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Previous studies found that stellar scattering by massive clumps can lead tothe formation of exponential profiles in galaxy discs, but details on how astar is moved around have not been fully explained. We use a GADGET-2simulation where an exponential profile forms from an initially Gaussian discin about 4 Gyr for a low-mass galaxy like a dwarf irregular. We find thatnearly all large angular momentum changes of stars are caused by star-clumpencounters with the closest approach less than 0.5 kpc. During star-clumpencounters, stars may increase their random motions, resulting in an increasein the average radial and vertical actions of the whole stellar population. Theangular momentum change and the radial action change of an individual star areinfluenced by the direction from which the star approaches a clump. A starinitially at a higher galactic radius relative to the scattering clump usuallygets pulled inwards and loses its angular momentum during the encounter, andone at a lower radius tends to shift outwards and gains angular momentum. Theincrease in the radial action is the largest if a star encounters a clump fromthe azimuthal direction, and is the smallest from a radial approach. Theangular momentum change due to encounters has an inward bias when the clumpprofile has a steep radial decline, and a shallow decline can make the biasoutwards. The stellar profile evolution towards an exponential seems to occurregardless of the direction of the bias.

Orbits and action changes during star-clump encounters responsible for the origin of exponential discs in dwarf galaxies