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We introduce a new stacking method in Keplerian disks that (1) enhancessignal-to-noise ratios (S/N) of detected molecular lines and (2) that makesvisible otherwise undetectable weak lines. Our technique takes advantage of theKeplerian rotational velocity pattern. It aligns spectra according to theirdifferent centroid velocities at their different positions in a disk and stacksthem. After aligning, the signals are accumulated in a narrower velocity rangeas compared to the original line width without alignment. Moreover, originallycorrelated noise becomes de-correlated. Stacked and aligned spectra, thus, havea 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) inthe protoplanetary disk around HD 163296. As a result, (1) the S/N of theoriginally 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, implyingone 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 dramaticallyenhanced S/N allow us to measure intensity distributions in all lines with highsignificance. The principle of our method can not only be applied to Kepleriandisks but also to any systems with ordered kinematic patterns.

Stacking Spectra in Protoplanetary Disks: Detecting Intensity Profiles from Hidden Molecular Lines in HD 163296