ExoMol line lists – XXIX. The rotation-vibration spectrum of methyl chloride up to 1200 K

A Owens,J Tennyson,A Yachmenev,W Thiel,S N Yurchenko,A Fateev

doi:10.1093/mnras/sty1542

Abstract

Comprehensive rotation-vibration line lists are presented for the two main isotopologues of methyl chloride, 12CH335Cl and 12CH337Cl. The line lists, OYT-35 and OYT-37, are suitable for temperatures up to |$T=1200\,$| K and consider transitions with rotational excitation up to J = 85 in the wavenumber range 0 – |$6400\,$| cm−1 (wavelengths |$\lambda \gt 1.56\,\mu$|m). Over 166 billion transitions among 10.2 million energy levels have been calculated variationally for each line list using a new empirically refined potential energy surface, determined by refining to 739 experimentally derived energy levels up to J = 5, and an established ab initio dipole moment surface. The OYT line lists show excellent agreement with newly measured high-temperature infrared absorption cross-sections, reproducing both strong and weak intensity features across the spectrum. The line lists are available from the ExoMol database and the cds database.

Highlights

The recent interstellar detection of methyl chloride around the protostar IRAS 16293-2422 and in the coma of comet 67P/Churyumov– Gerasimenko (67P/C-G) (Fayolle et al 2017) has undermined the possibility of CH3Cl as a realistic biosignature gas in the search for life outside our Solar system (Segura et al 2005; Seager, Bains & Hu 2013a,b)
We present newly computed rotation-vibration line lists, named OYT-35 and OYT-37, for the two main isotopologues of methyl chloride, 12CH335Cl and 12CH337Cl
The EXOMOL data structure has been adopted for the OYT line lists and a detailed description with illustrative examples can be found in Tennyson et al (2016)

Summary

Introduction

The recent interstellar detection of methyl chloride around the protostar IRAS 16293-2422 and in the coma of comet 67P/Churyumov– Gerasimenko (67P/C-G) (Fayolle et al 2017) has undermined the possibility of CH3Cl as a realistic biosignature gas in the search for life outside our Solar system (Segura et al 2005; Seager, Bains & Hu 2013a,b). Since 2012, the EXOMOL database (Tennyson & Yurchenko 2012; Tennyson et al 2016) has been generating molecular line lists and key spectroscopic data on a variety of small molecules deemed important for the characterization of hot astronomical atmospheres. Notable applications utilizing EXOMOL line lists include: the use of the 10to line list (Yurchenko & Tennyson 2014) to model methane in exoplanets (Beaulieu et al 2011; Yurchenko et al 2014; Tsiaras et al 2018) and the bright T4.5 brown dwarf 2MASS 055914 (Yurchenko et al 2014), and to assign lines in the near-infrared spectra of late T dwarfs (Canty et al 2015) in combination with the ammonia BYTe line list (Yurchenko, Barber & Tennyson 2011); the early detection of water using the BT2 line list (Barber et al 2006) in HD 189733b (Tinetti et al 2007) and HD 209458b (Beaulieu et al 2010); and the provisional identification of HCN in the atmosphere of super-Earth 55 Cancri e (Tsiaras et al 2016) and TiO in the atmosphere of hot Jupiter WASP-76 b (Tsiaras et al 2018). A tentative detection of NaH in the atmosphere of a brown dwarf was ruled out using a line list for this molecule (Rivlin et al 2015)

Methods

Results

Conclusion