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

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 ($\sim 600-800$), allowing the detection of the diffuse interstellar band (DIB) at 9577 Å 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 Å 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.

Accepted for publication in ApJL, June 2017

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• Searching for stable fullerenes in space with computational chemistry
• EDIBLES II. On the detectability of C60+ bands
• Laboratory photo-chemistry of covalently bonded fluorene clusters: observation of an interesting PAH bowl-forming mechanism
• The EDIBLES survey III. C2-DIBs and their profiles
• The EDIBLES survey V: Line profile variations in the $\lambda \lambda$5797, 6379, and 6614 diffuse interstellar bands as a tool to constrain carrier sizes
• Updated extraction of the APOGEE 1.5273 μm diffuse interstellar band: a Planck view on the carrier depletion in dense cores
• C${}_{60}^{+}$ diffuse interstellar band correlations and environmental variations
• The diffuse interstellar band around 8620 Å II. Kinematics and distance of the DIB carrier
• A high resolution study of near-IR diffuse interstellar bands, search for small scale structure, time variability and stellar features
• Near-Infrared Diffuse Interstellar Bands towards Her 36
• A Deep Search for a Strong Diffuse Interstellar Band in the Circumgalactic Medium
• Families and Clusters of Diffuse Interstellar Bands: a Data-Driven Correlation Analysis