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

Abstract

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 H (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.

Measuring molecular abundances in comet C/2014 Q2 (Lovejoy) using the APEX telescope

“…The HWHM was further reduced by 15 % to take into account the increase in line width due to thermal Doppler broadening, resulting in a value of 0.8 km s −1 . This expansion velocity agrees with the value determined using the IRAM 30 m telescope (Biver et al 2016).…”

“…We confirm that C/2014 Q2 is rather depleted in CO and has low HCN and H 2 CO abundances. The measured production rates are consistent with values derived for this object from other facilities at radio wavelengths (Biver et al 2016;Wirström et al 2016).…”

“…Since observations obtained with the Keck and IRAM telescopes have a smaller field of view than APEX, they cover the collisional region of the coma which has higher rotational temperature than the outer regions of the coma encompassed in our observations. The rotational temperatures derived from CH 3 OH multiplets are especially useful as they are intermediate between the gas kinetic temperature in the collisional region in the inner coma and the fluorescence equilibrium temperature in the outer parts of the coma (see e.g., Bockelée-Morvan et al 2004;de Val-Borro et al 2013;Biver et al 2016). Using the non-LTE CH 3 OH model described in Section 3.2, the inferred rotational temperature can be compared to the prediction from the model for a gas kinetic temperature equal to the observed rotational temperature.…”

“…To derive molecular abundances of the observed species relative to water, the H 2 O production rates were obtained from Odin observations and measurements of the OH radical maser lines at 18 cm with the Nançay radio telescope (Biver et al 2015(Biver et al , 2016. During the January 2015 12.8-16.8 observing period, an average water production rate of (5.0 ± 0.2) × 10 29 mol s −1 was measured.…”

“…The presence of complex organic molecules suggests that this object originates from the outskirts of the pre-solar nebula. The detection of HDO emission was accomplished in the millimetre range of wavelengths using the IRAM 30 m telescope (Biver et al 2016) and by infrared spectroscopy with the Keck Observatory (Paganini et al 2017), increasing the number of known HDO/H 2 O ratios in comets and confirming their diverse chemical composition.…”