Alignment of dense molecular core morphology and velocity gradients with ambient magnetic fields

TL;DR

This study analyzes the relationship between dense core shapes, velocity gradients, and magnetic fields using extensive observational data, revealing weak overall alignment but specific anti-alignments in protostellar cores.

Contribution

It provides a large, cross-matched catalog of dense cores with detailed morphological, kinematic, and magnetic field data, and investigates their relative orientations with novel statistical insights.

Findings

Most cores show no preferred orientation relative to magnetic fields.

Protostellar cores tend to be anti-aligned with magnetic fields, indicating magnetic influence during star formation.

Region-specific anti-alignment suggests local magnetic field order affects core orientation.

Abstract

Studies of dense core morphologies and their orientations with respect to gas flows and the local magnetic field have been limited to only a small sample of cores with spectroscopic data. Leveraging the Green Bank Ammonia Survey alongside existing sub-millimeter continuum observations and Planck dust polarization, we produce a cross-matched catalogue of 399 dense cores with estimates of core morphology, size, mass, specific angular momentum, and magnetic field orientation. Of the 399 cores, 329 exhibit 2D $\mathrm{v}_\mathrm{LSR}$ maps that are well fit with a linear gradient, consistent with rotation projected on the sky. We find a best-fit specific angular momentum and core size relationship of $J/M \propto R^{1.82 \pm 0.10}$, suggesting that core velocity gradients originate from a combination of solid body rotation and turbulent motions. Most cores have no preferred orientation between the axis of core elongation, velocity gradient direction, and the ambient magnetic field orientation, favouring a triaxial and weakly magnetized origin. We find, however, strong evidence for a preferred anti-alignment between the core elongation axis and magnetic field for protostellar cores, revealing a change in orientation from starless and prestellar populations that may result from gravitational contraction in a magnetically-regulated (but not dominant) environment. We also find marginal evidence for anti-alignment between the core velocity gradient and magnetic field orientation in the L1228 and L1251 regions of Cepheus, suggesting a preferred orientation with respect to magnetic fields may be more prevalent in regions with locally ordered fields.