Cordiner, M. A., Cox, N. L. J., Lallement, R., Najarro, F., Cami, J., Gull, T. R., Foing, B. H., Linnartz, H., Lindler, D. J., Proffitt, C. R., Sarre, P. J., Charnley, S. B. 2017. Searching for Interstellar C-60(+) Using a New Method for High Signal-to-noise HST/STIS Spectroscopy. Astrophysical Journal Letters 843, 1, DOI: 10.3847/2041-8213/aa78f7
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 mu m), 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 (similar to 600-800), allowing the detection of the diffuse interstellar band (DIB) at 9577 angstrom attributed to (+)(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 angstrom DIB strength and interstellar radiation field is identified, which suggests that more strongly irradiated interstellar sightlines will provide the optimal targets for future C-60(+) searches.