Episodic accretion constrained by a rich cluster of outflows
  The accretion history of protostars remains widely mysterious even though it
represents one of the best ways to understand the protostellar collapse that
leads to the formation of stars. Molecular outflows are here used to
characterize the protostellar accretion phase in W43-MM1. The W43-MM1
protocluster host a sufficient number of protostars to statistically
investigate molecular outflows in a single, homogeneous region. We used the
CO(2-1) and SiO(5-4) line datacubes, taken as part of an ALMA mosaic with a
2000 AU resolution, to search for protostellar outflows, evaluate the influence
that the environment has on these outflows' characteristics and put constraints
on outflow variability in W43-MM1. We discovered a rich cluster of 46 outflow
lobes, driven by 27 protostars with masses of 1-100 Msun. The complex
environment inside which these outflow lobes develop has a definite influence
on their length, limiting the validity of using outflows' dynamical timescales
as a proxy of the ejection timescale in clouds with high dynamics and varying
conditions. We performed a detailed study of Position-Velocity (PV) diagrams of
outflows that revealed clear events of episodic ejection. The time variability
of W43-MM1 outflows is a general trend and is more generally observed than in
nearby, low- to intermediate-mass star-forming regions. The typical timescale
found between two ejecta, about 500 yr, is consistent with that found in nearby
protostars. If ejection episodicity reflects variability in the accretion
process, either protostellar accretion is more variable or episodicity is
easier to detect in high-mass star-forming regions than in nearby clouds. The
timescale found between accretion events could be resulting from disk
instabilities, associated with bursts of inflowing gas arising from the
dynamical environment of high-mass star-forming cores.
