Jets in Low-Mass Protostars

TL;DR

This review addresses how protostellar jets and outflows regulate mass accretion and angular momentum transport in low-mass star formation, leveraging ALMA and JWST to study launching regions, shock fronts, and chemical signatures across molecular, atomic, and ionic tracers. The work synthesizes evidence for both X-wind and magneto-centrifugal disk-wind launching, highlights episodic ejection and rotation signatures, and emphasizes shock-driven chemistry and jet–environment feedback. Key contributions include detailed characterizations of mass-loss rates ($10^{-9}$ to $10^{-5}\,M_\odot\,\mathrm{yr^{-1}}$), momentum fluxes, rotation constraints, and chemical clocks, alongside a growing appreciation for disk winds and their interplay with jets. The findings advance our understanding of angular-momentum extraction, disk evolution, and the potential impact of jets on planet formation, demonstrating the indispensable synergy between JWST and ALMA for constraining jet launching physics in protostellar systems.

Abstract

Jets and outflows are key components of low-mass star formation, regulating accretion and shaping the surrounding molecular clouds. These flows, traced by molecular species at (sub)millimeter wavelengths (e.g., CO, SiO, SO, H$_2$CO, and CH$_3$OH) and by atomic, ionized, and molecular lines in the infrared (e.g., H$_2$, [Fe II], [S I]), originate from protostellar accretion disks deeply embedded within dusty envelopes. Jets play a crucial role in removing angular momentum from the disk, thereby enabling continued mass accretion, while directly preserving a record of the protostar's outflow history and potentially providing indirect insights into its accretion history. Recent advances in high-resolution, high-sensitivity observations, particularly with the James Webb Space Telescope (JWST) in the infrared and the Atacama Large Millimeter/submillimeter Array (ALMA) at (sub)millimeter wavelengths, have revolutionized studies of protostellar jets and outflows. These instruments provide complementary views of warm, shock-excited gas and cold molecular component of the jet-outflow system. In this review, we discuss the current status of observational studies that reveal detailed structures, kinematics, and chemical compositions of protostellar jets and outflows. Recent analyses of mass-loss rates, velocities, rotation, molecular abundances, and magnetic fields provide critical insights into jet launching mechanisms, disk evolution, and the potential formation of binary systems and planets. The synergy of JWST's infrared sensitivity and ALMA's high-resolution imaging is advancing our understanding of jets and outflows. Future large-scale, high-resolution surveys with these facilities are expected to drive major breakthroughs in outflow research.

Figures (7)

• Figure 1: G203.21-11.20W2 protostellar system located in the Orion molecular cloud, observed with ALMA Band 6. The RGB composite shows red: SiO (5–4), green: 1.3 mm continuum, and blue: CO (2–1). A narrow, collimated jet traced by SiO emission is visible along the axis of a wide-angle outflow cavity traced by CO emission. The protostar, embedded within the dense envelope traced by the 1.3 mm continuum, lies at the intersection of the bipolar outflow lobes. The SiO and CO maps have an angular resolution of approximately 150 AU, while the 1.3 mm continuum image has a resolution of approximately 2000 AU, with the maximum recoverable scale of $\sim$10,000 AU ($\sim$25$^{\prime\prime}$). This image has been reproduced with data from 2024AJ....167...72D.
• Figure 2: JWST/MIRI color composite image of the jet/outflow system HOPS 315. The image has been produced using JWST GO Cycle 1 data (Proposal ID: 1854; PI: Melissa McClure), following methods described in 2025dutta); red: H$_2$ 0-0 S(1) 17.0338 µ m; green: H$_2$ 0-0 S(3) 9.6635 µ m; blue: H$_2$ 0-0 S(7) 5.51 µ m. The outflow cavity is indicated by a solid parabola, the central axial jet by a dotted parabola, and the protostellar position is marked with an asterisk.
• Figure 3: Schematic diagram showing the detection ranges of various molecular, atomic and ionic species in outflow--jet--shocks, reproduced and expanded from the data in 2025dutta to include additional species. The distribution of individual species are indicated along the top axis by bidirectional arrows, while the corresponding colors illustrate their relative spatial extents as depicted in the schematic.
• Figure 4: ALMA SiO and CO maps of the protostar G208.55$-$19.68S2 (HOPS 10), showing episodic knots and the monopolar nature of the jets at $\sim$150 AU resolution, adapted from 2024AJ....167...72D. (a) integrated SiO emission map (background), with CO emission contours overlaid in white. (b) Position–velocity diagram with SiO emission as the background and CO contours in white. The location of the knots are marked with R1, R2, …, R11. The mean deprojected jet velocity is estimated to be $\sim 146^{+47}_{-46}$ km s$^{-1}$, assuming an inclination angle of $\sim 20^{+10}_{-5}$ degree 2024AJ....167...72D.
• Figure 5: Observed jet mass-loss rate ($\dot{M}j$) versus accretion rate ($\dot{M}{\rm acc}$) for protostellar jet sources. Blue symbols represent well-studied protostars with molecular jets from 2020AARv..28....1L, while red symbols denote molecular jets in the ALMASOP sample from 2024AJ....167...72D. Green symbols show more evolved T Tauri sources from 2013AA...551A...5E. Separate fits are shown for molecular jets (black line) and T Tauri stars (green line). All the data in this plot were adopted with permission of the lead authors.