| Abstract
| - Buckminsterfullerene (C 60) 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, C 60+. In addition, C 60+ 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 C 60. These 4 bands are observed only in this region of the nebula, while C 60 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 C 60+. The detection of C 60+ confirms the idea that large carbon molecules exist in the gas phase in these environments. In addition, the relative variation in the C 60 and C 60+ band intensities constitutes a potentially powerful probe of the physical conditions in highly UV-irradiated regions.
|
