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In recent ALMA surveys, the gas distributions and velocity structures of mostof the protoplanetary disks can still not be imaged at high S/N due to theshort integration time. In this work, we re-analyzed the ALMA 13CO (3-2) andC18O (3-2) data of 88 young stellar objects in Lupus with the velocity-alignedstacking method to enhance S/N and to study the kinematics and disk propertiestraced by molecular lines. This method aligns spectra at different positions ina disk based on the projected Keplerian velocities at their positions and thenstacks them. This method enhances the S/N ratios of molecular-line data andallows us to obtain better detections and to constrain dynamical stellar massesand disk orientations. We obtain 13CO detections in 41 disks and C18Odetections in 18 disks with 11 new detections in 13CO and 9 new detections inC18O after applying the method. We estimate the disk orientations and thedynamical stellar masses from the 13CO data. Our estimated dynamical stellarmasses correlate with the spectroscopic stellar masses, and in a subsample of16 sources, where the inclination angles are better constrained, the two massesare in a good agreement within the uncertainties and with a mean difference of0.15 Msun. With more detections of fainter disks, our results show that highgas masses derived from the 13CO and C18O lines tend to be associated with highdust masses estimated from the continuum emission. Nevertheless, the scatter islarge (0.9 dex), implying large uncertainties in deriving the disk gas massfrom the line fluxes. We find that with such large uncertainties it is expectedthat there is no correlation between the disk gas mass and the mass accretionrate with the current data. Deeper observations to detect disks with gas masses<1E-5 Msun in molecular lines are needed to investigate the correlation betweenthe disk gas mass and the mass accretion rate.

Stellar masses and disk properties of Lupus young stellar objects traced by velocity-aligned stacked ALMA 13CO and C18O spectra