DEEP IMPACT : SUBMILLIMETRE SPECTROSCOPIC HCN OBSERVATIONS OF 9P/TEMPEL-1 FROM JCMT. I. M. Coulson (i.coulson@jach.hawaii.edu), H. M. Butner, G. Moriarty-Schieven, Joint Astronomy Centre, Hilo, Hawaii, L. M. Woodney, California State University, San Bernardino, S. B. Charnley, S. D. Rodgers, NASA Ames Research Center, Moffett Field, California, J. Stuwe, Leiden University, Netherlands, R. Schultz, ESTEC, Noordwijk, Netherlands, K. Meech, Y. Fernandez, Institute for Astronomy, University of Hawaii, P. Vora, College of William & Mary, Williamsburg, Virginia. Abstract Submillimetre spectroscopic observations of the comet 9P/Tempel-1 before, during and after the impact by the NASA spacecraft Deep Impact on UT 04 July 2005 are reported. Sev- eral molecular species (HCN, CH3OH, CO, CS, HCO+) were targetted but none was detected. Upper limits on emission line strengths may weakly constrain the chemical composition of the ices in the excavated part of the comet nucleus, but it is the overall absence of molecular line emissions - relative to the copious amounts of dust released - that ought to provoke revisions to cometary nuclear models. Introduction The Deep Impact mission offered the first chance to ob- serve newly excavated and newly exposed nuclear material of a Jupiter-family comet. Models of comet nuclei, all variations on the theme of Whipple’s original (1950, ApJ 111, 375) dirty- snowball, suggested that the excavation of a crater some 30m deep (A’Hearn et al 2003, IAU Trans Vol 13) should vapourize essentially pristine nuclear ices which would expand into a coma in the days following the impact. The composition of the released gasses should be amenable to spectroscopy. De- pending upon the cohesion of the nucleus, the impact might also open up fissures that would outgas similarly over longer periods. The science proposals approved by the JCMT Time Allocation Groups were aimed at determining isotopic ratios and the deuteration fraction in these emissions from observa- tions of several molecular species. These would enable the origin of Tempel-1 to be located within the early solar neb- ula (Drouart et al, 1999 Icarus 140, 129). The observational challenge seemed merely to establish baseline emission levels, and to measure sufficient numbers of molecular emission lines during and after the impact in order to determine the physical and chemical characteristics of the vapourized gasses. Observations The present authors established a collaboration and set ob- servational priorities for each phase of the event; with enough flexibility to respond to the results obtained both at JCMT and elsewhere. Several nights of observing in the weeks prior to the impact were used to establish baseline emission levels of the molecules we hoped to follow through impact, and fol- lowup observations were scheduled for 3 weeks afterwards. Observations during impact were made using the JCMT re- ceiver ’B’ tuned to 338.5 GHz. The resolution of 0.625MHz (∼0.5km/s) allows a bandpass of approximately 0.5GHz, en- compassing 20 or more lines of the methanol CH3OH:J=7-6 ladder. Intensity ratios amongst these lines would allow the determination of the rotational temperature of the gas, a cru- cial factor in interpreting any detections of other molecular species. The observational plan thereafter was to observe these other species - both the normal and deuterated isotopomers - interspersed with methanol observations (there is another lad- der at ∼241.8GHz in the ’A’-band) to keep track of the gas temperature. Around the moment of impact and for an hour afterwards we observed with a resolution of 1 minute, relax- ing this to 10minutes once it became clear that there were no strong molecular emissions. The methanol observations were performed until ∼2hours after impact, and were followed by about an hour’s worth of observations at HCN, normally the strongest cometary emission species in the submillimetre. Ob- servations of the HCN:J=4-3 transition at 354.5GHz, and at a resolution of 0.15MHz (∼0.1km/s), were made until the comet dropped low in the sky, but resumed the following afternoon once the comet had risen into view from Mauna Kea. Initial non-detections were not met with too much alarm since we expected that it would take the putative gas coma ∼24 hours to expand and fill the JCMT beam. Nonetheless, no detections were made of any molecular species on UT 05 July either; the Figure shows the HCN:J=4-3 spectra obtained on UT July 04, 05, 06 & 07. These data also rule out any possible shielding of localized emission regions due to the 1.7-day rotation period of the nucleus. Analysis and Interpretation of the HCN data The obvious first result from these submillimetre observa- tions was how little molecular gas was released by the impact, suggesting an ice-poor surface layer. The entire JCMT dataset is being analyzed thoroughly to throw light on the gas/dust composition of the nucleus, but in this poster we show, in the Table, just the elements of the HCN observations and the de- rived preliminary 3σ upper limits on the HCN production rate, Q. The analysis adopts (from the preliminary results of other Deep Impact observers) a rotational temperature of 40K and an outflow velocity of 0.7km/s, and adopts also the Haser (1957 Bull. Acad. Roy. Belg. 43, 740) model of the coma density, although its applicability could be rightly questioned both in the case where production rates are vanishingly small and in the case of an impact. The data indicate that the impact had no effect upon molecular gas emissions, to within the detection limits of the observa- tions. They suggest a reasonably constant upper limit to the HCN production rate in the immediate post-impact phase of ∼2.5×10 24 mols/s. This is ∼4× smaller than Q(CN) during the same period (Schleicher et al 2005, priv comm), which may imply that HCN on its own is not the only source, by photodissociation, of the CN molecule. Protostars and Planets V 2005 8524.pdf