Whether star clusters are born with their most massive stars already concentrated toward the center (primordial mass segregation) or acquire this concentration dynamically within a few crossing times remains unsettled. Pavlík, Kroupa & Šubr (2019) concluded from idealized N-body experiments that the Orion Nebula Cluster (ONC) is compatible with having formed in a state of near-complete primordial mass segregation, and that the ALMA-observed protocluster Serpens South may show the same phenomenon at birth. Their own future-work agenda — testing whether that conclusion survives primordial binaries, multiple stellar populations, and self-consistent non-spherical geometry, and establishing observationally how widespread near-complete primordial segregation is — remains open. Later work has addressed the binary strand (it strengthens the primordial reading), split on what gas-dynamical formation simulations predict, and produced conflicting ALMA/(sub)millimeter surveys on prevalence, so a precise open core survives: the robustness of the ONC conclusion under realistic multi-population, binary-rich, non-spherical models, and the census of near-complete primordial segregation among newborn clusters.

Background

Mass segregation in a star cluster is the radial concentration of the most massive stars relative to lower-mass stars. In two-body relaxation — the slow exchange of energy between pairs of stars — massive stars lose energy and drift inward, so segregation is expected to develop dynamically on roughly a relaxation time, the half-mass relaxation time trht_{\mathrm{rh}}. Bonnell & Davies (1998) argued that if clusters younger than a relaxation time are observed to be segregated, that segregation cannot have been produced dynamically and must be primordial — imprinted by the formation process itself. A counterexample to the simplicity of that argument is rapid collisional segregation in cold, clumpy initial conditions (Allison et al. 2009). The Orion Nebula Cluster (ONC), the nearest massive embedded star-forming region (about 400 parsecs away and only a few million years old), is the canonical test case, and whether its observed strong segregation is primordial or dynamical is a key diagnostic between competing massive-star formation scenarios: monolithic core collapse (the cluster forms nearly at once from a single collapsing cloud core), competitive accretion (stars gain mass by accreting gas through the cluster's shared gravitational potential, favoring centrally located stars), and hierarchical subcluster assembly (the cluster builds up by merging smaller subclusters). Gaia DR2 astrometry combined with wide-field optical photometry further indicates that the ONC contains two or three distinct, centrally concentrated pre-main-sequence stellar populations about 1–2 Myr apart in age (Jeřábková et al. 2019) — the origin of the "three putative populations" invoked below — as well as a high primordial binary fraction. Pavlík, Kroupa & Šubr (2019) evolved idealized N-body models (the direct N-body code nbody6, which integrates the exact gravitational interactions of every star; spherical Plummer-sphere initial conditions, all-single stars, no gas, no stellar evolution) of initially fully mass-segregated clusters and measured the timescale on which primordial segregation is erased, ⟨τv⟩≈3.3 trh\langle\tau_v\rangle \approx 3.3\,t_{\mathrm{rh}}. Because this erasure is slow compared to the ONC age, they concluded that the present-day ONC is compatible with primordial (near-)complete mass segregation — a conclusion that became fully consistent with observations only after a deliberately dynamically inconsistent geometric elongation was imposed to mimic the ONC's observed elongated shape, while extinction-driven incompleteness (obscuration of low-mass stars by interstellar dust) played only a minor role. Independent support for birth segregation came from ALMA observations of the Serpens South protocluster, where the deeply embedded protostars (Plunkett et al. 2018) appear already segregated at the earliest observable stage. The authors acknowledged that their models are idealized — without a gaseous potential or other realistic features of very young clusters, without primordial binaries or stellar evolution, and using an elongation scaling that is not dynamically self-consistent — and listed what would be required to consolidate the conclusion: more detailed models that consider the evolution of non-spherical star clusters; models of the ONC incorporating its three putative stellar populations and a high initial binary population; and more observational data of newborn and young star clusters, to establish how widespread near-complete primordial segregation is and whether Serpens South is representative or exceptional. A standard observational diagnostic in this literature is the mass-segregation ratio ΛMSR\Lambda_{\mathrm{MSR}}, which compares the median separation of the NN most massive stars to that of equivalent random samples; values above 1 indicate segregation. The degree of primordial mass segregation is quantified by the parameter S∈[0,1]S\in[0,1] used by Pavlík, Kroupa & Šubr (2019): S=1S=1 is the completely mass-segregated state, in which the stars are sorted by mass along the energy ordering so that the most massive stars occupy the lowest-energy orbits, and S=0S=0 is an initially unsegregated state; "near-complete" denotes an initial state close to S=1S=1, so ΛMSR>1\Lambda_{\mathrm{MSR}}>1 indicates segregation but does not by itself establish that it is near-complete.

Problem Statement

The problem, as posed by the future work of Pavlík, Kroupa & Šubr (2019), has two jointly supportive parts. (1) Robustness of the ONC conclusion: construct self-consistent dynamical models of the Orion Nebula Cluster and determine whether the conclusion that the present-day ONC is consistent with primordial (near-)complete mass segregation survives. The ingredients the source's future work identifies as required are the self-consistent evolution of non-spherical (elongated/filamentary) initial geometry, the three putative stellar populations (the two confirmed and one suggested centrally concentrated pre-main-sequence populations), and a primordial binary population (high initial binary fraction). A gaseous background with residual gas expulsion, stellar evolution, and a treatment of extinction-driven observational incompleteness coupled to intrinsic cluster structure are optional realism extensions beyond these required ingredients. For a model set containing at minimum an initially near-completely segregated (S≈1S\approx1) run and an initially unsegregated (S=0S=0) run that each include all three required ingredients together, the quantity of interest is the erasure timescale in units of trht_{\mathrm{rh}} and its observational signatures; the conclusion survives if the erasure timescale still exceeds the estimated ONC age, so that the near-completely segregated initial state remains observationally consistent with the present-day ONC while the unsegregated state does not. (2) Prevalence: determine observationally how widespread near-complete primordial mass segregation is among newborn and very young clusters, and in particular whether the ALMA-observed strongly segregated cases such as Serpens South are representative of cluster formation or exceptional. The sample is the published newborn and very young clusters, of order a relaxation time old or younger (following the Bonnell & Davies argument), for which a radial segregation diagnostic has been measured, and a cluster counts as near-complete only if its segregation is consistent with the S≈1S\approx1 baseline rather than merely ΛMSR>1\Lambda_{\mathrm{MSR}}>1. The common objective that both parts serve is to decide whether star clusters form in a completely (or near-completely) mass-segregated way, or whether observed early segregation is dynamically assembled.

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