A single-dish survey of the HCO+, HCN, and CN emission toward the T Tauri disk population in Taurus

Context. The gas and dust content of protoplanetary disks evolves over the course of a few Myr. As the stellar X-ray and UV light penetration of the 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. Aims. We investigate specifically the evolution of the UV irradiation of the molecular gas in protoplanetary disks around a sample of classical T Tauri stars in Taurus that exhibit a wide range in grain growth and dust settling properties. Methods. We obtained HCO+ (J = 3-2), HCN(J = 3-2), and CN(J = 2-1) observations of 13 sources with the James Clerk Maxwell Telescope. Our sample has 1.3 mm fluxes in excess of 75 mJy, indicating the presence of significant dust reservoirs; a range of dust settling as traced through their spectral slopes between 6, 13, and 25 mu m; varying degrees of grain growth as extrapolated from the strength of their 10-mu m silicate emission feature; and complements data from the literature, essentially completing the molecular line coverage for the 21 brightest millimeter targets in the Taurus star-forming region. 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. Results. We detected HCO+ (3-2) toward 6 disks, HCN(3-2) from 0 disks, and CN(2-1) toward 4 disks, with typical 3 sigma upper limits of 150-300 mK (T-mb) in 0.2 kms(-1) channels. 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 slope. Modeling shows that fractional abundances of 10(-9) -10(-11) can explain the line intensities observed, although significant modeling uncertainties remain. For DG Tau, which we model in great detail, we find that two very different temperature and density disk structures produce very similar lines for the same underlying abundances. Instead, it is the gas properties (particularly T-kin and R-gas) and the projected kinematics (determined by M-* sin i) that have the largest impact on line appearance; and these system parameters are not well-constrained by current dust models, but instead will be probed directly with ALMA. Conclusions. Unresolved observations of the dust continuum provide neither a unique nor a complete picture of protoplanetary disks. Instead, gas-line measurements and resolved observations of dust and gas alike are needed to arrive at a full picture.