Asymmetric accretion through a streamer onto the pre-stellar core H-MM1

Abstract

Context. Dense cores inside molecular clouds are hubs of star formation. Cores have been thought to be isolated from their surrounding cloud. However, this idea is challenged by recent observations of streamers that show evidence of mass flow from outside the core onto the embedded protostar. Multi-component analysis using molecular line observations has also revealed the existence of subsonic material outside the traditional coherent boundary of dense cores.Aims. In this study, we aim to probe the extended subsonic region observed around the pre-stellar core H-MM1 in the L1688 molecular cloud in Ophiuchus using multi-component kinematical analysis of very high-sensitivity NH_{3 data.Methods. We used observations of NH3 (1,1) and (2,2) inversion transitions using the Green Bank Telescope (GBT). We then fitted up to two components towards the core and its surrounding molecular cloud.Results. We detect an extended region of subsonic turbulence in addition to the ambient cloud, which shows supersonic turbulence. This extended subsonic region is approximately 12 times the size of and more than two times as massive as the previously detected subsonic material. The subsonic region is further split into two well-separated, velocity-coherent components, one of which is kinematically and spatially connected to the dense core. The two subsonic components are red- and blue-shifted with respect to the cloud component. We also detect a flow of material onto the dense core from the extended subsonic region via a streamer of length ≈0.15 pc (≈30000 au).Conclusions. We find that the extended subsonic component kinematically associated with the dense core contains ≈27% more mass than the core. This material could be further accreted by the core. The other subsonic component contains a mass similar to that of the core mass, and could be tracing material in the early stage of core formation. The H-MM1 streamer is kinematically similar to the ones observed towards protostellar systems, but is the first instance of such an accretion feature onto a core in its pre-stellar phase. This accretion of chemically fresh material by the pre-stellar core challenges our current understanding of a core evolving with a mass that is unchanged since the time of its formation.}