2017MNRAS.468.2416M

A combined mid-IR spectrum of five colliding-wind, massive, dust-producing Population I Wolf-Rayet (WR) binaries shows a wealth of absorption and emission details coming from the circumstellar dust envelopes, as well as from the interstellar medium. The prominent absorption features may arise from a mix of interstellar carbonaceous grains formed in high- (e.g. 3.4, 6.8, 7.2 µm ) and low-temperature (e.g. 3.3, 6.9, 9.3 µm) environments. The broad emission complexes around ∼6.5, 8.0 and 8.8 µm could arise from ionized, small polycyclic aromatic hydrocarbon (PAH) clusters and/or amorphous carbonaceous grains. As such, these PAH emissions may represent the long sought-after precursors of amorphous carbon dust. We also detect a strong ∼10.0 µm emission in the spectra of WR48a and WR112 which we tentatively link to ionized PAHs. Upon examining the available archival spectra of prodigious individual WR dust sources, we notice a surprising lack of 7.7 µm PAH band in the spectrum of the binary WR19, in contrast to the apparent strength of the 11.2, 12.7 and 16.4 µm PAH features. Strong PAH emissions are also detected in the λ >10 µm spectrum of another dust-producing system, WR118, pointing to the presence of large, neutral, presumably interstellar PAH molecules towards WR19 and WR118.