SiO emission from low- and high-velocity shocks in Cygnus-X massive dense clumps

Context. The SiO molecule is formed through interstellar shocks and is often used as a tracer of high-velocity shocks from protostellar outflows. However, recent studies have suggested that low-velocity shocks in the interstellar medium can be responsible for a significant amount of SiO emission observed in star-forming regions.
Aims: We aim to investigate the existence of SiO emission that may or may not be associated with outflow shocks, within several massive dense clumps (MDCs), and explore how the SiO luminosities compare with the outflow properties estimated using CO.
Methods: We used observations of SiO (2-1) from the Plateau de Bure Interferomètre, towards a sample of six MDCs in Cygnus-X, and compared them to the CO high-velocity outflow emission, and to the velocity shears found in these regions.
Results: We find that most molecular outflows are detected in both SiO and CO, although there are some cases of CO outflows with no SiO counterpart. The data also shows narrow line SiO emission (σ_ν ≲ 1.5 km s^-1) which, in some cases, appears to be unrelated to outflows. The kinematics of this narrow emission often differs from those found by other high-density tracers such as H¹³CO⁺, and its extent varies from rather compact (∼0.03 pc) to widespread (∼0.2 pc). We find that the least centrally concentrated clumps with the least massive protostellar cores have the most widespread narrow SiO emission. The fraction of the total SiO luminosity that is not associated with outflows is highly variable in the different MDCs (from 10% to 90%); this might be a problem when extrapolating outflow properties from SiO luminosities without resolving individual outflows.
Conclusions: In line with previous evidence of SiO emission associated with low-velocity shocks, we propose an evolutionary picture to explain the existence and distribution of narrow SiO line profiles. In this scenario, the least centrally condensed MDCs are at an early stage where the SiO emission traces shocks from the large-scale collapse of material onto the MDC. This could be the case of CygX-N40, a MDC with a low-mass protostar at its centre, a weak outflow, and where 90% of the SiO emission is narrow and arises from the outskirts. As the MDC collapses, the SiO emission becomes more confined to the close surroundings of cores, tracing the post-shock material from the infalling MDC against the dense cores, such as in the small-scale converging flows of CygX-N3, N12, and N48. At later stages, when single massive protostars are formed, as for instance in CygX-N53 and N63, the SiO luminosity is largely dominated by powerful outflows, and the weaker narrow component shows perhaps the last remnants of the initial collapse.

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