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Tracing the Infall and the Accretion Shock in the Protostellar Disk: L1157

We present the first observational evidence supporting the presence of an accretion shock in a protostellar disk. This result was obtained from the spatially resolved methanol emission in the 50-40 (A) transition at 1 mm in the Class 0 protostellar object L1157. The high spatial resolution (with a beam of ~1.″1 × 1.″1) maps of methanol and continuum dust emission were observed with the Owens Valley Radio Observatory Millimeter Array. The structure and kinematics of the methanol emission indicate that the gas is tracing a warm layer in the infall-disk interface, consistent with an accretion shock. This result confirms an important prediction of theoretical models of infalling material feeding the growth of protostellar disks.