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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 ALMA and ourdata 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 wellresolved in the HCO+ (3-2) emission, and the stellar mass is estimated to be2.1+/-0.2 Msun with a disk inclination angle of 47 deg. The radial profiles ofthe HCO+ column density and excitation temperature are measured with the LTEanalysis of the two transitions of the HCO+ emission. An HCO+ gas gap at aradius of 30 au, where the column density drops by a factor of 4-8, is found inthe HCO+ column density profile, coincident with the dust gap traced by thecontinuum emission. No other clear HCO+ gas gap is seen. This HCO+ gas gap canbe opened by a planet with mass of 0.5-0.8 Mj, which is comparable to theplanet mass adopted in numerical simulations to form the dust gap at the sameradius in the HL Tau disk. In addition to the disk component, a one-arm spiralwith a length of ~3" (520 au) stretching out from the inner disk is observed inthe HCO+ (3-2) emission. The observed velocity structures along the spiralsuggest an infalling and rotating gas stream toward the inner disk.

HL Tau disk in HCO+ (3-2) and (1-0) with ALMA: gas density, temperature, gap, and one-arm spiral