Confirming interstellar C$_{60}^+$ using the Hubble Space Telescope
  Recent advances in laboratory spectroscopy lead to the claim of ionized
Buckminsterfullerene (C60+) as the carrier of two diffuse interstellar bands
(DIBs) in the near-infrared. However, irrefutable identification of
interstellar C60+ requires a match between the wavelengths and the expected
strengths of all absorption features detectable in the laboratory and in space.
Here we present Hubble Space Telescope (HST) spectra of the region covering the
C60+ 9348, 9365, 9428 and 9577 {\AA} absorption bands toward seven
heavily-reddened stars. We focus in particular on searching for the weaker
laboratory C60+ bands, the very presence of which has been a matter for recent
debate. Using the novel STIS-scanning technique to obtain ultra-high
signal-to-noise spectra without contamination from telluric absorption that
afflicted previous ground-based observations, we obtained reliable detections
of the (weak) 9365, 9428 {\AA} and (strong) 9577 {\AA} C60+ bands. The band
wavelengths and strength ratios are sufficiently similar to those determined in
the latest laboratory experiments that we consider this the first robust
identification of the 9428 {\AA} band, and a conclusive confirmation of
interstellar C60+.
