The EDIBLES survey II. The detectability of C₆₀⁺ bands

¹ GEPI, Observatoire de Paris, PSL Research University, CNRS, Place Jules Janssen, 92190 Meudon, France
e-mail: rosine.lallement@obspm.fr
² ACRI-ST, 260 route du Pin Montard, 06904 Sophia Antipolis, France
³ Department of Physics and Astronomy, The University of Western Ontario, London ON N6A 3K7, Canada
⁴ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁵ School of Astronomy, Institute for Research in Fundamental Sciences, 19395-5531 Tehran, Iran
⁶ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
⁷ Department of Physics, Catholic University of America, Washington, DC 20064, USA
⁸ Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁹ AAAS Science International, Clarendon House, Clarendon Road, Cambridge CB2 8FH, UK
¹⁰ George Washington University, Washington DC, USA
¹¹ ESTEC, ESA, Noordwijk, The Netherlands

Received: 15 January 2018
Accepted: 29 January 2018

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.