Vol. 671
6. Interstellar and circumstellar matter

A plausible molecular mechanism to explain near-infrared continuum emission: Recurrent fluorescence

by O. Lacinbala, F. Calvo, E. Dartois, C. Falvo, P. Parneix, A. Simon, and T. Pino 2023, A&A, 671, A89

The authors modeled neutral carbon clusters ranging about 24 to 60 atoms, which would be expected in regions of star formation, especially in reflection nebulae. They used a variety of structures and sampled the spectra using a Monte Carlo procedure to simulate the sort of mix that might be encountered in interstellar environments. The electronic and vibrational states were strongly coupled, so interactive excitations have resulted in a broad emission spectrum of the repeated fluorescence. Irradiated by a 20,000 K blackbody spectrum, taken as a proxy representation for a star that is not too hot, the emitted spectrum from all clusters resembles a blackbody with a color temperature of about 750 K. The emission bands are shown for the 1 to 10 micron band, which will be important for JWST and ground-based spectrophotometry. They also show the computed electronic absorption spectra in the ultraviolet (UV), which will be important for further modeling of star-forming regions. To obtain the spectrum and surface brightness observed in NGC 7023 — their paradigmatic reflection nebula — requires around 0.1 to 1% of the carbon to be sequestered in neutral clusters with between 24 and 60 atoms. This connects the UV absorption, extended red emission, and the near-infrared continuum emission.