Investigating the gas-to-dust ratio in the protoplanetary disk of HD 142527

Abstract

Abstract We present ALMA observations of the 98.5 GHz dust continuum and the ${}^{13}\mbox{CO}J = 1$–0 and $\mbox{C}^{18}\mbox{O}J = 1$–0 line emissions of the protoplanetary disk associated with HD 142527. The 98.5 GHz continuum shows a strong azimuthal-asymmetric distribution similar to that of the previously reported 336 GHz continuum, with a peak emission in dust concentrated region in the north. The disk is optically thin in both the 98.5 GHz dust continuum and the $\mbox{C}^{18}\mbox{O}J = 1$–0 emissions. We derive the distributions of gas and dust surface densities, $\Sigma _\mathrm{g}$ and $\Sigma _\mathrm{d}$, and the dust spectral opacity index, $\beta$, in the disk from ALMA Band 3 and Band 7 data. In the analyses, we assume the local thermodynamic equilibrium and the disk temperature to be equal to the peak brightness temperature of ${}^{13}\mbox{CO}\,J = 3$–2 with a continuum emission. The gas-to-dust ratio, $\mathrm{G/D}$, varies azimuthally with a relation $\mathrm{G/D} \propto \Sigma _\mathrm{d}^{-0.53}$, and $\beta$ is derived to be $\approx 1$ and $\approx 1.7$ in the northern and southern regions of the disk, respectively. These results are consistent with the accumulation of larger dust grains in a higher pressure region. In addition, our results show that the peak $\Sigma _\mathrm{d}$ is located ahead of the peak $\Sigma _\mathrm{g}$. If the latter corresponds to a vortex of high gas pressure, the results indicate that the dust is trapped ahead of the vortex, as predicted by some theoretical studies.