Exploring the volatile composition of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) with ALMA

Context. Comets formed in the outer and cold parts of the disk which eventually evolved into our solar system. Assuming that the comets have undergone no major processing, studying their composition provides insight in the pristine composition of the Solar Nebula. Aims. We derive production rates for a number of volatile coma species and explore how molecular line ratios can help constrain the uncertainties of these rates. Methods. We analyse observations obtained with the Atacama Large Millimeter/Submillimeter Array of the volatile composition of the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) at heliocentric distances of similar to 1.45 AU and similar to 0.56 AU, respectively. Assuming a Haser profile with constant outflow velocity, we model the line intensity of each transition using a 3D radiative transfer code and derive molecular production rates and parent scale lengths. Results. We report the first detection of CS in comet ISON obtained with the ALMA array and derive a parent scale length for CS of similar to 200 km. Due to the high spatial resolution of ALMA, resulting in a synthesised beam with a size slightly smaller than the derived parent scale length (0.59 x 0.39 corresponding to similar to(375 x 250) km at the distance of the comet at the time of observations), we are able to tentatively identify CS as a daughter species, i.e., a species produced in the coma and/or sublimated from icy grains, rather than a parent species. In addition we report the detection of several CH3OH transitions and confirm the previously reported detections of HCN, HNC and H2CO as well as dust in the coma of each comet, and report 3 sigma upper limits for HCO+. Conclusions. We derive molecular production rates relative to water of 0.2% for CS, 0.06-0.1% for HCN, 0.003-0.05% for HNC, 0.1-0.2% for H2CO and 0.5-1.0% for CH3OH, and show that the modelling uncertainties due to unknown collision rates and kinematic temperatures are modest and can be mitigated by available observations of different transitions of HCN.