Observations of sulfuretted species in HL Tau

TL;DR

This work presents NOEMA Band 2 observations of HL Tau targeting CS, H2S, H2CO, and SO2 to compare chemical compositions in the disk and envelope. By applying zeroth–second moment maps and Keplerian masking, the study derives column densities and, for SO2, a rotational diagram indicating $T_{rot} ≈ 56$–$60$ K and $N(SO_2) ≈ 3×10^{14}$ cm$^{-2}$. It finds a striking contrast in molecular ratios, especially $N(CS)/N(H_2S)$, which is ~40–50× higher in the envelope than in the disk, pointing to chemical reprocessing as material transitions from cloud to disk. The disk also shows azimuthal variations and a localized SO2 enhancement on the western side, likely tied to the impact of a large-scale streamer, suggesting shock-driven desorption and turbulence effects. Overall, the results imply that sulfur-bearing chemistry and related molecular ratios can differentiate envelope from disk gas and reveal chemical evolution during protoplanetary disk formation.

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.

Figures (11)

• Figure 1: Continuum map at 2 mm. Contour levels are 5, 25, and 50 times the rms.
• Figure 2: From left to right: Continuum-subtracted zeroth-, first-, and second-moment map of CS 3-2 (top), H$\rm _2$S 1$\rm _{10}$-1$\rm _{01}$ (middle), and H$\rm _2$CO $\rm 5_6-4_5$ (bottom) observed with NOEMA. Before the moment calculation, a four-sigma clipping mask was applied to the data cubes. The cubes were smoothed by a factor of three before computing the moment maps. Only channels in the range 3 to 11 km s$\rm ^{-1}$ and with a S/N larger than four have been used.
• Figure 3: From left to right: Integrated intensity maps of CS 3-2 (left), H$\rm _2$S 1$\rm _{10}$-1$\rm _{01}$ (middle), and H$\rm _2$CO $\rm 5_6-4_5$ (right) after a Keplerian mask has been applied. The black star in each map marks the position of the emission peak of the map.
• Figure 4: Continuum-subtracted average spectra of CS, H$\rm _2$CO, and H$\rm _2$S. The spectra shown have been integrated within a radius of 1.6$\arcsec$ from the center.
• Figure 5: Radial profiles of the species detected in our survey. The radial profiles were computed on deprojected maps assuming i=46.7$^\circ$ and PA=138$^\circ$. The shaded area around each profile accounts for the standard deviation at each radial bin. The dashed vertical lines mark the position of the radial emission peaks. The black line in the bottom right corner shows the average beam size.