Baumgardt, De Marchi & Kroupa (2008)

Abstract (paraphrased)¶

Demonstrates that primordial mass segregation can explain both the IMF-slope-vs-concentration trend and the depletion of low-mass stars in globular clusters. progenax uses this paper as the physical motivation for primordial segregation; the implemented energy-ordered IC machinery is exposed through the progenax.cluster layer.

Use in progenax¶

• Mass segregation — energy-ranked algorithm, two released routes

• progenax.energy_sorted_segregation — the PRIMORDIAL energy-ordered generator (public API)

• MultiComponentCluster.from_mass_segregation(delta) — the separate differentiable equilibrium route (not from this paper; equipartition law $w_j = \mu_j^{-\delta}$ after Gieles & Zocchi 2015)

Notes¶

The canonical primordial segregation IC algorithm: most massive stars onto the most bound orbits of an equilibrium pool, with bin widths proportional to cumulative mass.

Deliberate, documented departure from the published recipe. Baumgardt+2008 draw orbits randomly within cumulative-mass bins; progenax’s energy_sorted_segregation instead uses a deterministic isotonic rounding of the bin-centre targets to a strictly increasing orbit-index sequence. The random per-bin draw fails for steep IMFs — cumulative-mass bins collapse below one orbit, forcing many low-mass ranks onto the same orbit (coincident stars, $V = -\infty$). The deterministic assignment guarantees a distinct orbit per star for any mass spectrum; realisation variety comes from re-drawing the random orbit pool. See the module docstring of src/progenax/cluster/mass_segregation.py.

The historical λ_seg catalog blend that once smoothed this construction was retired in the 2026-06 unified redesign (its intermediate states drift from per-mass-group virial balance); differentiable segregation control now comes from the equilibrium MultiComponentCluster.from_mass_segregation route.