Fullerenes in the circumstellar medium of Herbig Ae/Be stars: insights from the Spitzer mid-infrared spectral catalog

This study presents the largest mid-infrared spectral catalogue of Herbig Ae/Be stars to date, containing the Spitzer Infrared Spectrograph spectra of 126 stars. Based on the catalogue analysis, two prominent infrared vibrational modes of C-60 bands at 17.4 and 18.9 mu m are detected in the spectra of nine sources, while 7.0 mu m feature is identified in the spectra of HD 319896. The spectral index analysis and the comparison of the known sources with C-60 features indicated that there exist two different types of emission classes among the sample of stars. The infrared spectra of six Herbig Ae/Be stars in this study resemble that of reflection nebulae, and their association with previously known reflection nebulae is confirmed. In the case of three Herbig Ae/Be stars, we report the tentative evidence of C-60 emission features originating from the circumstellar disc or nearby diffused emission region. The detection fraction of C-60 in the total HAeBe star sample is similar to 7 per cent, whereas the detection fraction is 30 per cent for HAeBe stars associated with nebulosity. In the catalog, C-60 is exclusively present in the circumstellar regions of B type Herbig Ae/Be stars, with no evidence of its presence detected in stars with later spectral types. The present study has increased the number of young stellar objects and reflection nebulae detected with C-60 multifold, which can help in understanding the excitation and formation pathway of the species.