Continuum and CO/HCO^+ Emission from the Disk Around the T Tauri Star LkCa 15

Abstract

We present Owens Valley Radio Observatory Millimeter Array λ = 3.4-1.2 mm dust continuum and spectral line observations of the accretion disk encircling the T Tauri star LkCa 15. The 1.2 mm dust continuum emission is resolved and gives a minimum diameter of 190 AU and an inclination angle of 57° ± 5°. There is a noticeable but, at present, poorly constrained decrease in the continuum spectral slope with frequency that may result from the coupled processes of grain growth and dust settling. Imaging of the fairly intense emission from the lowest rotational transitions of CO, ^(13)CO, and HCO^+ reveals a rotating disk substantially larger than that observed in the dust continuum. Emission extends to ~750 AU and the characteristic radius of the disk is determined to be ~425 AU (HWHM), based on model fits to the CO velocity field. The measured line ratios demonstrate that the emission from these species is optically thick, while that from C^(18)O and H^(13)CO^+ is optically thin, or nearly so. The disk mass derived from the CO isotopologues with typical dense cloud abundances is still nearly 2 orders of magnitude less than that inferred from the dust emission, the most probable explanation being extensive molecular depletion in the cold, dense disk midplane. Thus, while CO, HCO^+, and their isotopologues are excellent tracers of the disk velocity field, they are not reliable tracers of the disk mass. N_2H^+ 1 → 0 emission has also been detected which, along with HCO^+, sets a lower limit to the fractional ionization of 10^(-8) in the near-surface regions of protoplanetary disks. This first detection of N_2H^+ in circumstellar disks has also made possible a determination of the N_2/CO ratio (~2) that is at least an order of magnitude larger than those in the envelopes of young stellar objects and dense clouds. The large N_2/CO ratio indicates that our observations probe disk layers in which CO is depleted but some N_2 remains in the gas phase. Such differential depletion can lead to large variations in the fractional ionization with height in the outer reaches of circumstellar disks and may help to explain the relative nitrogen deficiency observed in comets.