Sulfur oxides tracing streamers and shocks at low-mass protostellar disk–envelope interfaces

Accretion shocks are thought to play a crucial role in the early stages of star and planet formation, but direct observational evidence of them remains elusive, particularly regarding the molecular tracers of these processes. In this work, we searched for features of accretion shocks by observing the emission of SO and SO₂ using ALMA in Band 6 toward nearby Class I protostars. We analyzed the SO and SO₂ emission from Oph IRS 63, DK Cha, and L1527, which have different disk inclination angles, ranging from nearly face-on to edge-on. SO emission is found to be concentrated in rings at the centrifugal barriers of the infalling envelopes. These rings are projected onto the plane of the sky as ellipses or parallel slabs, depending on the inclination angles. Spiral-like streamers with SO emission are also common, with warm (T_ex > 50 K) and even hot (T_ex ≳ 100 K) spots or segments of SO₂ observed near the centrifugal barriers. Inspired by these findings, we present a model that consistently explains the accretion shock traced by SO and SO₂, where the shock occurs primarily in two regions: (1) the centrifugal barriers, and (2) the surface of the disk-like inner envelope outside the centrifugal barrier. The outer envelope gains angular momentum through outflows, causing it to fall onto the midplane at or outside the centrifugal barrier, leading to a disk-like inner envelope that is pressure-confined by the accretion shock and that moves in a rotating and infalling motion. We classify the streamers into two types – those in the midplane and those off the midplane. These streamers interact with the inner envelopes in different ways, resulting in different patterns of shocked regions. We suggest that the shock-related chemistry at the surfaces of the disk and the disk-like inner envelope warrants further special attention.