The Perseus ALMA Chemical Survey (PEACHES)

III. Sulfur-bearing species tracing accretion and ejection processes in young protostars

¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
² Núcleo Milenio de Formación Planetaria (NPF), Valparaíso, Chile
³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Vitacura, Santiago, Chile
e-mail: Elizabeth.ArturdelaVillarmois@eso.org
⁴ RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
⁵ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904-4235, USA

Received: 21 April 2023
Accepted: 23 August 2023

Abstract

Context. Sulfur chemistry is poorly understood in the process of low-mass star and planet formation, where the main carriers of sulfur in both the gas and the dust phase are still unknown. Furthermore, the chemical evolution of sulfur-bearing species is not fully understood given that simple S-bearing molecules, such as SO and SO₂, are commonly seen in embedded Class 0/I sources but hardly detected in more evolved Class II disks. Despite the fact that simple S-bearing molecules are usually detected toward embedded sources, large surveys of S-bearing molecules with high angular resolution and sensitive observations are currently lacking.

Aims. The goal of this work is to present an unbiased survey of simple sulfur-bearing species in protostars and provide new statistics on detection rates, emitting regions, and molecular column densities. In addition, we investigate the role of S-bearing molecules in accretion processes and the connection between (non-)detection of complex organic molecules (COMs) and S-related species.

Methods. We present the observations of sulfur-bearing species (CS, SO,³⁴SO, and SO₂) that are part of the Perseus ALMA Chemical Survey (PEACHES). We analyzed a total of 50 Class 0/I sources with observations that have an average angular resolution of about 0″.6 (∼180 au) in ALMA band 6.

Results. Class 0 sources show detection rates of 97% for CS, 86% for SO, 31% for ³⁴SO, and 44% for SO₂, while Class I sources present detection rates of 71% for CS, 57% for SO, 36% for ³⁴SO, and 43% for SO₂. When ³⁴SO is detected, the SO/³⁴SO ratio is lower than the canonical value of 22, suggesting optically thick emission, and the lowest values are found for those sources that are rich in COMs. When SO₂ is detected, those sources that show CS and SO emission parallel to the outflow direction are usually very rich in COMs, while for sources where the CS and SO emission is perpendicular to the outflow direction, only a few or no COMs are detected. When CH₃OH and SO₂ are detected, the comparison between CH₃OH and SO₂ abundances shows a positive trend and CH₃OH is between 10 and 100 times more abundant than SO₂. The SO₂ abundances toward the PEACHES sample are, on average, two orders of magnitude lower than values from the Ophiuchus star-forming region and comparable with sources in Taurus.

Conclusions. The SO/³⁴SO ratio seems to be a good tracer of the inner high-density envelope and it could be used in the future to infer the presence of multiple COMs. The detection of multiple COMs seems to be related to the presence of collimated outflows (seen in CS and SO emission), where a high column density of warm material is expected close to the protostar, and SO₂ emission seems to trace the warm gas in those sources where CH₃OH is also detected. The difference in SO₂ abundances between different star-forming regions might indicate that the sulfur depletion in the gas-phase could depend on the external UV radiation toward the molecular cloud. Finally, the SO₂ emission detected in different evolutionary stages seems to arise from different physical mechanisms: high column density of warm material in Class 0 sources, shocks in Class I/II, and exposure to UV radiation from the protostar in more evolved Class II disks.