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Observations of gas flows inside a protoplanetary gap

Gaseous giant planet formation is thought to occur in the first few million years following stellar birth. Models predict that giant planet formation carves a deep gap in the dust component (shallower in the gas). Infrared observations of the disk around the young star HD142527, at ~140pc, found an inner disk ~10AU in radius, surrounded by a particularly large gap, with a disrupted outer disk beyond 140AU, indicative of a perturbing planetary-mass body at ~90 AU. From radio observations, the bulk mass is molecular and lies in the outer disk, whose continuum emission has a horseshoe morphology. The vigorous stellar accretion rate would deplete the inner disk in less than a year, so in order to sustain the observed accretion, matter must flow from the outer-disk into the cavity and cross the gap. In dynamical models, the putative protoplanets channel outer-disk material into gap-crossing bridges that feed stellar accretion through the inner disk. Here we report observations with the Atacama Large Millimetre Array (ALMA) that reveal diffuse CO gas inside the gap, with denser HCO+ gas along gap-crossing filaments, and that confirm the horseshoe morphology of the outer disk. The estimated flow rate of the gas is in the range 7E-9 to 2E-7 Msun/yr, which is sufficient to maintain accretion onto the star at the present rate.