tokens: of UV radiation (e.g., Jonkheid et al. 2007 ). Because the observed HCO + gas gap at a radius of 30 au coincides with the dust gap, there is a deficit of dust grains in the HCO + gas gap (Okuzumi et al. 2016 ; Pinte et al. 2016 ). Thus, the gas-to-dust mass ratio is higher in the gap, if there is no actual deficit of H 2 gas. As demonstrated in chemical models, the HCO + abundance in a protoplanetary disk with a higher gas-to-dust mass ratio of 1000 could be a factor of ten lower than in a disk with a gas-to-dust mass ratio of 100 (Jonkheid et al. 2007 ). Therefore, the HCO + column density possibly decreases in these dust gaps, even if there is no deficit of H 2 gas in the dust gaps. Future observations to measure the column density of neutral molecules in the HL Tau disk, whose abundances are less sensitive to the gas-to-dust mass ratio and UV radiation, are needed to examine this possibility. 5.2. Origins of the Infalling Spiral An infalling gas stream toward HL Tau is detected in the HCO + (3–2) emission with our ALMA observations, and its rotational motion gradually becomes dominant over its infalling motion. We note that the HCO + (3–2) line is optically thick (Figure 13 ), and the observed line-of-sight velocity of the spiral structure is inconsistent with the Keplerian velocity at radii larger than 03 (Figure 16 ). In addition, no counterpart of this spiral is detected in the continuum emission, which traces the disk mid-plane. These results could suggest that the infalling spiral is not in the mid-plane but is located above the disk surface. The velocity of the infalling spiral matches with the Keplerian velocity at a radius of 03, and there is a bright peak at a radius close to 03 (Figure 1 ). Thus, the infalling spiral could merge with the HL Tau disk at a radius of 03. Future observations of shock tracers, such as SO, SO 2 , and CH 3 OH, are needed to examine this scenario and the presence of accretion shock (Ohashi et al. 2014 ; Sakai et al. 2014 ; Yen et al. 2014 ; Artur de la Villarmois et al. 2019 ). Spiral structures connected with central disks have been observed in millimeter continuum emission and infrared reflected light around several young stellar objects (Liu et al. 2016 , 2018 ). Infalling motion along spiral structures has also been detected around other protoplanetary disks (Tang et al. 2012 ; Yen et al. 2014 ). An infalling spiral could form when a gas clump falls freely toward the center with a conserved angular momentum (e.g., Ulrich 1976 ). Free-falling material with a conserved angular momentum follows a parabolic trajectory, which appears as a spiral structure after the projection on the plane of the sky. This freefall model has successfully explained the morphology and velocity of the infalling spirals observed in the protostellar source, L1489 IRS (Yen et al. 2014 ). In HL Tau, we found that the morphology of the outer part of the infalling spiral at a radius larger than 05 can indeed be described by a
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