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 \farcs 1 obtained with ALMA and our data analysis of the 2.9 mm 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 \pm 0.2 M _ { \sun } with a disk inclination angle of 47 \arcdeg .
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 \sim 3 \arcsec ( 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 .