  Context. Dust polarization observations of the massive protocluster
G31.41+0.31 carried out at ~1'' (~3750 au) resolution with the SMA at 870
microm have revealed one of the clearest examples to date of an
hourglass-shaped magnetic field morphology in the high-massregime.
Additionally, ~0.24'' (~900 au) resolution observations with ALMA at 1.3 mm
have confirmed these results. The next step is to investigate whether the
magnetic field maintains its hourglass-shaped morphology down to circumstellar
scales. Aims. To study the magnetic field morphology toward the four
(proto)stars A, B, C, and D contained in G31.41+0.31 and examine whether the
self-similarity observed at core scales (1'' and 0.24'' resolution) still holds
at circumstellar scales, we carried out ALMA observations of the polarized dust
continuum emission at 1.3 mm and 3.1 mm at an angular resolution of ~0.068''
(~250 au), sufficient to resolve the envelope emission of the embedded
protostars. Methods. We used ALMA to perform full polarization observations at
233 GHz (Band 6) and 97.5 GHz (Band 3) with a synthesized beam of 0.072'' x
0.064''. We carried out polarization observations at two different wavelengths
to confirm that the polarization traces magnetically aligned dust grains and is
not due to dust self-scattering. Results. The polarized emission and the
direction of the magnetic field obtained at the two wavelengths are basically
the same, except for an area between the embedded sources C and B. In such an
area, the emission at 1.3 mm could be optically thick and affected by dichroic
extinction. ...
