Physical properties of accretion shocks toward the Class I protostellar system Oph-IRS 44

Abstract

Context. The final outcome and chemical composition of a planetary system depend on its formation history: the physical processes that were involved and the molecular species available at different stages. Physical processes such as accretion shocks are thought to be common in the protostellar phase, where the envelope component is still present, and they can release molecules from the dust to the gas phase, altering the original chemical composition of the disk. Consequently, the study of accretion shocks is essential for a better understanding of the physical processes at disk scales and their chemical output. Aims. The purpose of this work is to assess how the material from the infalling envelope feeds the disk and the chemical consequences thereof, particularly the characteristics of accretion shocks traced by sulfur-related species. Methods. We present high angular resolution observations (0″.1, corresponding to 14 au) with the Atacama Large Millimeter/submillimeter Array (ALMA) of the Class I protostar Oph-IRS 44 (also known as YLW 16A). The continuum emission at 0.87 mm is observed, together with sulfur-related species such as SO, SO2, and 34SO2. The non-local thermodynamic equilibrium (non-LTE) radiative-transfer tool RADEX and the rotational diagram method are employed to assess the physical conditions of the SO2 emitting region. Results. Six lines of SO2, two lines of 34SO2, and one line of SO are detected toward IRS 44. The emission of all the detected lines peaks at ~0″.1 (~14 au) from the continuum peak and we find infalling-rotating motions inside 30 au. However, only redshifted emission is seen between 50 and 30 au. Colder and more quiescent material is seen toward an offset region located at a distance of ~400 au from the protostar, and we do not find evidence of a Keplerian profile in these data. The SO2 emitting region around the protostar is consistent with dense gas (≥108 cm−3), temperatures above 70 K, high SO2 column densities between 0.4 and 1.8 × 1017 cm−2, line widths between 12 and 14 km s−1, and an abundance ratio SO2/SO ≥ 1, suggesting that some physical mechanism is enhancing the gas-phase SO2 abundance. Conclusions. Accretion shocks are the most plausible explanation for the high temperatures, high densities, and velocities found for the SO2 emission. The offset region seems to be part of a localized streamer that is injecting material to the disk-envelope system through a protrusion observed only in redshifted emission and associated with the highest kinetic temperature. When material enters the disk-envelope system, it generates accretion shocks that increase the dust temperature and desorb SO2 molecules from dust grains. High-energy SO2 transitions (Eup ~ 200 K) seem to be the best tracers of accretion shocks that can be followed up by future higher angular resolution ALMA observations and compared to other species to assess their importance in releasing molecules from the dust to the gas phase.