2015A&A...583A..98V

We re-examined photometry (VBLUW, UBV, uvby) of the yellow hypergiant HR5171A made a few decades ago. In that study no proper explanation could be given for the enigmatic brightness excesses in the L band (VBLUW system, λ_eff=3838Å). In the present paper, we suggest that this might have been caused by blue luminescence (BL), an emission feature of neutral polycyclic aromatic hydrocarbon molecules (PAHs), discovered in 2004. It is a fact that the highest emission peaks of the BL lie in the L band. Our goals were to investigate other possible causes, and to derive the fluxes of the emission. We used two-colour diagrams based on atmosphere models, spectral energy distributions, and different extinctions and extinction laws, depending on the location of the supposed BL source: either in Gum48d on the background or in the envelope of HR5171A. False L-excess sources, such as a hot companion, a nearby star, or some instrumental effect, could be excluded. Also, emission features from a hot chromosphere are not plausible. The fluxes of the L excess, recorded in the data sets of 1971, 1973, and 1977 varied (all in units of 10^–10W/m2/µm) between 1.4 to 21, depending on the location of the source. A flux near the low side of this range is preferred. Small brightness excesses in uv (uvby system) were present in 1979, but its connection with BL is doubtful. For the L fluxes we consider the lowest values as more realistic. The uncertainties are 20-30%. Similar to other yellow hypergiants, HR5171A showed powerful brightness outbursts, particularly in the 1970s. A release of stored H-ionization energy by atmospheric instabilities could create BL emitted by neutral PAHs.