# Confirming interstellar C$_{60}^+$ using the Hubble Space Telescope

## TL;DR

This study uses high-quality Hubble Space Telescope spectra to confirm the presence of interstellar C60+ ions by matching observed absorption bands with laboratory data, resolving previous debates.

## Contribution

It provides the first robust identification of the 9428 Å C60+ band in space, confirming interstellar C60+ using ultra-high signal-to-noise HST spectra.

## Key findings

- Detection of the 9365, 9428, and 9577 Å C60+ bands in space.
- Wavelengths and strength ratios match laboratory measurements.
- First conclusive confirmation of interstellar C60+.

## Abstract

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+.