---
title: 'OCCAM Survey: Mapping Open Cluster Chemistry'
url: https://www.emergentmind.com/topics/occam-survey
type: topic
---

# OCCAM Survey: Mapping Open Cluster Chemistry

OCCAM, the **Open Cluster Chemical Abundances and Mapping Survey**, is a Galactic-archaeology program built around open clusters and designed to produce “a comprehensive, uniform, infrared-based spectroscopic dataset for hundreds of open clusters” and to “constrain key Galactic dynamical and chemical parameters from this sample.” Its scientific premise is that open clusters are age-datable, approximately coeval, and chemically homogeneous stellar populations, so they can serve as unusually clean tracers of the Milky Way disk’s chemical structure, its temporal evolution, and the calibration of age-dating methods such as chemical clocks, gyrochronology, and asteroseismology [1308.4195][2206.13650][2203.05463]. Over time, OCCAM has expanded from an APOGEE-based infrared abundance survey into a broader platform that combines Gaia membership vetting, cluster-scale chemical mapping, empirical age calibration, and high-resolution optical follow-up of neutron-capture elements [2507.07264][2510.13014].

## 1. Survey scope and historical development

The first OCCAM science paper established the survey’s basic design in **SDSS DR10**, presenting analysis of **141 member stars in 28 open clusters**. That sample included “the first high-resolution metallicity measurements for 22 open clusters,” and it was used to examine the Galactic disk gradients of both \([M/H]\) and \([\alpha/M]\) from a single homogeneous APOGEE dataset [1308.4195]. The second contribution, based on **SDSS/DR14**, moved from an early proof-of-concept to “precision cluster abundances,” with **259 member stars** in **19 open clusters**, and it extended the survey from bulk metallicity to multi-element radial trends in \([{\rm O/Fe}], [{\rm Mg/Fe}], [{\rm Si/Fe}], [{\rm Mn/Fe}],\) and \([{\rm Ni/Fe}]\) [1807.09791].

A major expansion arrived with the **DR16** analysis. OCCAM IV assembled a raw sample of **128 open clusters** and **914 member stars**, of which **83** clusters passed the CMD-based “high quality” screen and **71** high-quality clusters remained in the main abundance-gradient analysis after excluding **12 very young clusters**. This release was important because it converted OCCAM from a moderate-size APOGEE cluster project into a large, homogeneous Galactic disk resource with gradients for **16 elements** and age-binned trend analysis [2002.08980]. OCCAM VI, based on **APOGEE DR17**, then presented the full APOGEE-2-era abundance-gradient analysis with **150 open clusters** and **2061 member stars** in the full sample, **94** “high quality” clusters, and **85** clusters in the final abundance-gradient sample [2206.13650].

The survey continued into the **SDSS-V/MWM DR19** era in OCCAM VIII. That release established a sample of **164 high quality open clusters** and **1083 member stars**, explicitly emphasizing that the cluster set had become large enough not only to refine radial abundance gradients but also, “for the first time using the OCCAM sample,” to investigate **Galactic azimuthal variations** [2507.07264]. In parallel, OCCAM broadened scientifically beyond the original APOGEE-accessible light, alpha, odd-\(Z\), and iron-peak species. The cerium study derived homogeneous Ce abundances for **218 stars in 42 open clusters** [2112.02196], OCCAM VII produced an empirical **\([{\rm C/N}]\)-age calibration** [2203.05463], and later optical follow-up papers used OCCAM as the membership and target-selection backbone for neutron-capture work inaccessible or unreliable in APOGEE [2510.13014][2607.00291].

## 2. Membership architecture and sample construction

OCCAM’s membership methodology evolved substantially across releases. In the earliest DR10 analysis, non-calibration clusters were pre-selected with an infrared strategy built around spatial location, color–magnitude behavior, and especially extinction filtering via the **Rayleigh-Jeans Color Excess (RJCE)** method. Candidate members then had to satisfy APOGEE photometric cuts, primarily
\[
(J-K_s)_0 \ge 0.5
\]
and
\[
7 \le H \le 12.2
\]
for a standard 3-hour field, with targeting possible down to \(H=13.8\) in deeper fields. Final membership relied mainly on radial velocity, with stars having radial-velocity membership probabilities \(>50\%\) accepted, followed by an iterative \(3\sigma\) metallicity cut [1308.4195].

By DR14, Gaia astrometry had become central. OCCAM II combined APOGEE radial velocities and metallicities with **Gaia DR2 proper motions**, and a star was considered a likely member if it lay within \(3\sigma\) of the cluster mean in **RV**, **[Fe/H]**, and **proper-motion** space. The abundance analysis was then restricted to likely **giant** members, approximately
\[
\log(g) \lessapprox 3.7,
\]
and a cluster entered the “high reliability” sample only if it had **at least 4 likely member giant stars** [1807.09791].

The DR16 and DR17 analyses preserved this multi-dimensional logic while intensifying quality control. OCCAM IV combined APOGEE spectroscopy with **Gaia DR2** astrometry and photometry, started from stars within \(2\times\) the cluster radius, and required \(3\sigma\) consistency in **RV**, \([\mathrm{Fe}/\mathrm{H}]\), and proper motion. It then applied a visual screen based on proper-motion-cleaned color–magnitude diagrams, retaining only clusters in which the APOGEE-selected members formed a plausible main sequence, subgiant branch, giant branch, or red clump [2002.08980]. OCCAM VI updated this structure with **Gaia EDR3**-era information, starting from about **26,700 stars** near known open clusters and retaining stars whose reported probabilities were \(>0.01\) in **RV**, **[Fe/H]**, and **PM** space within the \(3\sigma\) acceptance region [2206.13650].

The DR19 membership pipeline introduced a slightly different balance between Gaia and spectroscopy. For each cluster, OCCAM VIII selected stars within
\[
3 \times R_{50}
\]
and cross-matched them to **Cantat-Gaudin et al. (2020)** members with membership probability \(>70\%\). It then refined membership in **RV** and **[Fe/H]** space using Gaussian-kernel smoothing and retained stars with probability \(\ge 0.05\) in both, which the paper describes as a **\(2\sigma\)** cut for deriving bulk cluster parameters. Visual quality classes based on Gaia CMDs, Kiel diagrams, and PARSEC isochrones then separated calibration clusters, high-quality 5+ member clusters, high-quality 2–4 member clusters, and good one-star clusters from rejected systems [2507.07264]. This suggests a steady methodological trajectory from infrared field cleaning toward a fully multi-dimensional Gaia–APOGEE membership architecture.

## 3. Spectroscopic framework and abundance products

The original OCCAM survey is fundamentally an **APOGEE** survey. APOGEE provides high-resolution near-infrared spectroscopy in the \(H\)-band at \(R \sim 22{,}500\), which is especially valuable for open clusters in the dust-obscured Galactic plane because the infrared reduces extinction and allows observation of distant or reddened giant stars [1308.4195]. In OCCAM II and later papers, cluster abundances were not generally rederived from raw spectra by the OCCAM authors; instead, the survey relied on **ASPCAP** pipeline products, then converted member-star abundances into cluster means, using the observed scatter among vetted members rather than propagating individual-star uncertainties [1807.09791].

OCCAM IV standardized the use of DR16 “named tag” abundances and explicitly analyzed **16 chemical species**: Fe plus **O, Na, Mg, Al, Si, S, K, Ca, Ti, V, Cr, Mn, Co, Ni, Cu**. Cluster abundances were produced by determining reliable members, adopting APOGEE DR16 calibrated abundances for those stars, and computing cluster-level means and uncertainties from the member set [2002.08980]. OCCAM VI then updated this framework to DR17 and analyzed “16 reliable chemical species available in APOGEE DR17”: Fe, the \(\alpha\)-elements **O, Mg, Si, S, Ca, Ti**, the iron-peak elements **V, Cr, Mn, Co, Ni**, the odd-\(Z\) species **Na, Al, K**, and the neutron-capture element **Ce** [2206.13650].

The DR19 analysis extended the catalog structure further. OCCAM VIII used **SDSS-V / Milky Way Mapper DR19** abundances from **ASPCAP** within the **astra** framework and investigated **O, Mg, Si, S, Ca, Ti, Cr, Mn, Fe, Co, Ni, Na, Al, K, Ce, Nd**. It also released two value-added catalogs, `occam_member-DR19.fits` and `occam_cluster-DR19.fits`, containing member-level probabilities, bulk cluster chemistry, motions, and orbital parameters including \(R_{GC}\) and \(R_{Guide}\) [2507.07264]. Across releases, the survey repeatedly emphasized that its principal advantage lies in internal homogeneity rather than the mere size of the sample.

## 4. Galactic abundance gradients and disk structure

OCCAM’s most widely cited results concern the Milky Way’s radial abundance gradients. The first DR10 paper already found that a single global linear metallicity gradient was an incomplete description. Over
\[
7.9 \le R_{GC} \le 14.5 \ {\rm kpc},
\]
a single-line fit yielded
\[
\frac{d[M/H]}{dR_{GC}} = -0.09 \pm 0.03 \ {\rm dex \ kpc^{-1}},
\]
but splitting at
\[
R_{GC} = 10 \ {\rm kpc}
\]
produced
\[
\frac{d[M/H]}{dR_{GC}} = -0.20 \pm 0.08 \ {\rm dex \ kpc^{-1}}
\]
for \(7.9 \le R_{GC} \le 10\ {\rm kpc}\) and
\[
\frac{d[M/H]}{dR_{GC}} = -0.02 \pm 0.09 \ {\rm dex \ kpc^{-1}}
\]
for \(R_{GC} > 10\ {\rm kpc}\), while \([\alpha/M]\) showed
\[
\frac{d[\alpha/M]}{dR_{GC}} = -0.01 \pm 0.05 \ {\rm dex \ kpc^{-1}}
\]
[1308.4195].

OCCAM II remeasured the metallicity gradient with Gaia-refined membership and found the preferred
\[
\frac{d[{\rm Fe/H}]}{dR_{GC}} = -0.061 \pm 0.004 \ {\rm dex\ kpc}^{-1}
\]
after excluding **NGC 6791**, with significant positive gradients in \([{\rm O/Fe}], [{\rm Mg/Fe}], [{\rm Si/Fe}]\) and significant negative gradients in \([{\rm Mn/Fe}]\) and \([{\rm Ni/Fe}]\) [1807.09791]. OCCAM IV then used a larger DR16 sample and fit the break radius as a free parameter for the first time, obtaining an inner-disk metallicity gradient of
\[
\frac{d[\mathrm{Fe}/\mathrm{H}]}{dR_{\mathrm{GC}}} = -0.068 \pm 0.001\ \mathrm{dex\ kpc^{-1}}
\]
over \(6 < R_{\mathrm{GC}} < 13.9\) kpc, an outer gradient of
\[
-0.009 \pm 0.011\ \mathrm{dex\ kpc^{-1}},
\]
and a fitted break at
\[
R_{\mathrm{GC}} = 13.9\ \mathrm{kpc}.
\]
The same paper also found that older open-cluster populations show steeper \([\mathrm{Fe/H}]\) gradients, while younger populations are flatter [2002.08980].

OCCAM VI, using the final APOGEE DR17 sample, produced the survey’s definitive APOGEE-2 metallicity-gradient statement. For present-day Galactocentric radius, it found an inner-disk slope
\[
\frac{d[\mathrm{Fe/H}]}{dR_{GC}} = -0.073 \pm 0.002~\mathrm{dex\,kpc^{-1}}
\]
over
\[
6 < R_{GC} < 11.5~\mathrm{kpc},
\]
with a knee at
\[
11.5 \pm 0.09~\mathrm{kpc}
\]
and an outer slope
\[
\frac{d[\mathrm{Fe/H}]}{dR_{GC}} = -0.032 \pm 0.002~\mathrm{dex\,kpc^{-1}}.
\]
Using guiding-center radius,
\[
R_{guide} = \frac{L_z}{v_c(R)},
\]
it obtained a similar inner slope,
\[
\frac{d[\mathrm{Fe/H}]}{dR_{Guide}} = -0.074 \pm 0.002~\mathrm{dex\,kpc^{-1}},
\]
with a knee at
\[
12.2 \pm 0.12~\mathrm{kpc}
\]
and outer slope
\[
\frac{d[\mathrm{Fe/H}]}{dR_{Guide}} = -0.023 \pm 0.003~\mathrm{dex\,kpc^{-1}}.
\]
The same DR17 paper reported significant \([X/{\rm Fe}]\) gradients for **O, Mg, S, Ca, Mn, Na, Al, K, and Ce** [2206.13650].

OCCAM VIII revised the gradient picture in **DR19**. For the **164 high-quality open clusters**, it found an overall **linear** radial metallicity gradient
\[
\frac{d[\mathrm{Fe}/\mathrm{H}]}{dR_{GC}} = -0.075 \pm 0.006 \ \mathrm{dex\ kpc}^{-1}
\]
and
\[
\frac{d[\mathrm{Fe}/\mathrm{H}]}{dR_{Guide}} = -0.068 \pm 0.005 \ \mathrm{dex\ kpc}^{-1}.
\]
Although bilinear fits could still be obtained, the paper concluded from an Akaike Information Criterion comparison that the DR19 metallicity gradient was better described by a **single linear trend** than by a broken-linear model. For the first time in the OCCAM series, the sample size also allowed an azimuthal analysis, and the paper reported evidence of **azimuthal variations in the measured radial abundance gradient** in the Galactic disk [2507.07264]. A common misconception is therefore that OCCAM has converged on one immutable “true” break radius; the series instead shows that the inferred shape depends on release, radial coverage, coordinate choice, and model-comparison criterion.

## 5. Chemical clocks and neutron-capture extensions

OCCAM’s role in chemical-clock calibration is most explicit in OCCAM VII. Using APOGEE DR17 giant-star abundances in a final calibration sample of **49 clusters** and **530 stars**, the survey derived the empirical relation
\[
\log[Age({\rm yr})]_{\rm DR17} = 10.14 \, (\pm 0.08) + 2.23\,(\pm 0.19) \, {\rm [C/N]},
\]
usable for
\[
8.62 \leq \log(Age[{\rm yr}]) \leq 9.82,
\]
and described as applicable “primarily to metal-rich, thin and thick disk giant stars.” The paper also found no evidence for a significant difference between RGB-only and RC-only relations within the current uncertainties [2203.05463]. This gave OCCAM a direct role in field-star age inference rather than only cluster abundance cartography.

The survey’s neutron-capture extension began inside APOGEE space with the cerium study. Using **BACCHUS**, **MARCS** model atmospheres, and **Turbospectrum**, that paper derived Ce abundances from **seven Ce II lines** in **218 stars belonging to 42 open clusters**. Its central results were that, for **Ages \(< 4\) Gyr**, younger open clusters have higher \([{\rm Ce/Fe}]\) and \([{\rm Ce}/\alpha\text{-element}]\) than older clusters, that metallicity segregates clusters in the \([{\rm Ce}/X]\)-Age plane, and that these relations are therefore **not universal clocks**. It also derived, for the first time, age-binned radial gradients of \([{\rm Ce/H}]\) and \([{\rm Ce/Fe}]\), with
\[
\frac{d[\mathrm{Ce}/\mathrm{H}]}{dR_{GC}}<0
\]
and
\[
\frac{d[\mathrm{Ce}/\mathrm{Fe}]}{dR_{GC}}>0,
\]
and estimated that the \([{\rm Ce/Fe}]\) gradient becomes slightly steeper, changing by \(\sim +0.009\ {\rm dex\,kpc^{-1}\,Gyr^{-1}}\) [2112.02196].

The next step moved beyond APOGEE entirely. The conference proceeding **“Beyond OCCAM: Measuring Optical Neutron Capture Abundances of Open Cluster Stars”** explicitly states that it “utilize[s] the SDSS-IV/APOGEE-based OCCAM survey as the foundation for our optical follow-up observations.” It obtained **Keck I/HIRES** spectra at
\[
R \gtrsim 50{,}000
\]
with **signal-to-noise ratio at least 75 at 5500 Å** for targets with
\[
13 < V_{\rm mag} < 15.5.
\]
The observed sample comprised **29 stars across 8 clusters**, while the preliminary abundance analysis used **16 stars** and required **at least three high-quality members** per cluster, primarily from OCCAM and supplemented by **Cantat-Gaudin et al. (2020)** when needed. Abundances were derived with **BACCHUS**, **Turbospectrum**, and **MARCS model atmospheres**, starting from APOGEE stellar parameters, and the preliminary neutron-capture results focused on **Ce, Ba, Y, Mo, La, and Zr**. The main reported pattern was “a **non-linear trend of increasing abundance with cluster metallicity \([\mathrm{Fe}/\mathrm{H}]\)**,” visible across all measured species but “**shallower for molybdenum and zirconium**” [2510.13014].

OCCAM X then formalized the optical neutron-capture program on a larger scale. It used OCCAM as the foundation for new optical observations of **56 stars in 18 open clusters**, combining **Keck/HIRES** and **Magellan/MIKE** spectroscopy at **\(R > 50{,}000\)** and **high-S/N (\(>75\) at 5500Å)**. With **BACCHUS**, it derived abundances for **23 elements**, including **7 neutron capture abundances not measurable by APOGEE**, and characterized their radial distribution in the Milky Way. The paper reports that the **second-peak \(s\)-process and \(r\)-process abundances exhibit relatively flat gradients**, while the **first-peak \(s\)-process abundances also have slopes which are shallower than the alpha and iron-peak elements**. It further states that “a metallicity dependence of the AGB stars responsible for producing the heaviest s-process abundances may be necessary to consider in Galactic evolution models” [2607.00291]. In this phase, OCCAM functions both as a **membership catalog and target-selection engine** and as a contextual abundance/age database for high-resolution optical work.

## 6. Survey significance, boundaries, and related distinctions

OCCAM’s long-term significance lies in the fact that it turned open clusters into a reusable, internally consistent framework for Milky Way chemo-dynamical inference. By combining homogeneous APOGEE abundances, Gaia astrometry, cluster ages, and later optical follow-up, it created a bridge between classical open-cluster gradient work and survey-scale Galactic archaeology. The survey’s scientific outputs span radial metallicity structure, \([X/{\rm Fe}]\) gradients, age-binned trend analysis, empirical chemical-clock calibration, and neutron-capture abundances across multiple nucleosynthetic channels [2002.08980][2206.13650][2203.05463].

At the same time, the series is explicit about its limitations. OCCAM IV noted that the measured \([\mathrm{Fe/H}]\) slope can vary by **as much as \(\sim 15\%\)** with the adopted distance catalog, that the outer disk remained sparsely sampled, and that pipeline unreliability for very young stars required excluding clusters younger than **50 Myr** from abundance analysis [2002.08980]. OCCAM VI emphasized age uncertainties, possible abundance systematics for elements such as **V, Ti, S, Co,** and **Ce**, selection effects in cluster survival, and the unresolved discrepancy between DR17 ASPCAP cerium and BACCHUS-based or optical cerium abundances [2206.13650]. OCCAM VIII further showed that the inferred DR19 metallicity gradients depend on the membership catalog and that several element-by-element trends—especially **O, Ca, Ti, Co, Ni, Ce,** and **Nd**—can deviate considerably from other large spectroscopic surveys [2507.07264]. This suggests that OCCAM’s chief strength is not immunity to systematics, but the explicit attempt to control them within a uniform cluster framework.

A recurrent point of confusion in the literature concerns the distinction between **OCCAM** and **OCCASO**. They are not the same survey. OCCASO is the **Open Cluster Chemical Abundances from Spanish Observatories** survey, a distinct northern-hemisphere optical program using **FIES**, **HERMES**, and **CAFE**, designed to obtain homogeneous radial velocities, physical parameters, and chemical abundances for Milky Way open clusters [1603.00659]. OCCAM, by contrast, is the APOGEE- and Gaia-centered **Open Cluster Chemical Abundances and Mapping Survey** that later expanded into optical neutron-capture follow-up.

In cumulative terms, the survey’s trajectory is clear. Early OCCAM established that homogeneous cluster spectroscopy could sharpen the Milky Way’s metallicity-gradient problem. Mid-series OCCAM converted that insight into a large APOGEE resource for multi-element Galactic chemical evolution. Later OCCAM papers turned the survey into a calibration framework for \([{\rm C/N}]\)-based ages and a platform for multi-wavelength neutron-capture studies. A plausible implication is that OCCAM is no longer only an infrared cluster-abundance survey; it is a general cluster-based infrastructure for connecting ages, kinematics, and chemistry across multiple spectroscopic regimes.

Source: https://www.emergentmind.com/topics/occam-survey