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Transferring a significant fraction of the magnetic flux from a dense cloudcore is essential in the star formation process. A ring-like structure producedby magnetic flux loss has been predicted theoretically, but no observationalidentification has been presented. We have performed ALMA observations of theClass I protostar IRS 2 in the Corona Australis star-forming region andresolved a distinctive gas ring in the C$^{18}$O ($J$ = 2-1) line emission. Thecenter of this gas ring is $\sim$5,000 au away from the protostar, with adiameter of $\sim$7,000 au. The radial velocity of the gas is $\lesssim1$ kms$^{-1}$ blueshifted from that of the protostar, with a possible expandingfeature judged from the velocity-field (moment 1) map and position-velocitydiagram. These features are either observationally new or have been discoveredbut not discussed in depth because they are difficult to explain bywell-studied protostellar phenomena such as molecular outflows and accretionstreamers. A plausible interpretation is a magnetic wall created by theadvection of magnetic flux which is theoretically expected in the Class 0/Iphase during star formation as a removal mechanism of magnetic flux. Similarstructures reported in the other young stellar sources could likely becandidates formed by the same mechanism, encouraging us to revisit the issue ofmagnetic flux transport in the early stages of star formation from anobservational perspective.

An ALMA-resolved view of 7000 au Protostellar Gas Ring around the Class I source CrA-IRS 2 as a possible sign of magnetic flux advection