Main physical parameters corresponding to the HCN gas observed in M31-I(2) and M31-I-B(1). The optical depths are shown as function of the HCN column density (per km s^-1) and the molecular hydrogen density n_H2. The corresponding contours for these two clumps are shown with a dashed line for M31-I-B(1) and with a full line for M31-I(2). The vertical (green) line indicates the critical density computed for a collisional temperature of 20 K (see Table 3). The lower horizontal full (dashed) line shows the column density measured for M31-I(2) (M31-I-B(1)) at 9 K. The upper horizontal lines correspond to the column densities computed with standard abundances (Bergin et al. 1995) and the corresponding molecular hydrogen column densities. For M31-I-B(1), we derive an upper limit as the molecular hydrogen column density is only available for a smaller beam (see Table A.1) and we expect some dilution of the signal. The contour levels correspond to excitation temperatures of 9 K (full line), 15 K (dashed line), 20 K (dotted line), and 25 K (dash-dotted line).