CO J = 2 → 1 spectra obtained with the IRAM-30 m telescope. Fluxes are in Jy with respect to local standard of rest (LSR) velocity. The last box shows the cumulated spectrum made with spectra shifted in velocity to set the systemic velocity to zero (note that the flux scale is different from the individual spectra). The sources with a high rms or showing strong contamination (i.e., HD 15115, HD 21620, HD 42111 and HD 218396) are not used to produce the cumulated spectrum. The 3σ upper limit model for each spectrum is over-plotted in red (see sect. 3.3) except for the HD 23642 spectrum, where the best-fit model for the tentative detection is plotted in blue.