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

TL;DR

This study investigates how dust evolution in T Tauri star disks affects molecular emission lines of HCO+, HCN, and CN, revealing no clear correlation with dust properties, thus informing models of disk chemistry and evolution.

Contribution

It provides the first systematic survey of HCO+, HCN, and CN emission in a diverse sample of T Tauri disks, linking molecular line observations with dust evolution indicators.

Findings

HCO+ detected in 6 disks; HCN in none; CN in 4 disks.

No correlation between molecular line strengths and dust continuum or infrared slopes.

Results suggest complex chemical evolution not directly tied to dust properties.

Abstract

(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.