Panel a: m versus the total flux density of the clump. The red curves correspond to the case q = 0 and p = 0, while the black curves are for models allowing for temperature and density gradients. The dashed black curve has been obtained under the optically thin and RJ approximations from Eq. (11), whereas the dashed red curve is computed in the optically thin limit from Eq. (1). Panel b: mass ratios between all the curves in the top panel and the black solid curve.