Stellar Population Astrophysics (SPA) with the TNG: NLTE atmospheric parameters and abundances of giant stars in 33 Open Clusters

TL;DR

This work delivers NLTE atmospheric parameters ($T_{ m eff}$, $ abla g$, [Fe/H], $\xi$) and NLTE abundances for 11 elements in 95 giants spanning 33 open clusters, observed with HARPS-N as part of the SPA program. Using LOTUS for NLTE parameterization and TSFitPy for NLTE spectral synthesis, the authors produce a homogeneous, high-precision elemental census that includes nine clusters analyzed for the first time. They confirm NLTE corrections are essential, reveal age-related abundance trends in multiple element groups (e.g., $d([{ m Mg/Fe}])/d ext{age} eq 0$, $d([{ m Na/Fe}])/d ext{age} < 0$, $d([{ m Sr/Fe}])/d ext{age} > 0$, $d([{ m Y/Fe}])/d ext{age} < 0$, $d([{ m Eu/Fe}])/d ext{age} < 0$), and show overall agreement with GALAH and APOGEE trends. The resulting NLTE benchmark abundances will support future large-scale surveys such as 4MOST and WEAVE, enabling robust mappings of Galactic chemical evolution. The analysis highlights the interplay between Galactic chemical evolution and nucleosynthesis, validating NLTE corrections as a critical component of precise stellar chemistry in open clusters.

Abstract

Open clusters serve as important tools for accurately studying the chemical evolution of the Milky Way. By combining precise chemical data from high-resolution spectra with information on their distances and ages, we can effectively uncover the processes that have shaped our Galaxy. This study aims to derive NLTE atmospheric parameters and chemical abundances for approximately one hundred giant stars across 33 open clusters with near-solar metallicity. The clusters span a wide range of ages, enabling an assessment of the presence and extent of any age-related abundance gradients. In the Stellar Population Astrophysics (SPA) project, we acquired new high-resolution spectra of open clusters using the HARPS-N echelle spectrograph at the Telescopio Nazionale Galileo. We chemically characterized nine open clusters for the first time and reanalyzed previously studied SPA clusters, resulting in a consistent and homogeneous sample. We determined NLTE atmospheric parameters using the equivalent width method and derived NLTE chemical abundances through spectral synthesis for various elements, including alpha elements (Mg, Si, and Ti), light odd-Z elements (Na, Al), iron-peak elements (Mn, Co, and Ni), and neutron-capture elements (Sr, Y, and Eu). Our findings are compared with the existing literature, revealing good agreement. We examine the trends of [X/Fe] versus age, confirming previous observations and the enrichment patterns predicted by nucleosynthesis processes. Positive correlations with age are present for Mg, Si, Ti, Al, Mn, Co, Ni, and Sr, while Na and Y and Eu show a negative trend. This study emphasizes the significance of NLTE corrections and reinforces the utility of open clusters as tracers of Galactic chemical evolution. Furthermore, we provide a benchmark sample of NLTE abundances for upcoming open cluster surveys within large-scale projects such as 4MOST and WEAVE.

Figures (12)

• Figure 1: Color-magnitude diagrams with Gaia DR3 photometric data ($G$ vs. $G_{BP}-G_{RP}$) for the subset of clusters presented in this study for the first time as part of the SPA project. The red points highlight the member stars observed by SPA.
• Figure 2: Atmospheric parameters [Fe/H], $T_{\rm eff}$, and $\log g$ obtained with LOTUS for clusters containing more than five stars. The first row presents the NLTE results, while the second row shows the LTE results.
• Figure 3: Distribution of $\Delta T_{\rm eff}$ and $\Delta \log g$ comparing LOTUS results to those from previous SPA studies. The histograms were smoothed. The colors indicate the different authors. Mean differences and standard deviations are displayed in each panel.
• Figure 4: Comparison between our abundance of Fe, $\alpha$ and odd-z elements results and those from previous SPA studies for each open cluster. The lines represent different elements.
• Figure 5: Abundance [X/Fe] ratios as function of [Fe/H]. The gray diamonds represent the OCs compilation from GALAH (see Table \ref{['tab:galah_oc']}). Our results are color-coded according to the age.