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Results are presented from the first cometary observations using the AtacamaLarge Millimeter/Submillimeter Array (ALMA), including measurements of thespatially-resolved distributions of HCN, HNC, H$_2$CO and dust within the comaeof two comets: C/2012 F6 (Lemmon) and C/2012 S1 (ISON), observed atheliocentric distances of 1.5 AU and 0.54 AU, respectively. These observations(with angular resolution $\approx0.5''$), reveal an unprecedented level ofdetail in the distributions of these fundamental cometary molecules, anddemonstrate the power of ALMA for quantitative measurements of thedistributions of molecules and dust in the inner comae of typical brightcomets. In both comets, HCN is found to originate from (or within a few hundredkm of) the nucleus, with a spatial distribution largely consistent withspherically-symmetric, uniform outflow. By contrast, the HNC distributions areclumpy and asymmetrical, with peaks at cometocentric radii $\sim$500-1000~km,consistent with release of HNC in collimated outflow(s). Compared to HCN, theH$_2$CO distribution in comet Lemmon is very extended. The interferometricvisibility amplitudes are consistent with coma production of H$_2$CO and HNCfrom unidentified precursor material(s) in both comets. Adopting a Haser model,the H$_2$CO parent scale-length is found to be a few thousand km in Lemmon andonly a few hundred km in ISON, consistent with destruction of the precursor byphotolysis or thermal degradation at a rate which scales in proportion to theSolar radiation flux.

the astrophysical journal. letters

Mapping the release of volatiles in the inner comae of comets C/2012 F6  (Lemmon) and C/2012 S1 (ISON) using the Atacama Large  Millimeter/Submillimeter Array