2018A&A...620A..32S

We present results of the analysis of a sample of 22 stars of spectral types from O7 to B5 and luminosity classes I-V for which spectra from the Infrared Spectrograph (IRS) of Spitzer are available. The IRS spectra of these stars are examined for signs of excess infrared (IR) emission by comparison with stellar atmospheric spectra. We find that the spectra of half of the studied stars are dominated by excess emission in the far-IR, including all six super- and bright giants. In order to examine the origin of the far-IR excess, we supplement the Spitzer data with optical high-resolution echelle spectroscopy (λ/Δλ∼10⁵), near-IR high-contrast coronagraphic imaging taken with the SPHERE instrument at VLT with a spatial resolution of 0.05", and WISE and Herschel photometry. In the optical region, we detect various absorption and emission lines (H α, C III, and N III) irrespective of the far-IR excess. Pfund α and Humphrey α lines are observed at the same time as the far-IR excess. These lines are stronger in stars with far-IR excess than in stars without excess. A scattered-light disk in the central r≤2.5'' region of the far-IR excess stars HD 149404, HD 151804, and HD 154368 can be excluded from H band imaging down to a 1σ contrast of F(r)/F_*∼10^–6. The far-IR excess is fit either by a free-free component from ionized gas as for the winds of hot stars or a large (1pc) circumstellar dust shell. The putative dust envelopes required to explain the excess have a visual extinction as low as a few hundred µ-mag.