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Not All Stars Form on Circular Orbits

Because galaxies form "upside down," it's useful to group stars by when they formed relative to the disk's settling: before the disk formed (pre-disk, old stars), after it first became rotationally dominated (early-disk, moderately old stars), and after it fully settled into a rotationally supported disk (late-disk, younger stars). This figure shows the distribution of orbital eccentricities for each population at birth (solid lines) and today (dotted lines), with a value of 1 corresponding to a purely radial orbit and a value of 0 being perfectly circular. Most stars in FIRE do not form on perfectly circular orbits, which matters when trying to recover a star's birth conditions from its present-day orbit alone. Each population also occupies distinct eccentricities at both birth and today, tracing the disk's settling from turbulent to disky.

Orbital Heating Depends on Stellar Age and Migration Direction

This figure shows the median change in orbital eccentricity from birth to today, as a function of stellar age, for stars that migrated significantly radially outward (solid) or inward (dashed). The average transitions between the pre-disk, early-disk, and late-disk eras are marked. Dynamical heating is greatest for stars born near the early- to late-disk transition. Younger stars simply haven't had as much time to heat, while older stars were already born on dynamically hot orbits and had less room left to heat further. This shows that how much a star's orbit heats over time depends on when it formed relative to the settling of the disk. The strong assymetry between inward and outward migrators is likely due to the declining surface density with radius in the disk. Outward migrators move into lower-density regions where heating is less likely, while inward migrators enter denser environments where heating becomes increasingly unavoidable.

Stellar Orbits Can Become More Circular Since Birth

Top: the dynamically cooled fraction, or the fraction of stars that have become significantly more circular since birth, as a function of stellar age. This fraction increases with age, which makes sense since older stars were born on progressively more eccentric orbits and so have more room to cool. Notably, up to 23% of old stars have become more circular since birth.

Bottom: the fraction of these cooled stars that have also been significantly torqued radially outward or inward. Dynamical cooling is strongly associated with outward migration, particularly for older stars. This is a significant finding, since orbital evolution is often assumed to go one of two ways: stars either retain their birth orbit or become dynamically heated over time. These results show that meaningful orbital cooling also occurs, and is closely tied to radial redistribution.