2022MNRAS.509.4908N

Ever since they were first detected over 100 yr ago, the mysterious diffuse interstellar bands (DIBs), a set of several hundred broad absorption features seen against distant stars in the optical and near-infrared wavelength range, largely remain unidentified. The close match, both in wavelengths and in relative strengths, recently found between the experimental absorption spectra of gas-phase buckminsterfullerene ions (C₆₀⁺) and four DIBs at λ9632 Å, λ9577 Å, λ9428 Åand λ9365 Å(and, to a lesser degree, a weaker DIB at λ9348 Å) suggests that C₆₀⁺ is a promising carrier for these DIBs. However, arguments against the C₆₀⁺ identification remain and are mostly concerned with the large variation in the intensity ratios of the λ9632 Åand λ9577 ÅDIBs. In this work, we search for these DIBs in the X-shooter archival data of the European Southern Observatory's Very Large Telescope, and we identify the λ9632 Å, λ9577 Å, λ9428 Åand λ9365 ÅDIBs in a sample of 25 stars. While the λ9428 Åand λ9365 ÅDIBs are too noisy to allow any reliable analysis, the λ9632 Åand λ9577 ÅDIBs are unambiguously detected and, after correcting for telluric water vapour absorption, their correlation can be used to probe their origin. To this end, we select a subsample of nine hot, O- or B0-type stars of which the stellar Mg II contamination to the λ9632 ÅDIB is negligibly small. We find that their equivalent widths, after being normalized by reddening to eliminate their common correlation with the density of interstellar clouds, exhibit a tight, positive correlation, supporting C₆₀⁺ as the carrier of the λ9632 Åand λ9577 ÅDIBs.