Chemical segregation in L1157-B1. The maps are centred at , δ(J2000) = +68° 01′ 100. Left panel: p-H₂CO (2_0,2–1_0,1) integrated emission (grey; Benedettini et al. 2013), and SiO (2–1) (magenta) at low velocity (less than –10 km s^-1 with respect to v_sys; Gueth et al. 1998) on top of the present NH₂CHO line emission map (colour image, black contours). For clarity, for the H₂CO and SiO images, only the 3σ (1σ = 3.3 mJy beam^-1 km s^-1 and 50 mJy beam^-1 km s^-1 for H₂CO and SiO, respectively) contour is reported to show the overall B1 structure (see Fig. A.2 for the complete set of contours). The northern triangle at B1a identifies the youngest position in B1, where the precessing jet driven by L1157-mm impacts the cavity wall (Gueth et al. 1998; Busquet et al. 2014), while the southern triangle denotes the position of the oldest shock, which is identified by the so-called “finger” feature traced by SiO at low velocity. The H₂CO image is smoothed to the same angular resolution (red ellipse; see Fig. 1) of the NH₂CHO (4_1,4–3_1,3) line data. The synthesised beam of the SiO is 28 × 22, PA = 56°. Middle panel: same as in the left panel for the CH₃CHO (7_0,7–6_0,6 E+A) velocity-integrated emission (cyan contours; also smoothed to the same beam of the NH₂CHO map; first contour and step correspond to 3σ, 4 mJy beam^-1 km s^-1, and 1σ, respectively; Codella et al. 2015). Right panel: sketch of the three zones identified from the spatial distribution of formamide and acetaldehyde: SHOCK 1 (blue), the northern region, where CH₃CHO (and not NH₂CHO) is detected; SHOCK 2 (green), where both CH₃CHO and NH₂CHO are detected; and SHOCK 3 (red), the southern region, where only NH₂CHO (and not CH₃CHO) is detected. Time increases and chemistry evolves going from SHOCK 1 to SHOCK 3 (see dashed arrows in Fig. 1).