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

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.

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