Magnetic fields in star formation

Abstract

The question ‘how do massive stars obtain their mass?’ has been a topic of much debate. Most high-mass stars form in young stellar clusters, under conditions distinct from low-mass star formation. The two dominant theories - Turbulent Core and Competitive Accretion - propose contrasting views: the former suggests a slow, quasi-static formation, while the latter invokes dynamic, accretion-driven growth. Neither fully explains observations of massive star-forming regions.

This thesis investigates these models with the addition of magnetic fields, a fundamental component of molecular clouds. Magnetic fields inhibit collapse across field lines, shape cloud morphology, and may suppress fragmentation, which would facilitate the formation of high-mass prestellar cores. We simulate six magnetised molecular clumps spanning mass-to-flux ratios M/Φ = 3, 5, 10, 100, plus two purely hydrodynamical clouds for comparison: one virialised, one cold (T/U = 0.4).

Using an in-house potential-based clump-finding algorithm, we identify prestellar cores and track their evolution. We find that all cores begin as low-mass `seeds' and grow via mergers and accretion, becoming bound near the thermal Jeans mass - independent of the initial field strength. Growth prior to collapse suggests a Competitive Accretion-like scenario, regulated by the magnetic field.

The strength of the initial field modulates the nature of cloud collapse. Strong fields channel collapse along field-lines, whereas weaker fields initially collapse more spherically. These trends are reflected in the B–ρ^κ relation, with κ < 2/3 where fields dominate and κ ~ 2/3 in spherical collapse. These differences mean that by the time cores form, the field strength in the densest regions is comparable. Core morphological evolution confirms this: strong fields produce initially oblate cores evolving into prolate forms, while weaker fields favour filamentary/prolate structures from the outset, highlighting the field’s role in shaping both dynamics and morphology.

Date of Award	3 Jul 2026
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Ian Bonnell (Supervisor)