Kippenhahn diagram during the red giant phase. The central radiative zone and the convective envelope are represented by a white area and an area with black dots, respectively. The horizontal axis is the age of the star minus its main-sequence lifetime (t_MS). The four lines indicate the limit layers that can be reached by the planets of mass 1 M_J (red solid line), 5 M_J (blue dashed line), 10 M_J (green dash-dotted line), and 15 M_J (continuous cyan line). The vertical dashed lines labelled A, B, and C indicate when the engulfment occurs for the 15 M_J planet at an initial distance of 0.5 au, the 10 M_J planet at an initial distance of 1.0 au, and the 1 M_J planet at an initial distance of 1.5 au, respectively. All the other cases occur between the lines A and C. The left panel corresponds to a model star of 1.5 M_⊙ and the right panel to a mass of 2 M_⊙.