The EDIBLES survey II. The detectability of C60+ bands

R Lallement,M Elyajouri,P Ehrenfreund,H Linnartz,B H Foing,J Cami,A Fahrang,M A Cordiner,J Smoker,K T Smith,N L J Cox

doi:10.1051/0004-6361/201832647

Abstract

Gas phase spectroscopic laboratory experiments for the buckminsterfullerene cation C60+ have resulted in accurate rest wavelengths for five C60+ transitions that have been compared with diffuse interstellar bands (DIBs) in the near infra-red. Detecting these in astronomical spectra is difficult because of the strong contamination of ground-based spectra by atmospheric water vapor, to the presence of weak and shallow stellar lines and/or blending with other weak DIBs. The detection of the two strong bands has been claimed by several teams, and the three additional and weaker bands have been detected in a few sources. Certain recent papers have argued against the identification of C60+ based on spectral analyses claiming (i) a large variation in the ratio of the equivalent widths of the 9632 and 9577 Å bands, (ii) a large redshift of the 9632 Å band for the Orion star HD 37022, and (iii) the non-detection of the weaker 9428 Å DIB. Here we address these three points: (i) We show that the model stellar line correction for the 9632 Å DIB overestimates the difference between the strengths of the lines in giant and dwarf star spectra, casting doubts on the conclusions about the ratio variability. (ii) Using high quality stellar spectra from the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES), recorded with the ESO/Paranal Ultraviolet Echelle Spectrograph (UVES) in about the same atmospheric conditions, we find no wavelength shift in the 9632 Å band toward HD 37022. (iii) Using EDIBLES spectra and data from the Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) at CFHT we show that the presence of a weak 9428 Å band cannot be ruled out, even in the same observations that a previous study claimed it was not present.

Highlights

One of the longest standing spectroscopic mysteries in interstellar medium (ISM) studies is the identity of the carriers of the so-called diffuse interstellar bands (DIBs; see e.g., Herbig 1995; Sarre 2006; Snow 2014; Cami & Cox 2014, and references therein)
Ultraviolet Echelle Spectrograph (UVES) in about the same atmospheric conditions, we find no wavelength shift in the 9632 Å band toward HD 37022. (iii) Using ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) spectra and data from the Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) at Canada-France-Hawaii Telescope (CFHT) we show that the presence of a weak 9428 Å band cannot be ruled out, even in the same observations that a previous study claimed it was not present
We have considered with care the arguments used against the identification of the fullerene cation C+60 through its absorption bands

Summary

Introduction

One of the longest standing spectroscopic mysteries in interstellar medium (ISM) studies is the identity of the carriers of the so-called diffuse interstellar bands (DIBs; see e.g., Herbig 1995; Sarre 2006; Snow 2014; Cami & Cox 2014, and references therein). Weak stellar lines are present in the spectra of the early-type stars used for the DIB detections. A new technique using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST) has been developed by Cordiner et al (2017) and brought promising results Data from this technique that are free of telluric contamination, are expected to close the controversy about the C+60 identification. While some of the previously mentioned observations of reddened stars and their analyses found a convincing match between laboratory data and observed bands within observational uncertainties (Walker et al 2015, 2016, 2017), other recent studies based on the same high quality, high resolution spectra have questioned some of the claimed detections (Galazutdinov et al.2017; Galazutdinov & Krełowski 2017).

Stellar contamination of the 9632 Å DIB and the 9632 to 9577 DIB ratio

The 9632 Å DIB toward HD 37022

Search for the 9428 Å DIB

Conclusion and discussion