The magnetic properties of the protostellar core IRAS 15398-3359

TL;DR

This study uses polarisation observations to reveal the magnetic field structure of a young protostellar core, supporting theories of magnetically-influenced star formation and core collapse.

Contribution

First polarimetric measurement of IRAS 15398-3359 showing magnetic field morphology consistent with magnetically-driven collapse models.

Findings

Magnetic field lines are bent inward by gravity.

Magnetic field is ordered and aligned with large-scale field and outflow.

Core is in a transcritical regime with a magnetic field strength of 78 μG.

Abstract

Context. Magnetic fields can affect significantly the star formation process. The theory of the magnetically-driven collapse in a uniform field predicts that initially the contraction happens along the field lines. When the gravitational pull grows strong enough, the magnetic field lines pinch inwards, giving rise to a characteristic hourglass shape. Aims. We investigate the magnetic field structure of a young Class 0 object, IRAS 15398-3359, embedded in the Lupus I cloud. Previous observations at large scales suggest that this source evolved in an highly magnetised environment. This object thus appears an ideal candidate to study the magnetically driven core collapse in the low-mass regime. Methods. We have performed polarisation observations of IRAS 15398-3359 at 214$\mu$m using the SOFIA/HAWC+ instrument, thus tracing the linearly polarised thermal emission of cold dust. Results. Our data unveil a significant bend of the magnetic field lines due to the gravitational pull. The magnetic field appears ordered and aligned with the large-scale B-field of the cloud and with the outflow direction. We estimate a magnetic field strength of $B= 78 \mu$G, expected to be accurate within a factor of two. The measured mass-to-flux parameter is $\lambda= 0.95$, indicating that the core is in a transcritical regime.