Millimeter-wave observations sharpen view of protostar outflows

A low-mass star begins to form when a cloud of cold, molecular gas collapses under its own gravity. The more the cloud shrinks, the faster it rotates. Centrifugal forces would forestall further collapse but for the protostar’s ability to shed excess angular momentum via the formation of a rotating circumstellar accretion disk. The disk, in turn, creates another way to shed angular momentum: The circulating plasma in its inner regions forms magnetic funnels through which material can flow away from the protostar’s poles—like water spiraling down a bathtub drain.

Established in the early 1980s, that basic physical picture requires elaboration or modification to account for the fact that protostars are observed to have two types of polar flows: A slow, wide molecular outflow and a narrow, fast jet. How the two types might be related remains a mystery, but new observations by Yuko Matsushita of Kyushu University in Japan and her collaborators favor one previously proposed scenario over another.

Matsushita and company used the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile to observe a protostar in the constellation of Orion known as MMS 5 or OMC-3 (see the figure). Signals from ALMA’s 66 individual radio telescopes are combined interferometrically to yield images of unprecedented sharpness in the millimeter bandwidth. Viewed with ALMA, both types of flow in MMS 5 could be resolved for the first time. The observations revealed that the two flows were misaligned by 17°. From that and other measurements, the researchers deduced that the wide, molecular outflow started before the narrow jet and that the two flows launched from different protostellar radii.

The deduction is inconsistent with models that have the jets scooping up and entraining the molecular outflows along with them. But it’s consistent with a model developed in 2008 by Matsushita’s collaborator Masahiro Machida in which the two flows are launched in more or less the same way but at different stages of the protostar’s development.

In Machida’s model, the molecular outflow begins when the cloud reaches a particle density of about 5 × 10¹⁰ cm⁻³ and begins to collapse quickly and adiabatically rather than slowly and isothermally. At that stage, when the radius of the innermost disk is relatively large, the closest-in plasma circulates quickly enough to create magnetic funnels, but the outgoing flows are perforce wide and slow.

Jets are launched later when the protostar’s density exceeds the threshold, 10¹⁶ cm⁻³, at which a second abrupt collapse is triggered by the disassociation of molecular hydrogen. At that stage, the innermost region is closer in, circulation is faster, and the funnels are narrower. The outgoing flows are perforce narrow and fast.

The ALMA observations rule out the ongoing entrainment of molecular outflows by jets in MMS 5. Matsushita is careful to point out that the possibility remains that molecular outflows might have been driven in the past by a jet that is no longer visible: “Thus we cannot completely rule out the entrainment model.” (Y. Matsushita et al., Astrophys. J. 871, 221, 2019 .)