OCCAM: Open Cluster Chemical Abundances & Mapping

• OCCAM is a systematic effort that maps the Milky Way disk by measuring precise elemental abundances in open clusters using large spectroscopic surveys and Gaia astrometry.
• It reveals detailed radial and azimuthal abundance gradients that constrain models of nucleosynthesis, ISM mixing, and Galactic evolution.
• The project employs cross-survey calibrations and chemical clock techniques to accurately age-date clusters and refine our understanding of stellar evolution and Galactic history.

The Open Cluster Chemical Abundances and Mapping (OCCAM) project is a systematic effort to map the chemical, spatial, and dynamical structure of the Milky Way disk using large, homogeneous spectroscopic datasets of open cluster stars. OCCAM combines precision spectroscopic measurements—primarily from major surveys such as APOGEE and Milky Way Mapper (MWM)—with robust membership assignments leveraging Gaia astrometry, to deliver detailed elemental abundance patterns, chemical gradients, and evolutionary diagnostics across hundreds of open clusters. By treating open clusters as fundamental tracers—each serving as a “time-stamped” chemical probe—OCCAM enables the calibration of chemical clocks, the measurement of radial and azimuthal abundance gradients, and stringent tests of models of nucleosynthesis, ISM mixing, radial migration, and Galactic evolution.

1. Survey Design, Scope, and Data Framework

The OCCAM survey is built upon a foundation of massive spectroscopic campaigns targeting open clusters across a wide range of ages (from a few Myr to >8 Gyr) and Galactocentric radii (R$_{GC}$ ≈ 6–16 kpc) (Frinchaboy et al., 2013, Donor et al., 2018, 2206.13650, Otto et al., 9 Jul 2025). Key OCCAM data sources are:

• SDSS-III/IV/Apache Point Observatory Galactic Evolution Experiment (APOGEE), providing high-resolution (R ≈ 22,500) near-infrared spectra.
• SDSS-V Milky Way Mapper (MWM), supplementing and expanding APOGEE coverage, particularly for clusters in the southern hemisphere (Sinha et al., 4 Sep 2024).
• Ancillary medium- or high-resolution campaigns (e.g., WIYN-Hydra, CTIO/Hydra, Keck/HIRES), allowing cross-survey calibration—particularly crucial for elements dominantly measured in the optical (Ray et al., 2022, Myers et al., 14 Oct 2025).

Cluster membership is assigned with a combination of astrometric (Gaia DR2–DR3 positions, proper motions, parallaxes), photometric (cluster CMDs), and spectroscopic (radial velocity, [Fe/H]) criteria (Donor et al., 2018, 2206.13650, Otto et al., 9 Jul 2025). Data homogenization and the use of consistent reduction/analysis pipelines (e.g., APOGEE’s ASPCAP, The Cannon) are critical for minimizing systematics.

Elements studied span light elements (C, N, O, Na, Mg, Al), alpha elements (Si, Ca, Ti, S), iron-peak (Cr, Mn, Fe, Co, Ni), odd-Z (K), and neutron-capture species (Ce, Ba, Nd, etc.) (2206.13650, Sales-Silva et al., 2021, Myers et al., 14 Oct 2025). Both equivalent width and full spectral synthesis approaches are employed, with non-LTE (NLTE) corrections for select species as grids allow.

2. Galactic Chemical Gradients: Radial and Azimuthal Trends

OCCAM has delivered high-precision measurements of abundance gradients—both radially and, in recent work, azimuthally—across the Galactic disk:

Reference	[Fe/H] Radial Gradient	Sample Size	Method/Radius
(Frinchaboy et al., 2013)	–0.09 ± 0.03 dex/kpc	28 clusters	R$_{GC}$ (APOGEE DR10)
(Donor et al., 2018)	–0.061 ± 0.004 dex/kpc	19 clusters	R$_{GC}$ (DR14)
(Donor et al., 2020)	–0.068 ± 0.004 dex/kpc	71 clusters	R$_{GC}$ (DR16)
(2206.13650)	–0.073 ± 0.002 dex/kpc	94 clusters	R$_{GC}$ (DR17)
(Otto et al., 9 Jul 2025)	–0.075 ± 0.006 dex/kpc	164 clusters	R$_{GC}$ (DR19)

Characteristic findings include:

• A pronounced, linear [Fe/H] gradient in the disk interior (R$_{GC}$ ≲ 12 kpc), with typical slopes ≈ –0.06 to –0.08 dex kpc$^{-1}$; flattening or “knees” at large radii are sometimes detected, but recent DR19 OCCAM analysis prefers a simple linear model (Otto et al., 9 Jul 2025).
• For the first time, OCCAM DR19 establishes significant azimuthal variations in the value of the radial [Fe/H] gradient: azimuth slices through the disk reveal both steeper and flatter local gradients, implying that the Milky Way disk is not chemically axisymmetric (Otto et al., 9 Jul 2025).

Individual element gradients ([X/Fe])—including α-elements (O, Mg, Si, S, Ca, Ti), odd-Z, and neutron-capture elements—are mapped, generally showing:

• Mildly positive [O/Fe], [Mg/Fe], or [Si/Fe] gradients with radius (~+0.01 to +0.02 dex kpc$^{-1}$).
• Negative [Mn/Fe], [Na/Fe], and flat or mildly positive [Ce/Fe], [K/Fe], [Al/Fe] trends (2206.13650, Donor et al., 2020, Otto et al., 9 Jul 2025).
• Gradient strengths and orientations sometimes deviate significantly between OCCAM and other surveys (e.g., for [Ti/Fe]), highlighting ongoing challenges in cross-survey calibrations (Otto et al., 9 Jul 2025).

Evidence for temporal gradient evolution is weak or absent in the most recent large samples; i.e., [X/Fe] gradients do not show significant age dependence within the resolution of current datasets (Otto et al., 9 Jul 2025). However, older works and mono-age binning sometimes reveal steeper gradients in the ancient disk (Donor et al., 2020, Carbajo-Hijarrubia et al., 30 Apr 2024).

3. Elemental Abundance Patterns, Homogeneity, and Systematics

Quantification of chemical homogeneity both within and among open clusters is foundational for chemical tagging and Galactic archaeology. OCCAM analyses have demonstrated:

• Clusters are chemically homogeneous in key elements to a precision of ≤0.02 dex (3σ limits) for most elements (up to 20 measured), with neutron-capture and weak-line elements showing ≤0.2 dex due to larger measurement uncertainties (Sinha et al., 4 Sep 2024).
• Open cluster stars are ~0.01 dex more homogeneous than matched field giants, aligning with expectations if clusters form from a well-mixed ISM reservoir (Sinha et al., 4 Sep 2024).
• Simulations (e.g., FIRE-2) and OCCAM+APOGEE observations agree in low intra-cluster scatter for C, N, O, Ne, Mg, Si, S, Ca, Fe, but inter-cluster abundance signatures overlap extensively in this chemical space; effective “strong chemical tagging” is limited unless additional, less correlated species (especially heavier elements) are included (Bhattarai et al., 5 Aug 2024).

Systematic sources of abundance error include NLTE effects (notably for Na, Ba, K), stellar parameter uncertainties, line list variations, sampling bias (giants vs. main sequence), and data reduction pipeline differences. OCCAM’s practice of cross-survey scale calibration and method standardization seeks to minimize such systematics (Ray et al., 2022, Donor et al., 2018).

4. Age Diagnostics, Chemical Clocks, and Stellar Evolution

OCCAM clusters underpin the calibration of chemical clocks—empirical relations linking abundance ratios to age—enabling robust age-dating of field stars:

• The [C/N] ratio for red giants is linked to age via the relation:

$$\log[\text{Age}(\mathrm{yr})]_{\mathrm{DR17}} = 10.14\,(\pm0.08) + 2.23\,(\pm0.19)\,\mathrm{[C/N]}$$

This calibration holds for $8.62\leq \log(\text{Age}[\mathrm{yr}]) \leq 9.82$ and is validated against asteroseismic ages (Spoo et al., 2022).

• The addition of precise neutron-capture (e.g., Ce, Ba, Nd, Mo, Zr) and iron-peak element measurements further enhances chemical clock precision, addresses heavy element enrichment history, and improves age resolutions—particularly for old, metal-poor, or outer disk clusters (Sales-Silva et al., 2021, Myers et al., 14 Oct 2025).

Evolutionary effects such as atomic diffusion, turbulent and rotational mixing are directly detected in individual clusters (e.g., M67, NGC 2420), with differential abundances across evolutionary stages matching detailed stellar evolution models, indicating the necessity of evolutionary corrections when interpreting abundance-age relations (Souto et al., 2018, Semenova et al., 2020).

5. Nucleosynthetic Origin, Element Families, and Galactic Chemical Evolution

Open cluster abundance patterns elucidate the diverse nucleosynthetic pathways contributing to Galactic enrichment:

• α-elements (Mg, Si, Ca, Ti): produced primarily by core-collapse SNe; young inner disk clusters show [Mg/Fe], [Si/Fe] enhancements tied to recent massive star formation (Carbajo-Hijarrubia et al., 30 Apr 2024).
• Odd-Z and iron-peak: sensitivity to the relative rates and yields of Type Ia versus Type II SNe is manifest in negative [Mn/Fe], [Ni/Fe] gradients and generally flat [Cr/Fe], [Co/Fe] trends (Donor et al., 2018, 2206.13650).
• Heavy s-process: Ce, Ba, Nd, La abundances track AGB yields—with [Ce/Fe] increasing at low metallicities and in younger clusters, reflecting delayed release of s-process material and metallicity-dependent efficiency of neutron-capture (Sales-Silva et al., 2021, Myers et al., 14 Oct 2025). Radial [Ce/H] gradients are negative (–0.07 dex/kpc), while [Ce/Fe] gradients are positive; the latter steepen slightly with time.
• Constraints on AGB models and the temporal evolution of neutron-capture element gradients provide new diagnostics for timescales of Galactic chemical evolution and ISM mixing.

6. Implications for Galactic Structure, Dynamics, and Archaeology

OCCAM results have critical consequences for understanding the assembly and evolution of the Milky Way disk:

• The existence of a pronounced, nearly linear [Fe/H] gradient, modulated by azimuth and possibly time, argues for inside-out disk formation, ongoing radial migration, and non-axisymmetric dynamical influences (spiral arms, bar).
• Azimuthal abundance variations detected in OCCAM DR19 indicate that the disk is not well-mixed on all spatial scales, with local departures likely related to spiral arm dynamics or patchy star formation (Otto et al., 9 Jul 2025).
• The high level of intra-cluster chemical homogeneity substantiates the foundational assumption of chemical tagging, but the limited inter-cluster separation in light elements and the need for more discriminating abundances (e.g., neutron-capture) cautions against overreliance on simplistic tagging for reconstructing dispersed groups (Bhattarai et al., 5 Aug 2024, Sinha et al., 4 Sep 2024).
• Systematic, homogeneous open cluster work is crucial for anchoring field star abundance patterns and for numerical simulations to accurately calibrate ISM mixing, enrichment, and the impact of stellar feedback on disk chemical evolution.

7. Future Directions and Comparisons with Simulations

Further expansion of the OCCAM project is expected along multiple axes:

• Increased southern hemisphere sampling via MWM and deeper membership lists via advancing Gaia astrometry.
• Extension of high-resolution optical observations (e.g., with Keck/HIRES) to provide neutron-capture and r-process abundances not accessible in the near-IR, complementing APOGEE datasets (Myers et al., 14 Oct 2025).
• Ongoing calibrations against field star samples, benchmarks, and external surveys (GALAH, Gaia-ESO) to ensure cross-survey consistency and maximize the discriminatory power of chemical tagging.
• Continued comparison with cosmological simulations (e.g., FIRE-2) to reconcile ISM mixing, feedback, and cluster assembly predictions with observational constraints, while recognizing the challenge posed by the limited spread in abundance space for the most commonly measured light elements (Bhattarai et al., 5 Aug 2024).
• Exploration of additional azimuthal and vertical gradient variations, further quantification of gradient evolution with time and cluster age, and systematic resolution of abundance scale offsets between pipelines.

This larger-scale, homogeneous chemical mapping of open clusters is central to refining our models of the formation, structure, and dynamical evolution of the Milky Way disk. OCCAM’s sustained expansion and methodical approach continue to advance the field’s ability to decode the Milky Way’s chemical and dynamical history with unprecedented accuracy.