Extended MSTO (eMSTO) in Star Clusters

• Extended Main-Sequence Turn-Off (eMSTO) is a phenomenon where the MSTO region in a cluster’s CMD is broadened or split, indicating deviations from a simple, single-age population.
• High-precision photometry and isochrone fitting techniques, including FWHM measurements in CMDs, are used to discern intrinsic age spreads, stellar rotation effects, and chemical evolution signatures.
• Insights from eMSTO studies enhance our understanding of stellar evolution models by linking cluster mass, escape velocity, and internal dynamics to rotational mixing and self-enrichment processes.

An Extended Main-Sequence Turn-Off (eMSTO) is an observed broadening or splitting of the main-sequence turn-off (MSTO) region in the color–magnitude diagrams (CMDs) of young and intermediate-age star clusters, notably in the Magellanic Clouds but also in Galactic open clusters. This morphological feature is interpreted as evidence for complexities beyond a simple stellar population and is deeply linked to cluster formation history, internal stellar physics, and chemical evolution. The eMSTO has become a central diagnostic in studies of stellar evolution, cluster dynamics, and multiple stellar populations.

1. Observational Phenomenology and Definitions

The eMSTO is identified in CMDs by a region at the main-sequence turn-off that is significantly broader than expected from photometric uncertainties or binaries in a canonical single-age cluster. In some clusters, such as NGC 1846 or NGC 1856, the MSTO appears bifurcated; in others, it is simply broadened. The phenomenon is robust across a range of cluster masses (down to $\approx 10^3~M_\odot$), ages (spanning several 100 Myr to about 3 Gyr), and metallicities, and is now observed in both Magellanic Cloud and Galactic clusters (1102.1723, 1303.1361, Milone et al., 2015, Piatti et al., 2016, Marino et al., 2018, Cordoni et al., 2018).

Accurate photometry, often from HST or ground-based adaptive optics systems, is critical. Differential reddening and contamination must be carefully modeled and subtracted. Methodologies to parameterize the MSTO width typically involve defining regions in the CMD (parallelograms aligned with the reddening vector), interpolating stellar ages via isochrone fitting, and quantifying the full width at half maximum (FWHM) of the derived pseudo-age distribution (Milone et al., 2015, Souza et al., 21 Jul 2025). Intrinsic MSTO width is then assessed after subtracting the expected broadening from photometric errors and unresolved binaries.

2. Age Spreads, Extended Star Formation, and the Chemical Evolution Link

Initial interpretations pointed to genuine spreads in stellar ages—star formation episodes separated by tens to hundreds of Myr—as the origin of eMSTOs (1102.1723, 1303.1361, Milone et al., 2015). Observationally derived age spreads typically range from $\sim50$ to $300~\mathrm{Myr}$ in many LMC and SMC clusters. The correlation between the eMSTO phenomenon and clusters displaying dual red clumps further strengthens the case for an extended formation history (1303.1361, Milone et al., 2015).

This age spread scenario is strongly connected to the self-pollution hypothesis for the chemical evolution of ancient globular clusters (GCs) (1102.1723). The same age intervals invoked to explain eMSTOs match those required to yield the observed light element abundance inhomogeneities (notably in [O/Fe], [Na/Fe]) in old GCs. The paper (1102.1723) formalizes the mass budget for a second stellar generation:

$$M = \int_{M_{t=SG}}^{M_{min\text{\,SNe}}} (m_{initial} - m_{final}) \, m^{-\alpha} \, dm,$$

with $M_{t=SG}$ the turn-off mass at the epoch of second-generation (SG) star formation, $M_{min\,SNe}$ the upper mass limit for stars not disrupting the cluster, and Salpeter IMF slope $\alpha=2.35$. The processed gas from first-generation (FG) stars is insufficient to form a SG of comparable mass to the FG, implying the necessity of external gas accretion or fallback.

A critical prediction: eMSTO clusters should show light element abundance variations akin to those in ancient GCs, with SG stars more centrally concentrated and dynamically cooler (1102.1723). Preliminary abundance studies (e.g., NGC 1783, NGC 1978) find only modest spreads, but mass and metallicity effects may naturally suppress the amplitude relative to metal-poor GCs.

3. Alternative Physical Mechanisms: Stellar Rotation and Internal Mixing

Recent advances challenge the view that genuine age spreads are solely responsible for eMSTOs. It is now established that a spread in stellar rotation rates within a coeval population can mimic the CMD morphology of an eMSTO (Marino et al., 2018, Cordoni et al., 2018, Goudfrooij et al., 2017, Marino et al., 2018, Sun et al., 2019, Bastian et al., 2018, Maurya et al., 27 Jun 2024, Maurya et al., 19 Jul 2025). Fast rotation leads to gravity darkening and an overall redder appearance; rotational mixing extends main-sequence lifetimes, causing a spread in TO luminosities that, if interpreted as an age difference, can reach 200--300 Myr.

Empirical support comes from spectroscopic measurements showing that red, fainter eMSTO stars are systematically fast rotators ($v \sin i$ often exceeding $130$--$190~\mathrm{km~s^{-1}}$), while blue, brighter counterparts are slow rotators ($v \sin i$ around $80~\mathrm{km~s^{-1}}$ or lower) (Marino et al., 2018, Bastian et al., 2018, Maurya et al., 27 Jun 2024, Maurya et al., 19 Jul 2025). Gravity-darkening scaling, parameterized as $T_{eff}(\theta) \propto g(\theta)^{\beta}$ with $\beta \approx 0.25$, governs the surface temperature distribution over fast rotators.

Nonetheless, synthetic cluster simulations indicate that rotation alone (assuming random viewing angles and field-measured rotation distributions) cannot always reproduce the full observed MSTO width; rotational models can match $\sim40$–$60\%$ of the pseudo-age spread in $1~\mathrm{Gyr}$ clusters (Goudfrooij et al., 2017). Thus, a combination of a rotation distribution and a modest age spread is often required (Goudfrooij et al., 2017, Goudfrooij et al., 2018).

Other mechanisms include:

• Variable convective core overshooting (OVCC), where a spread in the overshooting parameter $\delta_{ov}$ can generate eMSTO morphologies, though OVCC alone does not cause significant MS splits in young clusters (Yang et al., 2017).
• Enhanced internal mixing unrelated to rotation (e.g., due to IGWs or radiative envelope mixing), as calibrated from asteroseismology and modeled using isochrone-cloud fitting, can enhance core masses and broaden the turn-off (Johnston et al., 2019).
• In the youngest clusters ($\lesssim 20$ Myr), variable stars and binary interactions, rather than only rotation, contribute to the broad MSTO (Li et al., 2019).
• In some clusters, modest intrinsic age spreads and binarity are also present and contribute to the morphology, but rotational and evolutionary effects account for the majority of the eMSTO signature.

4. Impact of Cluster Properties, Mass, and Dynamical Evolution

eMSTOs have historically been associated with massive, spatially extended clusters ($>5\times10^3~M_\odot$, $r_c\gtrsim2$ pc), invoking a link between deep potential wells and the ability to retain gas for multiple star formation episodes (1102.1723). However, discoveries in low-mass clusters ($\sim10^3~M_\odot$) now demonstrate that mass is not a strict requirement; eMSTOs are found wherever the present-day escape velocity is sufficient to affect stellar evolution timescales, with a lower limit to observable age spreads around $90~\mathrm{Myr}$ (Piatti et al., 2016, Souza et al., 21 Jul 2025).

A robust correlation exists between MSTO width (in age units), cluster mass, and escape velocity (Souza et al., 21 Jul 2025). Observational selection effects also restrict detection: in old clusters, age spreads become a small fraction of the total age, rendering eMSTO features photometrically undetectable (1102.1723, Piatti et al., 2016). Only clusters with particular structural properties (e.g., larger core radii resulting from early dynamical expansion or mass segregation) seem capable of hosting or revealing prominent eMSTOs (1102.1723, Piatti et al., 2016).

Environmental and dynamical factors, such as star-disk interaction during the pre-main-sequence (PMS) phase, further modulate rotation rate distributions; fast rotating, red eMSTO stars tend to be more centrally concentrated, consistent with earlier disk detachment in denser, central environments (Maurya et al., 27 Jun 2024, Maurya et al., 19 Jul 2025).

5. Broader Implications: Globular Cluster Analogues and Field Star Connections

eMSTOs provide a critical observational bridge between intermediate-age, chemically simple clusters and ancient, chemically inhomogeneous globular clusters (GCs). The same cluster evolutionary pathway that yields eMSTOs is postulated to be responsible for the multiple populations and light-element inhomogeneities of old GCs (1102.1723). The eMSTO phenomenon is thus a "missing link" in GC evolution, with strong predictions for chemical signatures: eMSTO clusters should show [O/Fe] and [Na/Fe] spreads and their SG stars should be centrally concentrated and dynamically cooler (1102.1723).

From the perspective of Galactic open clusters, the ubiquity of eMSTOs and broadened MSs (especially in clusters $<2.5$~Gyr) demonstrates that these systems, too, are not simple single-population objects (Cordoni et al., 2018, Marino et al., 2018, Bastian et al., 2018). This blurs the classical distinction between young open clusters and GCs regarding internal complexity.

The detection of eMSTOs in both low- and high-mass clusters and their strong dependence on stellar physics (rotation, mixing) and environment reinforce the universality of these processes, irrespective of host galaxy or metallicity. The eMSTO phenomenon thereby serves as a crucial benchmark for validating stellar evolutionary models, constraining rotation and mixing prescriptions, and guiding our understanding of cluster formation and self-enrichment pathways.

6. Open Questions, Controversies, and Future Directions

Key open issues remain regarding the physical origin of eMSTOs:

• What is the relative importance of true age spreads versus rapid stellar rotation and internal mixing in any given cluster context? The high-precision age-resolving potential of isochrone-cloud and asteroseismic modeling is promising but requires larger, more homogeneous samples (Johnston et al., 2019).
• In several clusters (e.g., Collinder 347 (Piatti et al., 2019)), the observed MSTO spread exceeds the combined effects of rotation, binarity, and observational uncertainties, suggesting real multiple stellar populations are present in at least some systems.
• The discovery of a pronounced kink in the MS at $M\sim1.45~M_\odot$, below which rotation has negligible effect due to magnetic braking, challenges interpretations that attribute all MSTO width to rotation alone (Goudfrooij et al., 2018). The eMSTO extends well below this mass in some clusters, indicating that age spreads must be invoked at least in part.

Emerging factors such as dust production from mass loss in fast rotators have been proposed to explain specific segregated features (e.g., UV-dim eMSTO stars in NGC 1783), with extinction-driven broadening à la

$$m_{abs} = m_{intrinsic} + \tau_{10} \, (\delta m_{\text{band}}/\delta\tau_{10}) \sin i$$

where $\tau_{10}$ is the circumstellar optical depth (D'Antona et al., 2023). However, UV studies in some open clusters (such as NGC 2355) find no evidence that dust extinction plays an appreciable role in the eMSTO (Maurya et al., 27 Jun 2024).

With the advent of large spectroscopic, photometric, and asteroseismic surveys, as well as improvements in stellar atmosphere and evolution modeling, the field is poised to resolve these issues. Refined rotation distributions, rotational mixing efficiency, binarity, environment-driven disk evolution, and improved chemical mappings (especially for light elements) are all active areas of research.

7. Summary Table: Key Drivers of the eMSTO Phenomenon

Proposed Mechanism	Observational Signature in eMSTO	Typical Diagnostic Evidence
Genuine Age Spread	Broad/wide MSTO region, possible dual RC	CMD modeling, pseudo-age distributions
Stellar Rotation	Red stars at MSTO are fast rotators; MS split	$v\sin i$–color correlation, synthetic CMDs
Variable Core Overshooting	MSTO broadening without MS split below TO	CMD shape, isochrone fitting
Binarity/Tidal Locking	Some slow rotators among blue eMSTO; SB2 binaries	Radial distribution, SB2 identification
Circumstellar Dust	UV-dim subgroup, extended/red eMSTO (in UV CMDs)	UV CMDs, extinction models
Star–Disc Interaction (PMS)	Central concentration of red/fast rotating stars	Radial distribution, cluster environment
Magnetic Braking (lMS)	Low $v\sin i$ below MSTO mass (Kraft break)	$v\sin i$ measurement, mass cutoff

Each mechanism's relative importance varies by cluster age, mass, metallicity, and environment. Contemporary analyses increasingly favor a hybrid scenario, where rotational broadening dominates for clusters where rotation and disk dispersal are significant, but with a residual role for genuine age spreads in some cases (especially where MSTO width exceeds rotational models' predictions). Circumstellar dust and quenching by magnetic braking modulate the lower-mass regime, while binarity contributes in cases where SB2s are observed. The eMSTO continues to be a cornerstone phenomenon for disentangling internal cluster complexity and stellar physics.