L1157 southern blueshifted lobe in CO (1–0) (white contours; Gueth et al. 1996). The precessing jet ejected by the central object L1157-mm (outside the frame and toward the northwest) excavated several clumpy cavities, named B0, B1, and B2, respectively. The maps are centred at α(J2000) = 20^h39^m10^s.2, δ(J2000) = + 68°01′10′′.5 (Δα = +25′′ and Δδ = −63′′.5 from the L1157-mm protostar). Top panel: map (in colour scale) of the emission due to the CH₃ OH(2_1,2–1_1,1) A transition (integrated over the whole velocity range). The first contour and step are 3σ (32 mJy beam⁻¹ km s⁻¹). For the CO image, the first contour and step are 6σ (1σ = 0.5 Jy beam⁻¹ km s⁻¹) and 4σ, respectively. The dashed circle shows the primary beam of the CH₃OH image (64′′). Yellow labels are for the B0 and B1 clumps (black triangles) previously identified using molecular tracers (see text, and e.g. Benedettini et al. 2007; Codella et al. 2009, 2017). The white and magenta ellipses depict the synthesised beams of the CO (3′′.65×2′′.96, PA = +88°), and CH₃ OH (2′′.97×2′′.26, PA = –155°) observations, respectively. Bottom panel: same as top panel but for CH₃CHO (5_0,4 –4_0,4) E. The first contour and step are 3σ (7 mJy beam⁻¹ km s⁻¹). The synthesised beams of the CH₃CHO image are similar to those of the CH₃OH map.