HL Tau disk in HCO+ (3-2) and (1-0) with ALMA: gas density, temperature,
  gap, and one-arm spiral
  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 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+/-0.2 Msun with a disk inclination angle of 47 deg. 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 Mj, 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.
