Top panel: 1.3 mm ALMA continuum emission map of the HMC G31. The contours are 5, 10, 15, 20, 40, 80, 160, and 300 times σ, where 1σ is 1.2 mJy beam⁻¹. The white dots mark the position of four embedded continuum sources observed at 1.4 mm and 3.5 mm (Beltrán et al., in prep.). The red cross indicates the position of the UC H II region imaged by Cesaroni et al. (1994). The synthesized beam is shown in the lower left-hand corner. Bottom panel: magnetic field segments (red lines), obtained by rotating 90° the polarization segments, overlapped on the 1.3 mm continuum emission map (contours). The segments are sampled following Nyquist sampling (every five pixels).