for maps larger than 300$\arcsec$ and 45$\arcsec$ for smaller maps. The (FWHM) beam size of the telescope was measured to be approximately 11$\arcsec$-12$\arcsec$ on maps of Uranus. The zenith atmospheric optical depth, monitored by ‘skydips’ every 1-2 hours, was between $\sim$ 0.15 and $\sim$ 0.5. The final co-added maps are about 7$\arcmin \times 7\arcmin$ in size and consist of $\sim$ 5–6 individual coverages taken at different hour angles to improve the reconstruction technique. The data were reduced with the IRAM software for bolometer arrays (“NIC”; cf. Broguière, Neri & Sievers 1995), which uses the EKH restoration algorithm (Emerson, Klein, & Haslam 1979). The typical rms noise was $\sim$ 8 mJy/11$\arcsec$beam.

The same dense cores listed in Table \[mm\] were also observed in the C$^{18}$O(1-0) molecular line at the IRAM 30 m telescope, in order to obtain estimates of the column density in the outer parts of the cores. We used the position switching mode along cuts of about 10$\arcmin$ length sampled every 20$\arcsec$ through each of the cores, and oriented along and perpendicular to the axis of the core. The OFF position, which was checked to be free of emission, was typically taken between 15$\arcmin$ and 30$\arcmin$ away from the 