Mass of the HMC G31.41+0.31 as a function of r_i. The input parameters are d = 7.9 kpc, R_o = 1.′′076, q = −0.77, p = −2, κ(217GHz) = 0.8, , S_{217 GHz} = 3.1 Jy. The black lines are for q = −0.77 and p = −2, whereas the red lines correspond to q = 0 and p = 0. Both dashed lines are obtained in the optically thin limit, while the black dashed line also assumes the RJ approximation. The blue dot corresponds to the value of the mass computed by BEL18. The dotted vertical line marks a plausible upper limit for r_i (see text).