NIKA2 observations around LBV stars: emission from stars and
  circumstellar material
  Luminous Blue Variable (LBV) stars are evolved massive objects, previous to
core-collapse supernova. LBVs are characterized by photometric and
spectroscopic variability, produced by strong and dense winds, mass-loss events
and very intense UV radiation. LBVs strongly disturb their surroundings by
heating and shocking, and produce important amounts of dust. The study of the
circumstellar material is therefore crucial to understand how these massive
stars evolve, and also to characterize their effects onto the interstellar
medium. The versatility of NIKA2 is a key in providing simultaneous
observations of both the stellar continuum and the extended, circumstellar
contribution. The NIKA2 frequencies (150 and 260 GHz) are in the range where
thermal dust and free-free emission compete, and hence NIKA2 has the capacity
to provide key information about the spatial distribution of circumstellar
ionized gas, warm dust and nearby dark clouds; non-thermal emission is also
possible even at these high frequencies. We show the results of the first NIKA2
survey towards five LBVs. We detected emission from four stars, three of them
immersed in tenuous circumstellar material. The spectral indices show a complex
distribution and allowed us to separate and characterize different components.
We also found nearby dark clouds, with spectral indices typical of thermal
emission from dust. Spectral indices of the detected stars are negative and
hard to be explained only by free-free processes. In one of the sources,
G79.29+0.46, we also found a strong correlation of the 1mm and 2mm continuum
emission with respect to nested molecular shells at 0.1 pc from the LBV. The
spectral index in this region clearly separates four components: the LBV star,
a bubble characterized by free-free emission, and a shell interacting with a
nearby infrared dark cloud.
