Photocenter analysis of the Hα decretion disk around β CMi. (a) 2D photocenter positions for each spectral channel. The colors indicate the Doppler velocities, and the gray ellipses show the 1σ uncertainties. The green line denotes the derived disk position angle, 126°. (b) Hα spectrum of β CMi (sum of all the 38 ports), showing a double-peaked line profile. The solid and dotted lines indicate the best-fit Keplerian velocity disk models with asymmetric and symmetric power-law intensity distributions. (c) and (d) Photocenter positions for each spectral channel, projected along the disk’s major and minor axes, respectively. The photocenter measurements from two separate observations (circular and triangular markers) show strong agreement, demonstrating the repeatability of the measurement. The photocenter shift along the major axis reveals disk rotation consistent with a Keplerian velocity field. Interestingly, at low velocities, a shift along the minor axis is observed, indicating nearside and farside asymmetry of the disk. (e) Best-fit intensity distribution of the asymmetric Keplerian model. (f) Best-fit velocity distribution of the asymmetric Keplerian model.