ALMA Survey Reveals Substructures in Protoplanetary Discs

The Ophiuchus DIsc Survey Employing ALMA (ODISEA) presents a complete flux-limited high-resolution study of approximately 100 protoplanetary discs, spanning fluxes of about 4-400 mJy at 225 GHz. The survey investigates substructures as a function of SED Class and disc mass using ALMA Band 8 continuum observations (410 GHz, 0.7 mm). It extends to faint discs containing as little as about 2 Earth masses of dust. Sources with flux >= 20 mJy were observed at about 20 au resolution, while fainter sources were observed at three times higher resolution. The Frankenstein code was used to fit non-parametric models to the visibilities, achieving sub-beam resolution. Substructures are classified into an evolutionary sequence linking morphology with stages of giant planet formation, from featureless discs (Stage 0) to inflection-point discs, gap-ring systems, and discs with central cavities. Despite higher optical depths, Band 8 efficiently traces substructures and recovers gaps and cavities seen at longer wavelengths with shorter integration times. Discs with dust masses above about 10 Earth masses show structures consistent with this sequence, even at modest resolution. The fraction of evolved substructures increases from 23 percent in Class I sources to at least 50 percent in Class II objects. Lower-mass discs rarely show such features, likely due to the steep flux-size relation and limited resolution. These results support a link between substructures in discs above about 10 Earth masses and giant planet formation, and highlight Band 8 as a powerful probe of disc substructures. This research provides valuable data for understanding the processes involved in planet formation and the evolution of protoplanetary discs.

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