The abundance of interstellar C₆₀ molecules toward BD+31°643 and other stars

¹ Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 January 2025
Accepted: 18 February 2026

Abstract

Context. The diffuse interstellar bands (DIBs) at 9577 and 9632 Å are attributed to electronic transitions of the C₆₀⁺ fullerene ion; thus, its column density in molecular clouds may be inferred. A comparison with the column density of the neutral C₆₀ fullerene makes it possible to determine local physical conditions in the clouds.

Aims. We aim to detect absorption lines of neutral C₆₀ in the visible spectra of stars toward interstellar clouds.

Methods. We searched for the γ₀, A₀, and A₁ absorption bands of C₆₀ near 6070 Å, 4024 Å, and 3980 Å using the Potsdam Echelle Polarimetric and Spectroscopic Instrument at the Large Binocular Telescope. We measured spectra toward eight background stars including BD+31°643.

Results. None of the γ₀, A₀, and A₁ absorption bands of C₆₀ were detected in our spectra. Toward BD+31°643, we determine an upper limit of (14 ± 11) × 10¹² cm⁻² for the column density of C₆₀, and values in the range of 10¹²−10¹⁴ cm⁻² toward other stars. Using literature data for C₆₀⁺, we derive upper limits of 0.8–3.8 for the C₆₀:C₆₀⁺ column density ratios.

Conclusions. The non-detection of the γ₀ absorption band toward BD+31°643 is in conflict with a measurement employing infrared (IR) emission bands of C₆₀. The discrepancy may indicate that free C₆₀ molecules are not the source of the observed IR emission bands toward the star and that the IR emission arises from C₆₀ molecules in dust grains instead. Alternatively, the oscillator strength of the γ₀ band may not be well constrained from the laboratory measurements.