Stellar Population Astrophysics (SPA) with the TNG. CNO abundances in 28 open clusters

Technical Summary: CNO Abundances in 28 Open Clusters and the [C/N]–Age Relation

Problem and Motivation
Accurate age determination for field stars remains a significant challenge in stellar astrophysics. While open clusters serve as excellent benchmarks due to their coeval nature and homogeneous chemical composition, calibrating "chemical clocks" for field stars requires a robust understanding of how chemical abundances evolve with age. Specifically, the surface carbon-to-nitrogen ratio ([C/N]) in evolved giants has emerged as a promising age indicator, driven by the first dredge-up (1DUP) and subsequent mixing processes. However, existing calibrations exhibit uncertainties, particularly at young and old age extremes, and often fail to distinguish between different evolutionary stages (e.g., first-ascent red giant branch vs. red clump stars). Furthermore, the extent to which non-standard mixing mechanisms (such as rotation-induced or thermohaline mixing) alter surface abundances in stars with varying turn-off masses requires empirical constraints from high-precision data.

Methodology
This study, part of the Stellar Population Astrophysics (SPA) programme at the Telescopio Nazionale Galileo (TNG), analyzed high-resolution spectra (R ≈ 115,000) obtained with the HARPS-N spectrograph. The sample consists of 88 evolved giants (RGB and Red Clump) belonging to 28 open clusters, plus two stars in the Theia 1214 association.

• Atmospheric Parameters: Effective temperature ($T_{\text{eff}}$), surface gravity ($\log g$), metallicity ([Fe/H]), and microturbulence ($\xi$) were derived using a differential model atmosphere method. Parameters were determined via the LOTUS pipeline, employing non-local thermodynamic equilibrium (NLTE) calculations to account for departures in cool giant atmospheres.
• Abundance Determination: Carbon, nitrogen, and oxygen abundances were derived using the Turbospectrum code with MARCS atmosphere models.
  • Oxygen: Derived from the forbidden [O I] line at 6300 Å.
  • Carbon: Derived from the C2 Swan band heads at 5135 and 5635.5 Å.
  • Nitrogen: Derived from CN features in the 6470–6490 Å region.
  • Iterative Solution: Due to the strong coupling of C, N, and O in cool stellar atmospheres via molecular equilibrium (CO and CN), an iterative process was employed to derive abundances consistent with observed spectral features.
• Evolutionary Classification: Stars were classified into three groups based on their position in the Kiel diagram ($\log g$ vs. $T_{\text{eff}}$) relative to PARSEC isochrones: first-ascent RGB (below the luminosity bump), RGB stars above the luminosity bump (RGB AB), and core helium-burning Red Clump (RC) stars.
• Galactic Context: Orbital parameters and Galactocentric distances were computed using the galpy package and Gaia DR3 data to map the clusters within the Galactic disk.

Key Results

1. Abundance Measurements: The study provides precise C, N, and O abundances for 88 stars across 28 open clusters. The sample spans a metallicity range near solar and covers ages from $\sim$0.5 to $\sim$10 Gyr, with Galactocentric radii between 7 and 9.5 kpc.
2. Comparison with Models:
   • For first-ascent giants with larger turn-off masses ($M_{\text{TO}} > 2.8 M_{\odot}$), the observed C/N ratios are slightly higher (by $\sim$0.20 dex) than predicted by standard 1DUP models. This suggests current models may slightly overestimate mixing efficiency or depth for intermediate-mass stars.
   • For post-bump giants and RC stars, the observed C/N ratios are lower than standard 1DUP predictions, indicating the presence of extra mixing. However, models relying solely on thermohaline mixing are insufficient to explain the full extent of C/N depletion in some clusters, implying a role for rotation-induced mixing or other mechanisms.
3. [C/N]–Age Calibration:
   • The study confirms that a single [C/N]–age calibration is insufficient for all giant stars. Separate calibrations are required for first-ascent giants and RC stars to account for additional abundance alterations during post-RGB evolution.
   • Linear Relations:
     • For RGB stars below the luminosity bump in older clusters: $\log \text{Age} = 3.509 \cdot [\text{C/N}] + 10.926$ (PCC=0.93).
     • For RC stars: $\log \text{Age} = 5.263 \cdot [\text{C/N}] + 12.105$ (PCC=0.71).
   • Young Age Regime: The [C/N] versus age relation for young, massive first-ascent giants is found to be flatter than relations derived in previous studies (e.g., those including field stars or globular clusters). This flattening aligns with theoretical expectations where the C/N–mass relation becomes flat for stars with masses $> \sim 3 M_{\odot}$.
4. Theia 1214 Association: Two stars in the Theia 1214 association were investigated to test their membership in the NGC 752 tidal tail. Despite occupying a similar region in the color-magnitude diagram, these stars exhibit significant discrepancies in CNO abundances, atmospheric parameters, and derived ages ($\sim$0.5 Gyr and $>6$ Gyr) compared to NGC 752 members. The authors conclude these are likely field stars unrelated to NGC 752.

Significance and Conclusions
The paper establishes that precise age dating using chemical clocks requires distinct calibrations for different evolutionary phases of giant stars. The separation of first-ascent giants from Red Clump stars is critical, as the latter undergo additional mixing processes that significantly alter surface [C/N] ratios. The findings challenge previous steep [C/N]–age relations in the young age regime, suggesting a flatter trend consistent with stellar evolution theory for massive turn-off stars. Furthermore, the study highlights the necessity of high-resolution spectroscopy to distinguish between cluster members and field stars in kinematically complex regions, as demonstrated by the reclassification of Theia 1214 stars. These results provide empirical constraints on mixing efficiencies and improve the reliability of [C/N] as an age diagnostic for the Galactic disk.