Results of determining the SO line through the same procedure using different samples and the results of fitting the SO line using polynomial functions from 2nd to 5th degrees. Panel (a) presents the results of determining the SO line using sample 1 with polynomial functions from 2nd to 5th degrees. The symbols and line styles have the same meanings as those in Fig. 3. The large blue asterisks represent the central points of the two contours. The dashed black curve represents the 2nd-degree fit. The solid blue curve represents the 3rd-degree fit. The dashed purple curve represents the 4th-degree fit. The dashed red curve represents the 5th-degree fit. Panels (b), (c), (d), (e), and (f) present the results of determining the SO line using samples 2–6 with a 3rd-degree fit, respectively. The symbols and line styles have the same meanings as those in panel (a).