# FAUST XXXII. Chemically layered structures and ionisation in the disk and envelope of NGC 1333 IRAS 4C

**Authors:** Shota Notsu, Nami Sakai, Yichen Zhang, Ana Lopez-Sepulcre, Claire J. Chandler, Ziwei E. Zhang, Cecilia Ceccarelli, Claudio Codella, Yuri Aikawa, Jaime E. Pineda, Brian Svoboda, Felipe O. Alves, Nadia Balucani, Eleonora Bianchi, Mathilde Bouvier, Gemma Busquet, Paola Caselli, Emmanuel Caux, Steven Charnley, Spandan Choudhury  
**arXiv:** [2610.28795](https://arxiv.org/abs/2610.28795)  
**Date:** 2026-10-20  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2610.28795v1/

## Key points

- Aim: Investigate whether chemical layering and radial ionisation gradients are already present in the disk and envelope of the very young Class 0 protostar NGC 1333 IRAS 4C.
- Observations: ALMA mapped molecular emission at approximately 45–80 au resolution; the rotationally supported disk and flattened infalling envelope cannot be cleanly separated.
- Vertical layering: C¹⁸O emission peaks approximately 50 au above the midplane, H₂CO at 50–150 au, and hydrocarbons CCH and cyclic-C₃H₂ at 150–300 au.
- Chemical interpretation: Hydrocarbon-deficient midplane regions support genuine chemical stratification rather than dust obscuration alone, with hydrocarbons tracing irradiated surface layers and H₂CO an intermediate layer.
- Radial structure: C¹⁸O peaks 50–100 au from the protostar, H¹³CO⁺ at 100–200 au, and DCO⁺ beyond approximately 300 au.
- Ionisation gradient: The HCO⁺/CO ratio increases outward, suggesting lower ionisation in the inner midplane; estimated rates are at most 10⁻¹⁸ s⁻¹ at 50–100 au versus approximately 10⁻¹⁷–10⁻¹⁶ s⁻¹ beyond 100 au.
- Deuteration: The DCO⁺/HCO⁺ ratio rises from below 0.1% inside 150 au to approximately 0.4–0.5% farther out.
- Implication: Chemical layering develops early, and disk formation may increase central gas density while reducing inner-midplane ionisation.
- Uncertainties: Absolute ionisation rates are uncertain by orders of magnitude and depend strongly on assumed line-of-sight depth; temperature, chemistry, optical depth, projection, and disk–envelope mixing introduce additional limitations.
- Next steps: Additional molecular transitions, ionisation tracers, and detailed chemical and physical modelling are needed; optically thick dust prevents reliable interpretation within approximately 50 au.

## Summary

This paper investigates whether chemical stratification and radial ionisation gradients are already present in the disks and envelopes of very young Class 0 protostars. Although these structures are well established in more evolved disks, they remain poorly explored during the earliest stages of disk formation. The study targets NGC 1333 IRAS 4C, a low-mass Class 0 protostar in Perseus, located 293 parsecs away and with a luminosity of 1.1 solar luminosities. Its disk is viewed approximately 25 degrees from edge-on, allowing the authors to examine variations in molecular emission with height above the midplane. Throughout the paper, “disk” refers to both the rotationally supported disk and the flattened, infalling envelope, which cannot be cleanly separated. The authors use ALMA observations from the FAUST large program, combining extended and compact 12-metre array observations with supplementary 7-metre array data for one frequency setting. Molecular lines near 216–262 GHz were mapped at angular resolutions of approximately 0.15–0.27 arcseconds, corresponding to roughly 45–80 astronomical units (au). The study compares C¹⁸O, formaldehyde (H₂CO), ethynyl (CCH), and cyclic-C₃H₂ to investigate vertical layering, and H¹³CO⁺, DCO⁺, and C¹⁸O to examine radial ionisation. Molecular column densities are derived assuming optically thin emission and an excitation temperature of 50 kelvin, while hydrogen column densities are estimated from dust emission. The observations reveal a clear vertical ordering of molecular emission. C¹⁸O lies closest to the midplane, with intensity peaks approximately 50 au above it. H₂CO peaks at heights of approximately 50–150 au, while CCH and cyclic-C₃H₂ peak at approximately 150–300 au. The hydrocarbon emission exhibits a dark lane along the midplane, including regions with weak dust emission. This pattern supports a genuine abundance deficit rather than obscuration by dust alone. Differences in excitation requirements also favour chemical stratification as the primary explanation. The authors interpret the hydrocarbons as tracers of irradiated, photochemically active surface layers, with H₂CO tracing an intermediate molecular layer. In contrast, the Class 0 source L1527 lacks comparable hydrocarbon emission from its disk surface. Radially, C¹⁸O emission peaks approximately 50–100 au from the protostar, H¹³CO⁺ at 100–200 au, and DCO⁺ beyond approximately 300 au. The inferred HCO⁺/CO column-density ratio increases from approximately 10⁻⁵–10⁻⁴ at 50–200 au to 10⁻⁴–2 × 10⁻⁴ beyond 200 au. Using a simplified ionisation–recombination balance and an assumed line-of-sight depth of 60,000 au, the authors estimate ionisation rates of approximately 10⁻¹⁷–10⁻¹⁶ s⁻¹ beyond 100 au, falling to at most 10⁻¹⁸ s⁻¹ at 50–100 au. The DCO⁺/HCO⁺ ratio also increases outward, from below 0.1% inside 150 au to approximately 0.4–0.5% farther out. These results suggest that chemical layering develops early and that disk formation may increase central gas densities while reducing ionisation in the inner midplane. However, the absolute ionisation rates remain uncertain by orders of magnitude: adopting a line-of-sight depth of 1,000 au increases the estimates by a factor of 60. Additional limitations include uncertainties in temperature structure and isotope ratios, simplified chemical assumptions, line optical depth, projection effects, and unresolved disk–envelope mixing. Optically thick dust also prevents reliable interpretation within approximately 50 au. The authors therefore recommend observations of additional molecular transitions and ionisation tracers, combined with detailed chemical and physical modelling, to test the inferred chemical structure and ionisation gradient.