# Searching for interstellar C60+ using a new method for high signal-to-noise HST/STIS spectroscopy

## TL;DR

This study introduces a novel high signal-to-noise spectroscopy method using HST/STIS to detect interstellar C60+ molecules, achieving unprecedented data quality and revealing potential correlations with interstellar radiation, though conclusive detection remains challenging.

## Contribution

The paper presents a new ultra-high S/N spectroscopic technique with HST/STIS in scan mode, enabling better detection of interstellar molecules like C60+ without telluric contamination.

## Key findings

- Achieved S/N of 600-800 in near-IR spectra of background stars.
- Detected the 9577 Å DIB attributed to C60+ in a heavily-reddened star.
- Identified a possible correlation between DIB strength and interstellar radiation field.

## Abstract

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.