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We present the first detection of¹³CCH in a protoplanetary disk (TW Hya). Using observations of C₂H, we measure CCH/¹³CCH = 65 ± 20 gas with a CO isotopic ratio of¹²CO/¹³CO = 21 ± 5. The TW Hya disk exhibits a gas phase C/O that exceeds unity, and C₂H is the tracer of this excess carbon. We confirm that the TW Hya gaseous disk exhibits two separate carbon isotopic reservoirs, as noted previously. We explore two theoretical solutions for the development of this dichotomy. One model represents TW Hya today with a protoplanetary disk exposed to a cosmic-ray ionization rate that is below interstellar as consistent with current estimates. We find that this model does not have sufficient ionization in cold (T< 40 K) layers to activate carbon isotopic fractionation. The second model investigates a younger TW Hya protostellar disk exposed to an interstellar cosmic-ray ionization rate. We find that the younger model has sources of ionization deeper in a colder disk that generates two independent isotopic reservoirs. One reservoir is¹²C-enriched carried by methane/hydrocarbon ices, and the other is¹³C-enriched carried by gaseous CO. The former potentially provides a source of methane/hydrocarbon ices to power the chemistry that generates the anomalously strong C₂H emission in this (and other) disk systems in later stages. The latter provides a source of gaseous¹³C-rich material to generate isotopic enrichments in forming giant planets, as recently detected in the super-Jupiter TYC 8998-760-1 b by Zhang et al.

Molecular emission is used to investigate both the physical and chemical properties of protoplanetary disks. Therefore, to derive disk properties accurately, we need a thorough understanding of the behavior of the molecular probes upon which we rely. Here we investigate how the molecular line emission of N₂H⁺, HCO⁺, HCN, and C¹⁸O compare to other measured quantities in a set of 20 protoplanetary disks. Overall, we find positive correlations between multiple line fluxes and the disk dust mass and radius. We also generally find strong positive correlations between the line fluxes of different molecular species. However, some disks do show noticeable differences in the relative fluxes of N₂H⁺, HCO⁺, HCN, and C¹⁸O. These differences occur even within a single star-forming region. This results in a potentially large range of different disk masses and chemical compositions for systems of similar age and birth environment. While we make preliminary comparisons of molecular fluxes across different star-forming regions, more complete and uniform samples are needed in the future to search for trends with birth environment or age.

It remains unclear what mechanism is driving the evolution of protoplanetary disks. Direct detection of the main candidates, either turbulence driven by magnetorotational instabilities or magnetohydrodynamical disk winds, has proven difficult, leaving the time evolution of the disk size as one of the most promising observables able to differentiate between these two mechanisms. But to do so successfully, we need to understand what the observed gas disk size actually traces. We studied the relation betweenR_CO,90%, the radius that encloses 90% of the¹²CO flux, andR_c, the radius that encodes the physical disk size, in order to provide simple prescriptions for conversions between these two sizes. For an extensive grid of thermochemical models, we calculateR_CO,90%from synthetic observations and relate properties measured at this radius, such as the gas column density, to bulk disk properties, such asR_cand the disk massM_disk. We found an empirical correlation between the gas column density atR_CO,90%and disk mass:$N_{\mathrm{gas}}{(R_{\mathrm{CO},90\%})\approx 3.73\times {10}^{21}(M_{\mathrm{disk}}/M_{\odot })}^{0.34}{\mathrm{cm}}^{-2}$. Using this correlation we derive an analytical prescription ofR_CO,90%that only depends onR_candM_disk. We deriveR_cfor disks in Lupus, Upper Sco, Taurus, and the DSHARP sample, finding that disks in the older Upper Sco region are significantly smaller (〈R_c〉 = 4.8 au) than disks in the younger Lupus and Taurus regions (〈R_c〉 = 19.8 and 20.9 au, respectively). This temporal decrease inR_cgoes against predictions of both viscous and wind-driven evolution, but could be a sign of significant external photoevaporation truncating disks in Upper Sco.

Observations of substructure in protoplanetary disks have largely been limited to the brightest and largest disks, excluding the abundant population of compact disks, which are likely sites of planet formation. Here, we reanalyze ∼0.″1, 1.33 mm Atacama Large Millimeter/submillimeter Array (ALMA) continuum observations of 12 compact protoplanetary disks in the Taurus star-forming region. By fitting visibilities directly, we identify substructures in six of the 12 compact disks. We then compare the substructures identified in the full Taurus sample of 24 disks in single-star systems and the ALMA DSHARP survey, differentiating between compact (R_eff,90%< 50 au) and extended (R_eff,90%≥50 au) disk sources. We find that substructures are detected at nearly all radii in both small and large disks. Tentatively, we find fewer wide gaps in intermediate-sized disks withR_eff,90%between 30 and 90 au. We perform a series of planet–disk interaction simulations to constrain the sensitivity of our visibility-fitting approach. Under the assumption of planet–disk interaction, we use the gap widths and common disk parameters to calculate potential planet masses within the Taurus sample. We find that the young planet occurrence rate peaks near Neptune masses, similar to the DSHARP sample. For 0.01M_J/M_⊙≲M_p/M_*≲0.1M_J/M_⊙, the rate is 17.4% ± 8.3%; for 0.1M_J/M_⊙≲M_p/M_*≲1M_J/M_⊙, it is 27.8% ± 8.3%. Both of them are consistent with microlensing surveys. For gas giants more massive than 5M_J, the occurrence rate is 4.2% ± 4.2%, consistent with direct imaging surveys.