Chemical inheritance versus reprocessing in protoplanetary disks: HL Tau as a case study

Planets form in protoplanetary disks, and their composition reflects the physical and chemical conditions of the regions where they are assembled. Protoplanetary disks partially inherit their chemical content from the parent molecular cloud, but this composition can be substantially altered by chemical reprocessing within the disk. Distinguishing between inheritance and in situ reprocessing is therefore a key step toward understanding the diversity of planetary atmospheres.

 

Class I sources provide an ideal framework to investigate this problem, as they simultaneously host a protoplanetary disk and a protostellar envelope. Since the envelope material is expected to retain a chemical composition closer to that of the natal molecular cloud, comparing the chemistry of the disk and the envelope can provide insights into the relative importance of inheritance and reprocessing. However, disentangling the disk and envelope contributions is challenging and often relies on detailed chemical modeling, where assumptions and uncertainties in the adopted parameters may affect the interpretation of the results.

 

HL Tau is an excellent laboratory in which to study these processes. It is a well-known Class I protostar in Taurus hosting a protoplanetary disk with prominent substructures in the form of concentric rings and gaps. The system also remains embedded within an infalling envelope. Furthermore, a streamer is feeding the system with fresh material. Understanding the role of these streamers in the mass buildup of young disks is crucial, as they may also alter their molecular composition.

We present interferometric observations of the molecular content of the HL Tau envelope and protoplanetary disk. These observations allow us to compare molecular abundances and abundance ratios between both components and investigate possible evolutionary signatures. The dataset traces sulfur-bearing molecules (CS, H₂S, and SO₂, see Figs. 1 and 2), formaldehyde, and deuterated species, all of which are sensitive to the evolutionary stage of the source. In particular, the SO2 emission traces gas released from grain surfaces, likely associated with the impact of the streamer onto the disk. We find strong variations in both column densities and molecular ratios between the envelope and the disk. In particular, the N(CS)/N(H₂S) ratio is nearly 50 times larger in the envelope than in the disk. This result suggests that molecular ratios can be used to disentangle envelope emission from that arising in the protoplanetary disk. Overall, the observed chemical differences point to significant chemical evolution between both components. Furthermore, we detect azimuthal variations in several molecular ratios across the disk, which may arise from local differences in the excitation conditions.

 

 

The detected deuterated species are DCN and DCO⁺ (J=4-3, see Fig. 3). The two molecules exhibit complementary spatial distributions: DCN shows centrally peaked emission, whereas DCO⁺ is predominantly concentrated toward the northern part of the disk. This contrasting morphology is consistent with the expected chemical differentiation between both species. DCN is concentrated toward the inner disk, where warmer gas-phase chemistry can efficiently enhance its abundance, while DCO⁺ preferentially traces colder regions in which deuterium fractionation remains efficient and CO is still present in the gas phase. The north-south asymmetry observed in DCO⁺ suggests that the physical conditions in the outer disk are not azimuthally uniform, possibly reflecting localized variations in temperature, density, or illumination. Overall, the distinct spatial distributions of DCN and DCO⁺ indicate that these molecules trace different thermal and chemical regimes within the HL Tau disk.

 

 

In summary, HL Tau provides evidence for chemical reprocessing at an early evolutionary stage, at least for the species surveyed in this work. A broader observational study is required to fully characterize the observed chemical differences. In addition, a detailed comparison with astrochemical models including deuteration and sulfur chemistry would provide important constraints on the relative roles of inheritance and chemical reprocessing.