  We introduce a new stacking method in Keplerian disks that (1) enhances
signal-to-noise ratios (S/N) of detected molecular lines and (2) that makes
visible otherwise undetectable weak lines. Our technique takes advantage of the
Keplerian rotational velocity pattern. It aligns spectra according to their
different centroid velocities at their different positions in a disk and stacks
them. After aligning, the signals are accumulated in a narrower velocity range
as compared to the original line width without alignment. Moreover, originally
correlated noise becomes de-correlated. Stacked and aligned spectra, thus, have
a higher S/N. We apply our method to ALMA archival data of DCN (3-2), DCO+
(3-2), N2D+ (3-2), and H2CO (3_0,3-2_0,2), (3_2,2-2_2,1), and (3_2,1-2_2,0) in
the protoplanetary disk around HD 163296. As a result, (1) the S/N of the
originally detected DCN (3-2), DCO+ (3-2), and H2CO (3_0,3-2_0,2) and N2D+
(3-2) lines are boosted by a factor of >4-5 at their spectral peaks, implying
one order of magnitude shorter integration times to reach the original S/N; and
(2) the previously undetectable spectra of the H2CO (3_2,2-2_2,1) and
(3_2,1-2_2,0) lines are materialized at more than 3 sigma. These dramatically
enhanced S/N allow us to measure intensity distributions in all lines with high
significance. The principle of our method can not only be applied to Keplerian
disks but also to any systems with ordered kinematic patterns.
