Isotopic ratios of H, C, N, O, and S in comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy)

The apparition of bright comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy) in March-April 2013 and January 2015, combined with the improved observational capabilities of submillimeter facilities, offered an opportunity to carry out sensitive compositional and isotopic studies of the volatiles in their coma. We observed comet Lovejoy with the IRAM 30 m telescope between 13 and 26 January 2015, and with the Odin submillimeter space observatory on 29 January-3 February 2015. We detected 22 molecules and several isotopologues. The H₂¹⁶O and H₂¹⁸O production rates measured with Odin follow a periodic pattern with a period of 0.94 days and an amplitude of ~25%. The inferred isotope ratios in comet Lovejoy are ¹⁶O/¹⁸O = 499 ± 24 and D/H = 1.4 ± 0.4 × 10^-4 in water, ³²S/³⁴S = 24.7 ± 3.5 in CS, all compatible with terrestrial values. The ratio ¹²C/¹³C = 109 ± 14 in HCN is marginally higher than terrestrial and ¹⁴N/¹⁵N = 145 ± 12 in HCN is half the Earth ratio. Several upper limits for D/H or ¹²C/¹³C in other molecules are reported. From our observation of HDO in comet C/2014 Q2 (Lovejoy), we report the first D/H ratio in an Oort Cloud comet that is not larger than the terrestrial value. On the other hand, the observation of the same HDO line in the other Oort-cloud comet, C/2012 F6 (Lemmon), suggests a D/H value four times higher. Given the previous measurements of D/H in cometary water, this illustrates that a diversity in the D/H ratio and in the chemical composition, is present even within the same dynamical group of comets, suggesting that current dynamical groups contain comets formed at very different places or times in the early solar system.

Based on observations carried out with the IRAM 30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).Odin is a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes) and the Centre National d'Études Spatiales (CNES, France). The Swedish Space Corporation is the prime contractor, also responsible for Odin operations.The spectra dataset is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/589/A78