Characterizing the physical and chemical properties of the Class I protostellar system Oph-IRS 44. Binarity, infalling streamers, and accretion shocks

TL;DR

This study uses high-resolution ALMA observations to explore the physical and chemical properties of the Class I protostellar system Oph-IRS 44, revealing binarity, infalling streamers, and evidence of accretion shocks affecting the surrounding chemistry.

Contribution

First detailed ALMA analysis of IRS 44 showing binarity, infalling streamers, and shock-related chemical processes in a Class I protostar.

Findings

Binary nature of IRS 44 observed at sub-millimeter wavelengths.

Infalling streamers identified at disk scales potentially causing accretion shocks.

Chemical environment is oxygen-rich with high SO and SO2 due to shocks and desorption.

Abstract

(Abridged) 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. Nevertheless, 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. We present new ALMA observations of IRS 44, a Class I source that has previously been associated with accretion shocks, taken at high angular resolution (0.1"). We target multiple molecular transitions of CO, H2CO, and simple sulfur-bearing species. In continuum emission, the binary nature of IRS 44 is observed for the first time at sub-millimeter wavelengths. Infalling signatures are seen for the CO 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. 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 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 release S-bearing species as well as promoting a lukewarm chemistry 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 be a consequence of two processes: direct thermal desorption from dust grains and gas-phase formation due to the availability of O and S.