Thirty years of cometary spectroscopy from McDonald Observatory

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2012-03, Vol.218 (1), p.144-168
• Main Authors: Cochran, A.L., Barker, E.S., Gray, C.L.
• Format: Article
• Language: English
• Subjects: Astronomy; Comets; Comets, Composition; Density; Depletion; Earth, ocean, space; Emission spectroscopy; Exact sciences and technology; Night; Solar system; Spectroscopy; Thresholds; Trends
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2011.12.010

► We present spectroscopic data from McDonald Observatory covering almost 30years and 130 comets. ► We find that 9% of the comets are carbon-chain depleted by the strictest definition (25% by a more customary definition). ► Jupiter Family comets are more likely to be depleted than Long Period comets. ► On average, there are few compositional differences with increasing orbital semi-major axis. We report on the results of a spectroscopic survey of 130 comets that was conducted at McDonald Observatory from 1980 through 2008. Some of the comets were observed on only one night, while others were observed repeatedly. For 20 of these comets, no molecules were detected. For the remaining 110 comets, some emission from CN, OH, NH, C3, C2, CH, and NH2 molecules were observed on at least one occasion. We converted the observed molecular column densities to production rates using a Haser (Haser, L. [1957]. Liege Inst. Astrophysics Reprint No. 394) model. We defined a restricted data set of comets that had at least three nights of observations. The restricted data set consists of 59 comets. We used ratios of production rates to study the trends in the data. We find two classes of comets: typical and carbon-chain depleted comets. Using a very strict definition of depleted comets, requiring C2and C3 to both be depleted, we find 9% of our restricted data set comets to be depleted. Using a more relaxed definition that requires only C2 to be below a threshold (similar to other researchers), we find 25% of the comets are depleted. Two-thirds of the depleted comets are Jupiter Family comets, while one-third are Long Period comets. 37% of the Jupiter Family comets are depleted, while 18.5% of the Long Period comets are depleted. We compare our results with other studies and find good agreement.