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tokens: this exercise is not to make a complete analysis of the origin of the spiral arm that we detect with our observations, but to test the viability of a scenario. We investigated the effect of each of the aforementioned parameters, and inspected the results visually. Figure 8 shows the time evolution for a given simulation, over a span of several hundreds years. For this example, we took t 0 = 2 yr and δt = 0.3 yr. Shortly after t = t 0 a spiral, devoid of particles, appears in the disk (where the particles originally were), but its width decreases for each orbit owing to Keplerian shear. On the other hand, the grains that experienced the wind are set on an elliptical orbit once the wind has stopped (and provided that β wind is not too strong), with their apocentre farther than the original radius of the disk. All these particles tend to travel all-together, therefore, a spiral arm appears in the image at two distinct moments per orbit: when the group of particles is entering or leaving the birth ring (middle and rightmost panels of Fig. 8 ). In our simulations, given that collisions are not taken into account, this phenomenon will persist over time. Nonetheless, if the optical depth is low enough, the train of particles may not collide too often with other particles of the birth ring, and thus the spiral could be long-lived. A movie of the simulation can be found online 4 . Fig. 8 N-body simulation of the effect of anisotropic stellar winds on an ensemble of massless particles, creating a spiral arm in the disk. Nonetheless, the δt timescale is far too long compared to the stellar rotation period for the CME to stay in one preferential direction (0.3 yr compared to ~5 days; Watson et al. 2006 ). The stellar wind would most likely sweep across the entire disk multiple times. If we require δt to be much lower than a typical stellar rotation period, or of the order of the typical duration of the flaring event reported by Uzawa et al. (2011) , then the corresponding β wind value becomes much higher. As an example, for δt = 0.5 days, we find that a similar spiral arm requires β wind = 7500, a value that seems unrealistically high. Indeed, β wind is directly proportional to stellar mass loss rate and inversely proportional to grain sizes following the relation ( Augereau Beust 2006 ): with C D a factor close to 2, Ṁ ⋆ the stellar mass loss rate, v sw the speed of the stellar wind, G the graviational constant, and ρ the dust density. Even though TWA 7 is younger than AU Mic, and therefore potentially more active, a β wind = 7500 value seems rather improbable, as it would require extremely small dust grains (which would not scatter the stellar light efficiently in H -band) or an extremely large mass loss rate. For AU Mic, Augereau Beust (2006) found Ṁ ⋆ ~ 47 Ṁ ⊙ in quiescent state, ~ 2450 Ṁ ⊙ during flares (leading to β wind ~ 40−50), which averaged to ~ 300 Ṁ ⊙ (assuming flares occur 10% of the time). This means that the flares of TWA 7 should be