The protoplanetary disk population in the ρ-Ophiuchi region L1688 and the time evolution of Class II YSOs

Context. Planets form during the first few Myr of the evolution of the star-disk system, possibly before the end of the embedded phase. The properties of very young disks and their subsequent evolution reflect the presence and properties of their planetary content. 
Aims. We present a study of the Class II/F disk population in L1688, the densest and youngest region of star formation in Ophiuchus. We also compare it to other well-known nearby regions of different ages, namely Lupus, Chamaeleon I, Corona Australis, Taurus and Upper Scorpius. 
Methods. We selected our L1688 sample using a combination of criteria (available ALMA data, Gaia membership, and optical and near-IR spectroscopy) to determine the stellar and disk properties, specifically stellar mass (M_⋆), average population age, mass accretion rate (Ṁ_acc) and disk dust mass (Ṁ_dust). We applied the same procedure in a consistent manner to the other regions. Results. In L1688 the relations between Ṁ_acc and M⋆, M_dust and M_⋆, and Ṁ_acc and M_dust have a roughly linear trend with slopes 1.8–1.9 for the first two relations and ~1 for the third, which is similar to what found in the other regions. When ordered according to the characteristic age of each region, which ranging from ~ 0.5 to ~5 Myr, Ṁ_acc decreases as t⁻¹, when corrected for the different stellar mass content; M_dust follows roughly the same trend, ranging between 0.5 and 5 Myr, but has an increase of a factor of ~3 at ages of 2–3 Myr. We suggest that this could result from an earlier planet formation, followed by collisional fragmentation that temporarily replenishes the millimeter-size grain population. The dispersion of Ṁ_acc and M_dust around the best-fitting relation with M⋆, as well as that of Ṁ_acc versus M_dust are equally large. When adding all the regions together to increase the statistical significance, we find that the dispersions have continuous distributions with a log-normal shape and similar widths (~0.8 dex). 
Conclusions. This detailed study of L1688 confirms the general picture of Class II/F disk properties and extends it to a younger age. The amount of dust observed at ~1 Myr is not sufficient to assemble the majority of planetary systems, which suggests an earlier formation process for planetary cores. The dust mass traces to a large extent the disk gas mass evolution, even if the ratio M_dust/M_disk at the earliest age (0.5–1 Myr) is not known. Two properties are still not understood: the steep dependence of Ṁ_acc and M_dust on M_⋆ and the cause of the large dispersion in the three relations analyzed in this paper, in particular that of the Ṁ_acc versus M_dust relation.