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

Abstract

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 C60 bands at 17.4 and 18.9 μm are detected in the spectra of nine sources, while 7.0 μm feature is identified in the spectra of HD 319896. The spectral index analysis and the comparison of the known sources with C60 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 C60 emission features originating from the circumstellar disc or nearby diffused emission region. The detection fraction of C60 in the total HAeBe star sample is ∼7 per cent, whereas the detection fraction is 30 per cent for HAeBe stars associated with nebulosity. In the catalog, C60 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 C60 multifold, which can help in understanding the excitation and formation pathway of the species.