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

Abstract

We present ALMA observations of the $98.5\mathrm{GHz}$ dust continuum and the ${}^{13}\mathrm{CO}J=1-0$ and ${\mathrm{C}}^{18}\mathrm{O}J=1-0$ line emissions of the protoplanetary disk associated with HD~142527. The $98.5\mathrm{GHz}$ continuum shows a strong azimuthal-asymmetric distribution similar to that of the previously reported $336\mathrm{GHz}$ continuum, with a peak emission in dust concentrated region in the north. The disk is optically thin in both the $98.5\mathrm{GHz}$ dust continuum and the ${\mathrm{C}}^{18}\mathrm{O}J=1-0$ 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 ${}^{13}\mathrm{CO}J=3-2$ with a continuum emission. The gas-to-dust ratio, $G/D$, varies azimuthally with a relation $G/D\propto {\Sigma }_{\mathrm{d}}^{-0.53}$, 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.

32 pages, 15 figures

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