Chemical Portrait of a Young Star: When Infalling Gas Paints Patterns express
The star AB Aur is young, about 4 million years old. Around it swirls a dense disk of gas and dust. Recently, a long gas stream, a flow of material from the surrounding cloud, crashed into it. Astronomers pointed the NOEMA radio telescope at this disk and saw that its chemistry is not uniform: the northern part smells of sulfur, while the southern part smells of hydrocarbons.
In the north, where the infalling stream struck, there are many SO (sulfur monoxide) molecules. There, the gas is hotter and denser. It seems the impact heated the dust, and icy grains released sulfur and oxygen, like defrosting a freezer, making smells stronger. In the south, by contrast, there is a lot of C2H (a molecule of two carbon atoms and one hydrogen atom). There, the chemistry is richer in carbon. The reason is not just temperature — possibly, ultraviolet radiation from the star or heat from a forming planet breaks down more complex molecules, leaving light hydrocarbons.
The ring of CS (carbon sulfide) molecules remains smooth and cold throughout: it is like an upper floor of the disk, where splashes from the impact do not reach. Meanwhile, HCO+ (formyl ion) shines brightly where dust has gathered into a dense ring. The carbon isotope ratio of 12C to 13C in the disk's cavity is higher than usual — a clear sign that the star's harsh ultraviolet light selectively destroys heavier molecules.
Comparison with chemical models showed that in the southern, hydrocarbon-rich sector, there is more carbon than oxygen in the gas. This is a rare state for protoplanetary disks. In the northern sector, by contrast, there is more oxygen — ice evaporated and mixed into the gas. Two mechanisms, the infall of external gas and the influence of a growing planet, together paint a chemical portrait of the disk, where north and south differ like day and night. Future planets, born under such different conditions, may inherit different chemical compositions.
Original abstract
as written by the authors arXiv:2607.18683
Late infall episodes are emerging as an important driver of disk evolution. Observed as filamentary streamers in molecular lines and scattered light, such accretion perturbs disk structures, yet its chemical consequences remain unconstrained. We present NOEMA 1.2 mm observations of AB Aur, a structured young Herbig disk showing evidence for ongoing infall and planet formation. We detect azimuthal chemical diversity: SO emission is enhanced in the northern disk near the inferred streamer-disk interaction region, while C$$_2$$H peaks on the opposite southern side; CS forms a nearly axisymmetric ring. HCN and HCO$$^+$$ peak near the dust continuum overdensity in the dust ring. Rotational diagram analyses show higher SO rotational temperatures and column densities in the north, whereas CS remains axisymmetric with lower rotational temperatures, suggesting that the species probe different disk layers. For C$$_2$$H, temperature variations may contribute to but cannot fully explain the asymmetries. The HCO$$^+$$/H$$^{13}$$CO$$^+$$ line ratio indicates that HCO$$^+$$ is optically thick across the molecular ring, while the elevated ratio inside the cavity suggests enhanced gas-phase $$^{12}$$C/$$^{13}$$C, consistent with isotope-selective photodissociation. Comparison with chemical models favors gas-phase C/O ratios near or above unity, with higher effective C/O in the C$$_2$$H-bright sector. We discuss two origins for the chemical asymmetries: (i) infall-induced heating and desorption of O-bearing ices enhance SO and lower gas-phase C/O near the streamer's impact site, and (ii) planet-driven substructures and localized heating or enhanced UV irradiation promote hydrocarbon-rich chemistry in the southern disk. These results highlight that environmental accretion and planet formation can jointly imprint azimuthal variations in disk chemistry, with potential impacts on forming planets' compositions.
On Earth, the SO molecule is sulfur dioxide, a pungent gas you smell when striking a match. In the AB Aur disk, it does not smell, but it serves as a marker of shock heating of ices.