A Monopolar Jet from Protostar HOPS 10: Evidence for Asymmetric Magnetized Launching

TL;DR

This work reports a uniquely monopolar high-velocity protostellar jet in HOPS 10, detected across CO and SiO tracers, while the accompanying low-velocity disk wind is bipolar. Through dual-band ALMA observations, the authors quantify jet and wind mass-loss rates and reveal a rotating inner envelope that implies a magnetized disk capable of launching outflows. The near-equality of total mass loss on both sides, despite a one-sided HV jet, supports intrinsic asymmetry in jet launching rather than projection or extinction effects, consistent with magneto-centrifugal wind models where hemispheric differences in mass loading or magnetic topology produce a persistent one-sided jet. This finding has significant implications for angular-momentum transport and jet launching physics in young protostars, and motivates high-resolution polarimetric studies and advanced MHD modeling to constrain the magnetic structure driving monopolar launching.

Abstract

A fundamental challenge in star formation is understanding how a protostar accretes mass from its circumstellar disk while removing excess angular momentum. Protostellar jets are widely invoked as the primary channels for angular momentum removal, yet the mechanism by which they are launched and extract angular momentum remains poorly constrained. Here we report high-resolution ALMA Band 7 (345 GHz) and Band 6 (230 GHz) observations of CO (3-2), CO (2-1), and SiO (5-4) emission from the protostar HOPS 10 (G209.55-19.68S2). The combined data trace both the entrained outflow and the collimated jet with excellent spatial and velocity resolution, revealing a uniquely monopolar protostellar jet, the clearest example reported to date. The system exhibits a distinctly unipolar high-velocity jet with velocity offsets of +44 to +66 km s-1, unlike the predominantly bipolar morphology characteristic of most protostellar jets. While the low-velocity outflow, with velocity offsets of -20 to +30 km s-1, is detected in both directions, the high-velocity jet appears only on one side, and this monopolarity is consistent across all tracers. Given the nearly edge-on geometry and low submillimeter extinction, comparable emission would normally be expected from both lobes. The shock tracer SiO emission confirms a genuine, highly collimated jet rather than cloud contamination, and no ambient structure is capable of obscuring a counterjet. We argue that intrinsically asymmetric mass loading along the disk magnetic field lines provides the most plausible explanation for the observed monopolarity.

Figures (5)

• Figure 1: Left column: Integrated-intensity maps of $^{12}$ CO(2--1), $^{12}$ CO(3--2), and SiO(5--4), from top to bottom at $\sim$ 160 AU resolution. The background images show the integrated intensity of the high-velocity (HV) components ($V_{\rm HV, off} = V_{\rm obs} - V_{\rm sys} = +44$ to $+66~\mathrm{km\,s^{-1}}$), selected based on the position--velocity diagram in the right column. The sensitivities of the three maps are 3, 13, and 2.6 $\mathrm{K\,km\,s^{-1}}$, respectively. Contours trace the $^{12}$ CO(2--1) emission integrated over the low-velocity (LV) range ($V_{\rm LV,off} = -20$ to $+30~\mathrm{km\,s^{-1}}$) above the $3\sigma$ level, and are drawn at $3n\sigma$ (where $n = 1, 2, 4, \ldots$ and $\sigma = 3~\mathrm{K\,km\,s^{-1}}$). The jet axis (PA = 30$\degr$ from North to East) is shown as a cyan dashed line, and the source position is at $(0,0)$. The synthesized beam is shown in the lower left of each panel. Right column: Position--velocity (PV) diagrams extracted along the jet axis. The rms noise levels are $\sim$0.25 K for $^{12}$ CO(2--1), $\sim$1.3 K for $^{12}$ CO(3--2), and 0.24 K for SiO(5--4). $^{12}$ CO(2--1) contours are overplotted in all three panels at 5, 10, 20, and 40% of the peak emission. The source location is at $(0,0)$. The velocity and angular resolutions are indicated in red in the lower left corner of each panel.
• Figure 2: Background image showing the low-velocity (LV) $^{12}$CO (3--2) emission integrated over $\rm V_{LV,off}$ = $-20$ to $+30~\mathrm{km\,s^{-1}}$, having a sensitivity of $\sim 13~\mathrm{K\,km\,s^{-1}}$. Black contours indicate the $^{12}$CO (2--1) emission over the same LV range. The contours are drawn above the $3\sigma$ level at $3 n_1 \sigma$ (where $n_1 = 1, 2, 4, \ldots$ and $\sigma = 2.5~\mathrm{K\,km\,s^{-1}}$). Red contours trace the high-velocity (HV) $^{12}$CO (2--1) emission, $\rm V_{HV,off}$ = $+44$ to $+66~\mathrm{km\,s^{-1}}$, drawn at $3 n_2 \sigma$ (where $n_2 = 2, 3, 4, \ldots$ and $\sigma = 2.6~\mathrm{K\,km\,s^{-1}}$).
• Figure 3: (Left panel) Integrated blue- and red-shifted ${}^{13}$CO emission contours overplotted on the 0.87 mm continuum emission. Black contours show the continuum at levels of $3 n_1 \sigma$, where $n_1 = 1,2,3,4,\ldots$ and sensitivity $\sigma = 1.33\times10^{-4}~\mathrm{Jy\,beam^{-1}}$. Blue contours are integrated over $V_{\rm obs} = 5.84$ to $7.6~\mathrm{km\,s^{-1}}$ and drawn at $(70, 78, 83, 88, 93)\sigma$ with sensitivity $\sigma = 10~\mathrm{K\,km\,s^{-1}}$. Red contours are integrated over $V_{\rm obs} = 8.92$ to $10.68~\mathrm{km\,s^{-1}}$ and drawn at $(40, 48, 56, 64)\sigma$ with sensitivity $\sigma = 6~\mathrm{K\,km\,s^{-1}}$. A tentative sense of rotation is indicated by the curved arrow from red- to blue-shifted emission. The synthesized beam is shown in the lower left corner, and the red arrow marks the direction of the redshifted jet axis. (Right panel) Position--velocity (PV) diagram of the ${}^{13}$CO emission along the disk major axis. White contours are drawn at 20, 30, 40, and 50% of the peak emission. Keplerian rotation curves for several representative central masses are overplotted. Velocities are shown as offsets from the systemic velocity, $V_{\rm off} = V_{\rm obs} - v_{\rm sys}$. The spatial and velocity resolutions are indicated in red in the lower left corner.
• Figure A1: Spectra of the CO(2--1), CO(3--2), and SiO(5--4) emission toward HOPS10 at $\sim$ 160 AU resolution, extracted from a $13\arcsec \times 1\arcsec$ region centered on the continuum peak and aligned with the jet axis (position angle $30^\circ$ east of north). All spectra are smoothed with a Gaussian kernel of $\sigma=2$ channels. The vertical dashed line marks the systemic velocity, $\rm V_{sys}=8.29~{km~s^{-1}}$.
• Figure A2: ACA $^{12}$CO(2--1) channel maps at low resolution ($\sim$2000 AU). The velocity range of each integrated channel, shown in the observed LSR (Local Standard of Rest) frame and not offset from the systemic velocity ($\rm V_{sys}=8.29~{km~s^{-1}}$), is indicated above the corresponding panel. The synthesized beam and the scalebar are shown in the first panel. Jet axis directions are indicated with red arrows for redshifted emission and cyan arrows for blueshifted emission. The continuum center is at the cross mark of the dotted lines, where the arrows originated.