Searching for interstellar C60+ using 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): C60+ (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 (S/N) spectroscopy of background stars in the
near infrared (at wavelengths 0.9-1 micron), using the Hubble Space Telescope
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 C60+ bands
lie, and terrestrial water vapor contamination is severe. Our STIS spectrum of
the heavily-reddened B0 star BD63\,1964 reaches an unprecedented S/N for this
instrument ($\sim600-800$), allowing the detection of the diffuse interstellar
band (DIB) at 9577 \AA\ attributed to C60+ as well as new DIBs in the near-IR.
Unfortunately, the presence of overlapping stellar lines, and the unexpected
weakness of the C60+ bands in this sightline, prevents conclusive detection of
the weaker C60+ bands. A probable correlation between the 9577 \AA\ DIB
strength and interstellar radiation field is identified, which suggests that
more strongly-irradiated interstellar sightlines will provide the optimal
targets for future C60+ searches.
