Thirty years of cometary spectroscopy from McDonald Observatory

Abstract

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, C 3 , C 2 , CH, and NH 2 molecules were observed on at least one occasion. We converted the observed molecular column densities to production rates using a Haser (1957) model. We defined a restricted data set of comets that had at least 3 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 C 2 and C 3 to both be depleted, we find 9% of our restricted data set comets to be depleted. Using a more relaxed definition that requires only C 2 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. marked difference being the amount of continuum present. This simple observation leads to the question of whether all comets share the same composition or if there are comets with fundamentally different compositions. If differences are seen, then an additional question of the origin of the differences would be raised: are differences the result of different formational scenarios or different evolutionary scenarios? These

The Main Belt Comets and ice in the Solar System

“…This OH emission is often used to estimate the water production rate of comets, via spectroscopy or photometry. Low-resolution spectroscopy gives total production rates (e.g., Cochran et al 2012), while higher resolution with either large telescopes or from space can improve signal-to-noise (S/N) for bright comets and reveal the structure within the band (e.g., Weaver et al 2003;Jehin et al 2006). Using narrow-band filters that isolate the region around the OH emission can give production rates and can be used to image the 2D structure of the gas coma, with OH production rates as low as 2.2 × 10 27 mol s −1 detected for comet 88P/Howell at 2 au from the Sun using the 60 cm TRAPPIST telescope (Opitom et al 2017, in prep.…”

Jet Morphology and Coma Analysis of Comet 103P/Hartley 2

“…Thus, the scale lengths from Cochran et al (2012) serve as a starting point for narrowing the field of likely parents.…”

“…This is because the surface temperature of the comet nucleus is nearly inversely proportional to its distance from the Sun, so ejection speed of a gas is inversely proportional to the square root of the nucleus heliocentric distance, or vgas  Rh -0.5 (Delsemme, 1982). Empirical data by Cochran et al (2012) provide evidence that most coma gases travel at radial speeds close to 0.85 km/s for a comet nucleus near Rh = 1 AU. Thus, an estimate for the radial velocity of gases ejected directly from the nucleus is…”

“…Note that although dP was provided, this does not imply that the exact parentage of the listed daughter is known. Values were acquired from Haser modeling with a multiplicity of comet comae by Cochran et al (2012). The parent scale length dP assumes a single source, but this may actually represent multiple sources contributing to produce the daughter species.…”

“…Identifying the source for this radical is complicated since expected parent molecules have scale lengths or fractional decays that are disparate from the required values in models. Adopted scale lengths published by Cochran et al (2012) show that a C3 gas source with an ejection speed defined by Eq. 5.11 should have a lifetime of about τ = 3.65×10 3 s at 1 AU.…”