  The infall of material onto a protostar, in the case of optically thick line
emission, produces an asymmetry in the blue- and red-wing line emission. For an
angularly resolved emission, this translates in a blue central spot in the
first-order moment (intensity weighted velocity) map. An analytical expression
for the first-order moment intensity as a function of the projected distance
was derived, for the cases of infinite and finite infall radius. The effect of
a finite angular resolution, which requires the numerical convolution with the
beam, was also studied. This method was applied to existing data of several
star-forming regions, namely G31.41+0.31 HMC, B335, and LDN 1287, obtaining
good fits to the first-order moment intensity maps, and deriving values of the
central masses onto which the infall is taking place (G31.41+0.31 HMC: 70-120
$M_\odot$; B335: 0.1 $M_\odot$; Guitar Core of LDN 1287: 4.8 $M_\odot$). The
central-blue-spot infall hallmark appears to be a robust and reliable indicator
of infall.
