Stellar masses and disk properties of Lupus young stellar objects traced
  by velocity-aligned stacked ALMA 13CO and C18O spectra
  In recent ALMA surveys, the gas distributions and velocity structures of most
of the protoplanetary disks can still not be imaged at high S/N due to the
short integration time. In this work, we re-analyzed the ALMA 13CO (3-2) and
C18O (3-2) data of 88 young stellar objects in Lupus with the velocity-aligned
stacking method to enhance S/N and to study the kinematics and disk properties
traced by molecular lines. This method aligns spectra at different positions in
a disk based on the projected Keplerian velocities at their positions and then
stacks them. This method enhances the S/N ratios of molecular-line data and
allows us to obtain better detections and to constrain dynamical stellar masses
and disk orientations. We obtain 13CO detections in 41 disks and C18O
detections in 18 disks with 11 new detections in 13CO and 9 new detections in
C18O after applying the method. We estimate the disk orientations and the
dynamical stellar masses from the 13CO data. Our estimated dynamical stellar
masses correlate with the spectroscopic stellar masses, and in a subsample of
16 sources, where the inclination angles are better constrained, the two masses
are in a good agreement within the uncertainties and with a mean difference of
0.15 Msun. With more detections of fainter disks, our results show that high
gas masses derived from the 13CO and C18O lines tend to be associated with high
dust masses estimated from the continuum emission. Nevertheless, the scatter is
large (0.9 dex), implying large uncertainties in deriving the disk gas mass
from the line fluxes. We find that with such large uncertainties it is expected
that there is no correlation between the disk gas mass and the mass accretion
rate with the current data. Deeper observations to detect disks with gas masses
<1E-5 Msun in molecular lines are needed to investigate the correlation between
the disk gas mass and the mass accretion rate.
