Interstellar C₆₀^+⋆

¹ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France
e-mail: olivier.berne@gmail.com
² CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Cagliari, strada 54, località Poggio dei Pini, 09012 Capoterra (CA), Italy

Received: 13 November 2012
Accepted: 29 November 2012

Abstract

Buckminsterfullerene (C₆₀) has recently been detected through its infrared emission bands in the interstellar medium (ISM), including in the proximity of massive stars, where physical conditions could favor the formation of the cationic form, C₆₀⁺. In addition, C₆₀⁺ was proposed as the carrier of two diffuse interstellar bands in the near-IR, although a firm identification still awaits gas-phase spectroscopic data. We examined in detail the Spitzer IRS spectra of the NGC 7023 reflection nebula, at a position close (7.5′′) to the illuminating B star HD 200775, and found four previously unreported bands at 6.4, 7.1, 8.2, and 10.5 μm, in addition to the classical bands attributed to polycylic aromatic hydrocarbons (PAHs) and neutral C₆₀. These 4 bands are observed only in this region of the nebula, while C₆₀ emission is still present slightly farther away from the star, and PAH emission even farther away. Based on this observation, on theoretical calculations we perform, and on laboratory studies, we attribute these bands to C₆₀⁺. The detection of C₆₀⁺ confirms the idea that large carbon molecules exist in the gas phase in these environments. In addition, the relative variation in the C₆₀ and C₆₀⁺ band intensities constitutes a potentially powerful probe of the physical conditions in highly UV-irradiated regions.