Observations of sulfuretted species in HL Tau

TL;DR

This study investigates the chemical differences between the envelope and the protoplanetary disk of HL Tau by analyzing molecular line emissions, revealing significant variations in molecular ratios indicative of chemical reprocessing during disk formation.

Contribution

It provides new NOEMA observations of HL Tau's molecular species and compares their spatial distribution and column densities in the disk and envelope, highlighting chemical evolution.

Findings

Large differences in molecular column density ratios between disk and envelope.

Variation in ratios attributed to different excitation and UV-irradiation regimes.

Evidence of chemical reprocessing during disk evolution.

Abstract

Protoplanetary disks inherit their chemical composition from their natal molecular cloud, but the extent to which this material is preserved versus reset through chemical reprocessing remains an open question. Understanding this balance is a major topic in astrochemistry. Comparing the chemical composition of the envelope and the protoplanetary disk is key to solving the topic. The goal of this paper is to investigate the chemical differences between the disk and the surrounding envelope by comparing the column density ratios of a few selected species in each region. The source we focus on is HL Tau. We present new NOEMA observations of HL Tau targeting the following species: CS, H2CO, H2S, and SO2. We produced zeroth-, first-, and second-moment maps for the species where emission was detected and used them to analyze the spatial distribution and kinematic properties of the different molecules in the disk and the envelope. We derived the column densities and compared the values derived for the envelope and disk. We also computed the rotational diagram for the SO2 detected transitions. Assuming two different temperature regimes, 17 and 58 K, we derived column densities for the species surveyed in the disk and compared them with values derived for the envelope. We find large differences in the derived column density ratios of the surveyed molecules, especially for N(CS)/N(H2S), which is 40 to 50 times larger in the envelope. We attribute these variations to the different excitation and UV-irradiation regimes in the disk and envelope. We also note strong gradients in the ratios between different positions of the disk and tentatively attribute them to different levels of turbulence at different azimuths. The observed differences in molecular ratios in the envelope and the disk are suggestive of chemical reprocessing of the gas during the formation and evolution of the protoplanetary disk.