Looking for the weak members of the C$_{60}^+$ family in the interstellar medium

TL;DR

This study uses high-resolution spectra to investigate the spectral features attributed to C60+ in the interstellar medium, revealing variability in feature strength ratios and challenging previous identifications of certain bands.

Contribution

It provides new observational evidence of variability in C60+ spectral features and refines the rest wavelength of the strongest diffuse band, questioning prior identifications.

Findings

Variable strength ratios of C60+ features observed.

Lack of the 9428 Å feature contradicts laboratory spectra.

Refined rest wavelength of the strongest diffuse band to 9576.8 Å.

Abstract

We demonstrate, using the high resolution spectra from the ESPADONS spectrograph, fed with the 3.6m CFH telescope, that the strength ratios of the strong--to--weak spectral features, attributed to C$_{60}^+$, are variable. We found that in the range of expected 9366~\AA\ C$_{60}^+$ feature there are two diffuse bands centered at 9362.0$\pm$0.1 and 9365.3$\pm$0.1 \AA\ with variable intensity ratio. We confidently confirm the lack of 9428~\AA\ feature which, in the laboratory spectra of C$_{60}^+$, is stronger than 9366~\AA. The weakest laboratory feature, near 9348.4~\AA\, remains below the level of detection in all spectra. The intensity ratio 9577/9365 is variable. These facts contradict to their common origin and so -- the identification of some interstellar spectral features as being carried by the cation of the "soccer ball". We also refined the rest wavelength position of the strongest diffuse band in this range: it is 9576.8$\pm$0.1~\AA.