C₆₀⁺– looking for the bucky-ball in interstellar space

Abstract

The laboratory gas-phase spectrum recently published by Campbell et al. has reinvigorated attempts to confirm the presence of the C|$_{60}^+$| cation in the interstellar medium, through an analysis of the spectra of hot, reddened stars. This search is hindered by at least two issues that need to be addressed: (i) the wavelength range of interest is severely polluted by strong water-vapour lines coming from the Earth's atmosphere; (ii) one of the major bands attributed to C|$_{60}^+$|⁠, at 9633 Å, is blended with the stellar Mg ii line, which is susceptible to non-local thermodynamic equilibrium effects in hot stellar atmospheres. Both these issues are carefully considered here for the first time, based on high-resolution and high signal-to-noise ratio echellé spectra for 19 lines of sight. The result is that the presence of C|$_{60}^+$| in interstellar clouds is brought into question.