E. C. Fayolle, K. I. Öberg, J. K. Jørgensen, K. Altwegg, H. Calcutt, H. S. P. Müller, M. Rubin, M. H. D. van der Wiel, P. Bjerkeli, T. L. Bourke, A. Coutens, E. F. van Dishoeck, M. N. Drozdovskaya, R. T. Garrod, N. F. W. Ligterink, M. V. Persson, S. F. Wampfler, and the ROSINA team
reported on the
Protostellar and Cometary Detections of Organohalogens
Nat. Astron. 1 (2017), 703–708.
The J = 13 – 12 transitions with K = 0 to 4 of methyl chloride, CH₃Cl, were detected with ALMA near 345.4 and 340.1 GHz for the ³⁵Cl and ³⁷Cl isotopologs, respectively, in emission toward the Solar-type protostar IRAS 16293–2422B. CH₃³⁵Cl was also seen toward the companion IRAS 16293–2422A. The ³⁵Cl/³⁷Cl ratio is about 2.1, the same as observed for HCl in that source, but slightly lower than 3.13 on Earth.
Methyl chloride is also known as chloromethan or Freon-40. It is a trace constituent in Earth's atmosphere. The larger part originates from industrial processes, but a significant fraction comes from natural sources. Therefore, astrobiologists considered it to be an indicator for the presence of extraterrestrial life. The finding of the molecule in the molecular cloud surrounding two protostars makes it unsuitable for the purpose of a biomarker.
Moreover, the finding of methyl chloride in the comet 67P/Churiumov-Gerassimenko combined with model calculations suggest that comets may have brought an amount of CH₃Cl to Earth that is a fair fraction of the part from natural sources. And this may well apply to exo-planets.
The author also report the tentative detection of methyl fluoride, CH₃F, via its J = 7 – 6 transitions with K = 0 to 3. However, the lines with K = 2 and 3 are blended with some much stronger lines, and the other two lines are quite weak.
See also ESO press releases in English or in German or the press release of the Universität zu Köln (in German).

Contributor(s): H. S. P. Müller 10, 2017