Effect of the Keplerian shifting on the line spectrum. Top row: Disk-integrated spectrum of ¹²CO (J=2–1), C₂H (3_7/2−2_5/2) and H₂CO (3_2,1 − 2_2,0), respectively, on a circular de-projected region of radius 3″ for the ¹²CO and of 2″ for the other ones. The blue line is the shifted spectrum, obtained by correcting for the Doppler shift due to the Keplerian rotation of the disk, while the gray line corresponds to the un-shifted spectrum. Bottom row: Spectra of ¹²CO (J=2–1), C₂H (3_7/2−2_5/2) and H₂CO (3_2,1−2_2,0) as a function of radius, obtained after the Keplerian shifting: the spectra are expressed in brightness temperature, using the Rayleigh-Jeans approximation. The hyperfine components of the C₂H transition are visible in the middle column.