HCO+ Detection of Dust-Depleted Gas in the Inner Hole of the LkCa 15
  Pre-Transitional Disk
  LkCa 15 is an extensively studied star in the Taurus region known for its
pre-transitional disk with a large inner cavity in dust continuum and normal
gas accretion rate. The most popular hypothesis to explain the LkCa 15 data
invokes one or more planets to carve out the inner cavity, while gas continues
to flow across the gap from the outer disk onto the central star. We present
spatially unresolved HCO+ J=4-3 observations of the LkCa 15 disk from the JCMT
and model the data with the ProDiMo code. We find that: (1) HCO+ line-wings are
clearly detected, certifying the presence of gas in the cavity within <50 AU of
the star. (2) Reproducing the observed line-wing flux requires both a
significant suppression of cavity dust (by a factor >10^4 compared to the ISM)
and a substantial increase in the gas scale-height within the cavity (H_0/R_0 ~
0.6). An ISM dust-to-gas ratio (d:g=10^-2) yields too little line-wing flux
regardless of the scale-height or cavity gas geometry, while a smaller
scale-height also under predicts the flux even with a reduced d:g. (3) The
cavity gas mass is consistent with the surface density profile of the outer
disk extended inwards to the sublimation radius (corresponding to mass M_d ~
0.03 M_sun), and masses lower by a factor >10 appear to be ruled out.
