HCO+ Detection of Dust-Depleted Gas in the Inner Hole of the LkCa 15 Pre-Transitional Disk

TL;DR

This study detects gas within the dust-depleted cavity of the LkCa 15 disk using HCO+ observations, revealing significant gas presence and structural details that support planet-induced cavity formation.

Contribution

It provides the first spatially unresolved HCO+ detection of gas in the cavity, modeling the gas and dust distribution to explain the observed line-wing flux and cavity structure.

Findings

Gas exists within <50 AU of the star in the cavity.

Significant dust depletion (>10^4) and increased gas scale-height are required to match observations.

Cavity gas mass is consistent with an extension of the outer disk, around 0.03 solar masses.

Abstract

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.