A train of shocks at 3000-au scale? Exploring the clash of an expanding bubble into the NGC 1333 IRAS 4 region. SOLIS XIV

There is evidence that the star formation process is linked to the intricate net of filaments in molecular clouds, which may be also due to gas compression from external triggers. We studied the southern region of the Perseus NGC 1333 molecular cloud, known to be heavily shaped by similar external triggers, to shed light on the process that perturbed the filament where the Class 0 IRAS4 protostars lie. We use new IRAM-NOEMA observations of SiO and CH₃OH, both known to trace violent events as shocks, towards IRAS 4A as part of the Large Program Seeds Of Life In Space (SOLIS). We detected three parallel elongated (>6000 au) structures, called fingers, with narrow-line profiles (~1.5 km s^-1) peaked at the cloud systemic velocity, tracing gas with high density ((5-20) × 10⁵ cm^-3) and high temperature (80-160 K). They are chemically different, with the northern finger traced by both SiO and CH₃OH ([CH₃OH]/[SiO] ~ 160-300), while the other two only by SiO ([CH₃OH]/[SiO] ≤ 40). Among various possibilities, a train of three shocks, distanced by ≥5000 yr, would be consistent with the observations if a substantial fraction of silicon, frozen on to the grain mantles, is released by the shocks. We suggest that the shock train is due to an expanding gas bubble, coming behind NGC 1333 from the south-west and clashing against the filament where IRAS 4A lies. Finally, we propose a solution to the two-decades-long debate on the nature and origin of the widespread narrow SiO emission observed in the south part of NGC 1333, namely that it is due to unresolved trains of shocks.