The GALAH survey: Improving chemical abundances using star clusters

TL;DR

This study uses open and globular clusters, plus young associations, as empirical benchmarks to diagnose systematic trends in GALAH DR4 stellar abundances across a wide Teff range. By fitting element-abundance trends with cubic splines, separating dwarfs and giants, and stacking cluster data after metallicity-relative shifts, the authors derive detrended abundances and provide a corrected DR4 catalogue. They test potential origins by re-fitting spectra with SME and Korg and conducting targeted tests of photometric priors, continuum treatment, linelists, and modeling codes, finding that trends persist across clusters but originate from a combination of measurement and modelling effects rather than simple stellar physics. The work demonstrates that clusters are powerful benchmarks for large surveys, informing more reliable chemical tagging and astrochemical analyses, even as the exact cause of the temperature-dependent trends remains to be fully understood.

Abstract

Large spectroscopic surveys aim to consistently compute stellar parameters of very diverse stars while minimizing systematic errors. We explore the use of stellar clusters as benchmarks to verify the precision of spectroscopic parameters in the 4. data release (DR4) of the GALAH survey. We examine 58 open and globular clusters and associations to validate measurements of temperature, gravity, chemical abundances, and stellar ages. We focus on identifying systematic errors and understanding trends between stellar parameters, particularly temperature and chemical abundances. We identify trends by stacking measurements of chemical abundances against effective temperature and modelling them with splines. We also refit spectra in three clusters with the Spectroscopy Made Easy and Korg packages to reproduce the trends in DR4 and to search for their origin by varying temperature and gravity priors, linelists, and spectral continuum. Trends are consistent between clusters of different ages and metallicities, can reach amplitudes of ~0.5 dex and differ for dwarfs and giants. We use the derived trends to correct the DR4 abundances of 24 and 31 chemical elements for dwarfs and giants, and publish a detrended catalogue. While the origin of the trends could not be pinpointed, we found that: i) photometric priors affect derived abundances, ii) temperature, metallicity, and continuum levels are degenerate in spectral fitting, and it is hard to break the degeneracy even by using independent measurements, iii) the completeness of the linelist used in spectral synthesis is essential for cool stars, and iv) different spectral fitting codes produce significantly different iron abundances for stars of all temperatures. We conclude that clusters can be used to characterise the systematic errors of parameters produced in large surveys, but further research is needed to explain the origin of the trends.

Figures (21)

• Figure 1: Kiel diagram of the stars used to compute the trends. Unflagged stars are plotted with colours and flagged stars with grey crosses. Gray background distribution are all stars in GALAH DR4. Regions delineating dwarfs, giants, and the overlap region are marked in red. The equations of all red lines are specified in Equation \ref{['eq:lines']}.
• Figure 2: Trends for several clusters illustrated for a small selection of elements. Top: trends for seven open clusters and associations. Last row shows the combined measurements. Bottom: trends for eight open and globular clusters. Quantities in the first column are shifted by the value of $[Fe/H]$ given under the cluster's name, so they fit into the same plotting range. Colored points are unflagged stars, grey crosses are flagged stars. Solid line is a fit for unflagged stars, dashed line is a fit for all stars.
• Figure 3: Effects of keeping $T_\mathrm{eff}$ and $\log g$ free parameters or keeping them fixed at the photometric value. Symbols mark the stars belonging to clusters Melotte 25 (circles), Melotte 22 (diamonds), and NGC 2632 (squares). Crosses are measurements for the stars without a trustworthy fit. The solid line is the trend derived in Section \ref{['sec:fitted_trends']} for the GALAH DR4 iron abundance in dwarfs.
• Figure 4: Effect of two different continuum fits on the measured iron abundances. Symbols mark the stars belonging to clusters Melotte 25 (circles), Melotte 22 (diamonds), and NGC 2632 (squares). Crosses are measurements for the stars without a trustworthy fit. The solid line is the trend derived in Section \ref{['sec:fitted_trends']} for the GALAH DR4 iron abundance in dwarfs.
• Figure 5: Top: Iron abundances computed with linelists of different fidelities. Symbols mark the stars belonging to clusters Melotte 25 (circles), Melotte 22 (diamonds), and NGC 2632 (squares). Crosses are measurements for the stars without a trustworthy fit. The solid line is the trend derived in Section \ref{['sec:fitted_trends']} for the GALAH DR4 iron abundance in dwarfs. Bottom: Difference of iron abundances between the computations using the XL linelist and other three linelists. Solid line shows the median difference in each temperature bin, and the dashed lines show the 16th and 84th percentiles of the differences in each bin.