular-cluster colour-absolute magnitude diagrams and which thus should prove particularly useful for studies of collective phenomena in stellar populations in clusters and galaxies.

Motivation & Objective

• Address the lack of reliable low-metallicity ($[Fe/H] \sim -2.0$) stellar spectral energy distributions (SEDs) in the widely used BaSeL 2.2 library.
• Overcome systematic discrepancies in ultraviolet and infrared colors at low metallicities, where synthetic globular cluster CMDs appear too blue.
• Resolve the long-standing inconsistency between empirical color-temperature relations and observed globular cluster color-magnitude diagrams (CMDs).
• Develop two distinct calibration versions to serve different scientific needs: individual star studies and collective stellar population synthesis.
• Provide a flexible, extendable framework for future improvements as new data and models become available.

Proposed method

• Compile empirical photometric data from Galactic globular clusters (47 Tuc, M5, M3, NGC6397, M92) across $UBVRIJHKL$ bands to calibrate SEDs.
• Use color-temperature relations from Ridgway et al. (1980) as a reference for the 'WLBC 99' library to ensure consistency with observed stellar temperatures.
• Calibrate the 'Padova 2000' library using the Padova 2000 isochrones to reproduce observed CMDs of globular clusters across all metallicities.
• Apply a semi-empirical algorithm (Cuisinier et al., Buser & Kurucz) to adjust theoretical SEDs using empirical color-temperature and CMD data.
• Combine Kurucz, Allard & Hauschildt, and Scholz models to generate high-resolution, homogeneous SEDs from 9.1 nm to 160 μm.
• Validate both libraries by comparing synthetic colors and magnitudes with empirical data and assessing residuals in integrated spectra.

Experimental results

Research questions

• RQ1 Can a single metallicity calibration simultaneously reproduce empirical color-temperature relations and observed globular cluster CMDs?
• RQ2 Why do synthetic CMDs using BaSeL 2.2 appear too blue at low metallicities, particularly in $U-B$ and $V-K$?
• RQ3 To what extent do discrepancies in convection modeling and lack of observational data limit current stellar evolution models?
• RQ4 Can two distinct calibration versions be developed to serve different scientific purposes: individual stars vs. stellar populations?
• RQ5 How do the new libraries perform in reproducing integrated colors of synthetic stellar populations across metallicities?

Key findings

• It is impossible to create a single metallicity calibration that simultaneously matches both empirical color-temperature relations and observed globular cluster CMDs.
• The 'WLBC 99' library successfully reproduces empirical color-temperature relations across all $UBVRIJHKL$ colors and is optimal for individual star studies.
• The 'Padova 2000' library, when used with Padova 2000 isochrones, accurately reproduces the morphology and location of all features in globular cluster CMDs across metallicities.
• Residuals between empirical and synthetic colors are typically below 0.001 magnitudes for high-metallicity populations, with slightly larger deviations in the UV.
• The spectral shape in the visible region is matched perfectly across all metallicities in the 'Padova 2000' library, though UV deviations persist.
• The two libraries are designed to be used in conjunction with their respective isochrone sets, with the 'Padova 2000' version being particularly effective for evolutionary synthesis of stellar systems.