2026ApJ..1000...35J

GY 91, commonly categorized as a Class I young stellar object, is notable for disk dust substructures that have been hypothesized to trace early planet formation. Using the Atacama Large Millimeter/submillimeter Array 12 m and Atacama Compact Array, we present new Band 7 dust continuum and molecular line observations of GY 91 at an angular resolution of $\sim 0\mathop{.}\limits^{^{\prime\prime} }3$ (40 au). We report detections of CS J = 6-5, N₂H⁺J = 3-2, C¹⁸O J = 3-2, H₂CS ${J}_{{K}_{a},{K}_{c}}={8}_{1,7}-{7}_{1,6}$, H₂CO ${J}_{{K}_{a},{K}_{c}}={4}_{0,4}-{3}_{0,3}$, and H₂CO ${J}_{{K}_{a},{K}_{c}}={4}_{2,3}-{3}_{2,2}$, as well as a tentative detection of ¹³C¹⁸O J = 3-2. We observe azimuthal asymmetry in CS and H₂CS emission, as well as radially structured H₂CO 4_0,4-3_0,3 emission outside the dust continuum. C¹⁸O and H₂CO 4_0,4-3_0,3 show significant cloud contamination, while CS and N₂H⁺ are good tracers of Keplerian rotation originating from the disk. Envelope emission does not appear to contribute significantly either to the continuum or molecular line observations. GY 91's chemical properties appear in large part to resemble those of Class II disks, although observations of additional molecular probes should be obtained for a fuller comparison. With CS, we estimated a dynamical stellar mass of 0.58 M_⊙, which is higher than previous estimates from stellar evolutionary models (0.25 M_⊙). Using both radiative transfer modeling of the dust continuum and comparison of the C¹⁸O and N₂H⁺ fluxes to literature thermochemical models, we estimate a disk mass of ∼0.01 M_⊙.