Millimetre continuum from luminous blue variable stars and their environs. NIKA2 observations and Virtual Observatory data

Luminous blue variables (LBVs) represent a brief transitional phase in the evolution of massive stars. Multi-wavelength studies of their circumstellar environments are essential to quantify their feedback at Galactic scales. Dominant emission mechanisms at millimetre wavelengths are, however, still poorly understood. Stellar winds, circumstellar dust, and ionised gas have not been explored together in the case of LBVs. We aim to study the millimetre continuum emission of Galactic LBVs to disclose the presence of these components, to describe their morphology, and to measure their relevance in the mass and energy injection to the interstellar medium. We used the NIKA2 continuum camera at the IRAM 30 m radio telescope to observe and analyse 1.15 and 2 mm continuum from the LBVs HD168607 HD168625 GKF2010 MN87 GKF2010 MN101 and G79.29+0.46 . We used the Virtual Observatory to complement our observations with archival data from optical, infrared, millimetre, and centimetre wavelengths. With this information, we built complete spectral energy distributions (SEDs) for the five sources that cover six decades of the electromagnetic spectrum. All targets except MN87 were detected at both wavelengths, with features including compact sources, extended nebular emission, shells, and unrelated background structures. The spectral indices of compact sources are consistent with thermal emission from stellar winds. We modelled the SEDs and successfully reproduced the emission from stellar photospheres, circumstellar dust, thermal stellar winds, and enshrouding Hii regions. Our models, in agreement with previous literature results, reveal unresolved hot dust very close to the stars and provide the first estimates for the fundamental parameters of MN101. This pilot study highlights the great potential of millimetre continuum studies of LBVs and possibly other evolved massive stars. The millimetre spectral window bridges the far-IR and radio regimes and can disclose the relative contribution of dust and free-free emission in this type of source.