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Although gravitational collapse is supposed to play an essential role in thestar formation process, infall motions have been always elusive to detect. Sofar, only a few observational signatures have been commonly used to claim forthe presence of infall. Often these features consist in either"blue-asymmetries" or absorption at red-shifted velocities (e.g., inverseP-Cygni profiles). Both signatures are based only on the shape of the lineprofile and they do not guarantee by themselves the presence of dominant infallmotions. More robust "mapping signatures" can be obtained from images thatangularly resolve the infalling gas. Here we present VLA observations of theammonia inversion transitions (2,2), (3,3), (4,4), (5,5), and (6,6) towards thehot molecular core (HMC) near G31.41+0.31 that show the signatures ofprotostellar infall theoretically predicted by Anglada et al. (1991). Theintensity of the ammonia emission is compact and sharply increases towards thecentre in the blue-shifted velocity channel maps, while it shows a moreflattened distribution in the red-shifted velocity channels. Additionally, theemission becomes more compact with increasing (relative) velocity for both redand blue-shifted channels. We introduce a new infall signature, the "centralblue spot", easily identifiable in the first-order moment maps. We show thatrotation produces an additional, independent signature, making the distributionof the emission in the channel maps asymmetric with respect to the centralposition, but without masking the infall signatures. All these mappingsignatures, which are identified here for the first time, are present in theobserved ammonia transitions of G31 HMC.

Signatures of infall motions in the images of the molecular emission of G31 HMC