A single-dish survey of the HCO+, HCN, and CN emission toward the T
  Tauri disk population in Taurus
  (Abridged) As the stellar X-ray and UV light penetration of a protoplanetary
disk depends sensitively on the dust properties, trace molecular species like
HCO+, HCN, and CN are expected to show marked differences from photoprocessing
effects as the dust content in the disk evolves. We investigate the evolution
of the UV irradiation of the molecular gas in a sample of classical T Tauri
stars in Taurus that exhibit a wide range in grain growth and dust settling
properties. We obtained HCO+ (J=3-2), HCN (J=3-2), and CN (J=2-1) observations
of 13 sources with the JCMT. Our sample has 1.3mm fluxes in excess of 75mJy,
indicating the presence of significant dust reservoirs; a range of dust
settling as traced through their spectral slopes between 6, 13, and 25 microns;
and varying degrees of grain growth as extrapolated from the strength of their
10-micron silicate emission features. We compare the emission line strengths
with the sources' continuum flux and infrared features, and use detailed
modeling based on two different model prescriptions to compare typical disk
abundances for HCO+, HCN, and CN with the gas-line observations for our sample.
We detected HCO+ (3-2) toward 6 disks, HCN (3-2) from 0 disks, and CN (2-1)
toward 4 disks. For the complete sample, there is no correlation between the
gas-line strengths or their ratios and either the sources' dust continuum flux
or infrared slopes.
