2016ApJ...820L..25Y

We have analyzed the HCO⁺ (1-0) data of the Class I-II protostar, HL Tau, obtained from the Atacama Large Millimeter/submillimeter Array long baseline campaign. We generated the HCO⁺ image cube at an angular resolution of ∼0".07 (∼10 au) and performed azimuthal averaging on the image cube to enhance the signal-to-noise ratio and measure the radial profile of the HCO⁺ integrated intensity. Two gaps at radii of ∼28 and ∼69 au and a central cavity are identified in the radial intensity profile. The inner HCO⁺ gap is coincident with the millimeter continuum gap at a radius of 32 au. The outer HCO⁺ gap is located at the millimeter continuum bright ring at a radius of 69 au and overlaps with the two millimeter continuum gaps at radii of 64 and 74 au. On the contrary, the presence of the central cavity is likely due to the high optical depth of the 3 mm continuum emission and not the depletion of the HCO⁺ gas. We derived the HCO⁺ column density profile from its intensity profile. From the column density profile, the FWHM widths of the inner and outer HCO⁺ gaps are both estimated to be ∼14 au, and their depths are estimated to be ∼2.4 and ∼5.0. These results are consistent with the expectation from the gaps opened by forming (sub-)Jovian mass planets, while placing tight constraints on the theoretical models solely incorporating the variation of dust properties and grain sizes.