Searching for Interstellar C⁺₆₀Using a New Method for High Signal-to-noise HST/STIS Spectroscopy

Due to recent advances in laboratory spectroscopy, the first optical detection of a very large molecule has been claimed in the diffuse interstellar medium (ISM): C⁺₆₀ (ionized Buckminsterfullerene). Confirming the presence of this molecule would have significant implications regarding the carbon budget and chemical complexity of the ISM. Here we present results from a new method for ultra-high signal-to-noise ratio (S/N) spectroscopy of background stars in the near-infrared (at wavelengths of 0.9-1 μm), using the Hubble Space Telescope (HST) Imaging Spectrograph (STIS) in a previously untested STIS scan mode. The use of HST provides the crucial benefit of eliminating the need for error-prone telluric-correction methods in the part of the spectrum where the C⁺₆₀ bands lie and where the terrestrial water vapor contamination is severe. Our STIS spectrum of the heavily reddened B0 supergiant star BD+63 1964 reaches an unprecedented S/N for this instrument (∼600-800), allowing the detection of the diffuse interstellar band (DIB) at 9577 Å attributed to C⁺₆₀ , as well as new DIBs in the near-IR. Unfortunately, the presence of overlapping stellar lines, and the unexpected weakness of the C⁺₆₀ bands in this sightline, prevents conclusive detection of the weaker C⁺₆₀ bands. A probable correlation between the 9577 Å DIB strength and interstellar radiation field is identified, which suggests that more strongly irradiated interstellar sightlines will provide the optimal targets for future C⁺₆₀searches.