Local Enhancement of the Surface Density in the Protoplanetary Ring Surrounding HD 142527

Abstract We report on ALMA observations of the dust continuum, and ^13 CO (J=3-2), and C^18 O (J=3-2) line emission toward a gapped protoplanetary disk around HD 142527. The outer horseshoe-shaped disk shows a strong azimuthal asymmetry in the dust continuum with a ratio of 30 to 1 at 336 GHz between the northern peak and the southwestern minimum. In addition, the maximum brightness temperature of 24K at its northern area is exceptionally high at 160 au from a star. To evaluate the surface density in this region, the grain temperature needed constraining, and was estimated from the optically thick ^13 CO (J=3-2) emission. The lower limit of the peak surface density was then calculated to be 28 g cm ^-2 by assuming a canonical gas-to-dust mass ratio of 100. This finding implies that the region is locally too massive to withstand self-gravity, since Toomre's Q 1-2, and thus it may collapse into a gaseous protoplanet. Another possibility is that the gas mass is low enough to be gravitationally stable, and only dust grains are accumulated. In this case, a lower gas-to-dust ratio by at least 1 order of magnitude is required, implying the possible formation of a rocky planetary core.