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

Abstract

(Abridged) 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. The purpose of this work is to assess the characteristics of accretion shocks traced by sulfur-related species. We present ALMA high angular resolution observations (0.1") of the Class I protostar Oph-IRS 44. The continuum emission at 0.87 mm is observed, together with sulfur-related species such as SO, SO${}_2$, and ${}^{34}$SO${}_2$. Six lines of SO${}_2$, two lines of ${}^{34}$SO${}_2$, 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. Accretion shocks are the most plausible explanation for the high temperatures, high densities, and velocities found for the SO${}_2$ emission. When material enters the disk--envelope system, it generates accretion shocks that increase the dust temperature and desorb SO${}_2$ molecules from dust grains. High-energy SO${}_2$ transitions (~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.

16 pages, 16 figures

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