2026A&A...706A.256A

Context. In the low-mass star formation process, theoretical models predict that material from the infalling envelope could be shocked as it encounters the outer regions of the disk. This is followed by an increase in the dust temperature and sublimation, into the gas phase, of molecular species that will otherwise remain locked on dust grains. Although accretion shocks are predicted by theoretical models, only a few protostars show evidence of these shocks at the disk-envelope interface, and the main formation path of shocked-related species is still unclear. They can be formed entirely on dust surfaces and then sublimated, or through reactions in the gas phase, or a combination of both.
Aims. The goal of this work is to assess the chemistry associated with accretion shocks and the formation path of molecules that are usually associated with these dense and warm regions.
Methods. We present new observations of IRS 44, a Class I source with a resolved disk that has previously been associated with accretion shocks, taken at high angular resolution (0_⋅^''1, corresponding to 14 au) with the Atacama Large Millimeter/submillimeter Array (ALMA). We observe three different spectral settings in bands 6 and 7, targeting multiple molecular transitions of CO, H₂CO, and simple sulfur-bearing species (such as CS, SO, SO₂, H₂S, OCS, and H₂CS).
Results. In continuum emission, the binary nature of IRS 44 is observed for the first time at sub-millimeter wavelengths and the emission agrees with the optical and infrared counterparts. Infalling signatures are seen for the CO 2-1 line and the emission peaks at the edges of the continuum emission around IRS 44 B, the same region where bright SO and SO₂ emission is seen. Weak CS and H₂CO emission is observed, while OCS, H₂S, and H₂CS transitions are not detected.
Conclusions. IRS 44 B seems to be more embedded than IRS 44 A, indicating a non-coeval formation scenario or the rejuvenation of source B due to late infall. CO 2-1 emission is tracing the outflow component at large scales, infalling envelope material at intermediate scales, and two infalling streamer candidates are identified at disk scales. Infalling streamers might produce accretion shocks when they encounter the outer regions of the infalling-rotating envelope. These shocks heat the dust and efficiently release S-bearing species (such as H₂S, SO, and SO₂), as well as promoting a lukewarm chemistry (∼200 K) in the gas phase. With the majority of carbon locked in CO, there is little free C available to form CS and H₂CS in the gas, leaving an oxygen-rich environment. The high column densities of SO and SO₂ might therefore be a consequence of two processes: direct thermal desorption from dust grains and gas-phase formation due to the availability of O and S. IRS 44 is an ideal candidate with which to study the chemical consequences of accretion shocks and the dynamical connections between the envelope and the disk, through infalling streamers.