Main physical parameters corresponding to the HCO+(1–0) gas observed in M31-I(2) and M31-I-B(1). The optical depths are shown as function of the HCO+(1–0) column density (per km s^-1) and the molecular hydrogen density n_H2. 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 column densities measured for M31-I(2) (M31-I-B(1)) at 8 K. The upper horizontal line corresponds 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, as is the case for M31-I(2). The contour levels correspond to excitation temperatures of 9 K (full line), 10 K (dashed line), 15 K (dotted line), and 20 K (dash-dotted line).