Multiple shells driven by disk winds: ALMA observations in the HH 30 outflow

TL;DR

This study uses ALMA observations to analyze the multiple shell structures in the HH 30 outflow, revealing episodic ejections likely driven by magnetocentrifugal disk winds from the protostellar system.

Contribution

It provides the first detailed characterization of multiple shells and their episodic origins in the HH 30 outflow, supporting the disk wind launching mechanism.

Findings

Identified three shells with constant expansion and possible rotation signatures.

Estimated dynamical ages for shells, indicating episodic ejections over centuries.

Supported magnetocentrifugal disk wind as the driving mechanism.

Abstract

We present archive Atacama Large Millimeter/Submillimeter Array (ALMA) Band 6 observations of the $^{13}$CO (J=2-1) and $^{12}$CO (J=2-1) molecular line emission of the protostellar system associated with HH 30. The $^{13}$CO molecular line shows the accretion disk while the molecular outflow is traced by the emission of the $^{12}$CO molecular line. We estimated a dynamical mass for the central object of $0.45\pm0.14$ M$_\odot$, and a mass for the molecular outflow of $1.83\pm0.19\times10^{-4}$ M$_\odot$. The molecular outflow presents an internal cavity as well as multiple outflowing shell structures. We distinguish three different shells with constant expansion ($\sim4-6$ km s$^{-1}$) and possible rotation signatures ($\leq0.5$ km s$^{-1}$). We find that the shells can be explained by magnetocentrifugal disk winds with launching radii $R_\mathrm{launch}\lesssim4$ au and a small magnetic lever arm $\lambda\sim1.6-1.9$. The multiple shell structure may be the result of episodic ejections of the material from the accretion disk associated with three different epochs with dynamical ages of $497\pm15$ yr, $310\pm9$ yr, and $262\pm11$ yr for the first, second, and third shells, respectively. The outermost shell was ejected $187\pm17$ yr before the medium shell, while the medium shell was launched $48\pm14$ yr before the innermost shell. Our estimations of the linear and angular momentum rates of the outflow as well as the accretion luminosity are consistent with the expected values if the outflow of HH 30 is produced by a wide-angle disk wind.