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, thorough 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 9633Å, is blended with the stellar Mgii line, which is susceptible to non-local-thermodynamic equilibrium effects in hot stellar atmospheres. Both these issues are here 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.

Searching for Interstellar Using a New Method for High Signal-to-noise HST/STIS Spectroscopy

“…Galazutdinov et al (2017) were unable to confirm the presence of the weakest 3 features in a sample of 19 heavily reddened Galactic sightlines observed from the ground. Instead of a constant ratio for the two strongest C 60 + DIBs (λ9577 and λ9632), as expected for electronic transitions arising from a A u 2 1 ground vibronic state, Galazutdinov et al (2017) found that the interstellar band ratio was highly variable among different lines of sight. We note, however, that if the observed transitions involve lower levels above the ground state (such as the split levels arising from Jahn-Teller distortion), a variable ratio could potentially occur.…”

“…Based on the laboratory measurements, five absorption features are expected (at 9348.4, 9365.2, 9427.8, 9577.0, and 9632.1 Å, with strength ratios 0.07:0.2:0.3:1.0:0.8; Campbell et al 2016b). Galazutdinov et al (2017) were unable to confirm the presence of the weakest 3 features in a sample of 19 heavily reddened Galactic sightlines observed from the ground. Instead of a constant ratio for the two strongest C 60 + DIBs (λ9577 and λ9632), as expected for electronic transitions arising from a A u 2 1 ground vibronic state, Galazutdinov et al (2017) found that the interstellar band ratio was highly variable among different lines of sight.…”

“…Measurements of the NIR electronic transitions of C 60 + (in absorption) would also provide a unique complement to the discovery of mid-infrared C 60 and C 60 + emission bands in circumstellar and interstellar environments (Cami et al 2010;Sellgren et al 2010;Berné et al 2013). Despite dedicated observational studies (Walker et al 2015(Walker et al , 2016Galazutdinov et al 2017), the case for interstellar C 60 + has not yet been proven beyond reasonable doubt. Based on the laboratory measurements, five absorption features are expected (at 9348.4, 9365.2, 9427.8, 9577.0, and 9632.1 Å, with strength ratios 0.07:0.2:0.3:1.0:0.8; Campbell et al 2016b).…”

“…We note, however, that if the observed transitions involve lower levels above the ground state (such as the split levels arising from Jahn-Teller distortion), a variable ratio could potentially occur. Although Walker et al (2016) claimed interstellar detections of all five C 60 + bands, the three weaker C 60 + bands are problematic as they fall in a wavelength region heavily obscured by absorption due to water vapor in the Earth's atmosphere (see Galazutdinov et al 2000Galazutdinov et al , 2017. Telluric-correction methods for such weak interstellar absorption features are error-prone as a result of incomplete telluric line cancelation, and from the possible presence of weak (unseen) stellar lines in the telluric standard spectrum.…”

The Behavior of Selected Diffuse Interstellar Bands with Molecular Fraction in Diffuse Atomic and Molecular Clouds

“…While a number of such small molecules have been considered over the years, most detailed comparisons between laboratory and astronomical spectra have not yielded adequate matches (e.g., Tulej et al 1998;Motylewski et al 2000;Lakin et al 2000;McCall et al 2001;Guthe et al 2001;Salama et al 2011). Some five near-IR DIBs were recently attributed to the fullerene cation C 60 + Walker et al 2015; though see Galazutdinov et al 2017). Along with laboratory work, correlation studies have been performed to investigate the nature of carriers of the DIBs, by comparing DIB equivalent widths (EWs) with different interstellar parameters such as E B-V, the column densities of interstellar species (N(X)), and the strengths of other DIBs (e.g.…”