The EDIBLES survey

V. Line profile variations in the λλ5797, 6379, and 6614 diffuse interstellar bands as a tool to constrain carrier sizes

¹ Department of Physics and Astronomy, The University of Western Ontario, London, ON N6A 3K7 Canada
e-mail: jcami@uwo.ca
² Institute for Earth and Space Exploration, The University of Western Ontario, London, ON N6A 3K7, Canada
³ SETI Institute, 189 Bernardo Ave, Suite 100, Mountain View, CA 94043, USA
⁴ ACRI-ST, 260 route du Pin Montard, 06904 Sophia Antipolis, France
⁵ School of Astronomy, Institute for Research in Fundamental Sciences, 19395-5531 Tehran, Iran
⁶ European Southern Observatory, Alonso de Cordova 3107 Vitacura, Santiago, Chile
⁷ GEPI, Observatoire de Paris, PSL Research University, CNRS, Université Paris-Diderot, Sorbonne Paris Cité, Place Jules Janssen, 92195 Meudon, France
⁸ School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
⁹ Astrochemistry Laboratory, NASA Goddard Space Flight Center, Code 691, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
¹⁰ Department of Physics, The Catholic University of America, Washington, DC 20064, USA
¹¹ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹² ESTEC, ESA, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹³ Lennard-Jones Laboratories, Keele University, ST5 5BG UK
¹⁴ INAF – Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius, Italy
¹⁵ AAAS Science International, Clarendon House, Clarendon Road, Cambridge CB2 8FH, UK

Received: 15 September 2021
Accepted: 28 January 2022

Abstract

Context. Several diffuse interstellar bands (DIBs) have profiles with resolved sub-peaks that resemble rotational bands of large molecules. Analysis of these profiles can constrain the sizes and geometries of the DIB carriers, especially if the profiles exhibit clear variations along lines of sight probing different physical conditions.

Aims. Using the extensive data set from the ESO Diffuse Interstellar Bands Large Exploration Survey we searched for systematic variations in the peak-to-peak separation of these sub-peaks for three well-known DIBs in lines of sight with a single dominant interstellar cloud.

Methods. We used the spectra of twelve single-cloud sight lines to examine the λλ5797, 6379, and 6614 DIB profiles. We measured the peak-to-peak separation in the band profile substructures for these DIBs. We adopted the rotational contour formalism for linear or spherical top molecules to infer the rotational constant for each DIB carrier and the rotational excitation temperature in the sight lines. We compared these to experimentally or theoretically obtained rotational constants for linear and spherical molecules to estimate the DIB carrier sizes.

Results. All three DIBs have peak separations that vary systematically between lines of sight, indicating correlated changes in the rotational excitation temperatures. The rotational constant B of the λ6614 DIB was determined independently of the rotational excitation temperature; we derived B₆₆₁₄ = (22.2 ± 8.9) x 10⁻³ cm⁻¹, consistent with previous estimates. Assuming a similar rotational temperature for the λ6614 DIB carrier and assuming a linear carrier, we found B₅₇₉₇^linear = (5.1 ± 2.0) × 10⁻³ cm⁻¹ and B₆₃₇₉^linear = (2.3 ± 0.9) × 10⁻³ cm⁻¹. If the carriers of those DIBs are spherical species, on the other hand, their rotational constants are half that value, B₅₇₉₇^spherical = (2.6 ± 1.0) × 10⁻³ cm⁻¹ and B₆₃₇₉^spherical = (1.1 ± 0.4) × 10⁻³ cm⁻¹.

Conclusions. Systematic variations in the DIB profiles provide the means to constrain the molecular properties. We estimate molecule sizes that range from 7–9 carbon atoms (λ6614 carrier, linear) to 77–114 carbon atoms (λ6379, spherical).