2019ApJ...880...69Y

We present our observational results of the 1.1 mm continuum and the HCO⁺ (3-2) line in HL Tau at angular resolutions of 0.''1 obtained with Atacama Large Millimeter/submillimeter Array and our data analysis of the 2.9 and 1.1 mm continuum and the HCO⁺ (3-2) and (1-0) lines of the HL Tau disk. The Keplerian rotation of the HL Tau disk is well resolved in the HCO⁺ (3-2) emission, and the stellar mass is estimated to be 2.1 ± 0.2 M_☉ with a disk inclination angle of 47°. The radial profiles of the HCO⁺ column density and excitation temperature are measured with the LTE analysis of the two transitions of the HCO⁺ emission. An HCO⁺ gas gap at a radius of 30 au, where the column density drops by a factor of 4-8, is found in the HCO⁺ column density profile, coincident with the dust gap traced by the continuum emission. No other clear HCO⁺ gas gap is seen. This HCO⁺ gas gap can be opened by a planet with mass of 0.5-0.8 M_J, which is comparable to the planet mass adopted in numerical simulations to form the dust gap at the same radius in the HL Tau disk. In addition to the disk component, a one-arm spiral with a length of ∼3'' (520 au) stretching out from the inner disk is observed in the HCO⁺ (3-2) emission. The observed velocity structures along the spiral suggest an infalling and rotating gas stream toward the inner disk.