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Matt Dennis Grew up in and Around the Greater St Production Rates and Spin Temperatures of Volatiles in Comet C/2007 N3 (Lulin) Matthew A Dennis University of Missouri-St. Louis Adviser: Dr. Erika Gibb Abstract We examine spectra taken of comet C/2007 N3 (Lulin) in an effort to identify production rates of organic molecules present within the comet’s coma. We also indentify the temperature range at which the comet formed by examining the ortho/para ratio of the comet’s water ices. These observations will contribute to a growing catalog of chemical conditions within comets, which will in turn lead to a better understanding of the conditions in the early solar system. Introduction Comets are icy bodies thought to have coalesced during the solar system’s formation. Comets are mostly made of water and dust, but it is now known that they also harbor many of the volatile chemicals necessary for life. The study of volatiles in comets is an important link to understanding the formation of the solar system. By studying the composition of comets, astronomers gain many insights into conditions under which the comet formed, thereby strengthening our models for the behavior of the early protoplanetary disk. Many astronomers assert that comets may be responsible for the current life-friendly conditions on Earth (via the delivery of water and volatiles to the planet), and perhaps even for life itself. Models of the formation of the solar system have to include the appearance of water and organic molecules on Earth, which facilitated the development of life. Ongoing research into the composition, formation, and movements of comets and other small bodies is key to the development of these models. Comet C/2007 N3 (Lulin) is a comet thought to have formed in the Jovian region of the solar system, and then ejected by the gravitational influence of the gas giants into the outer solar system. There it became part of the “Oort Cloud,” a spherical reservoir of comets in a region somewhere between 2000 and 100,000 AU from the Sun, nearly half the distance to the nearest star. Current models of the Oort Cloud suggest that the bodies it is comprised of have mostly stayed at this distance since the Cloud was formed approximately 4.6 billion years ago.[1] This means that comets from the Oort Cloud, like Lulin, are rare opportunities to study relatively pristine examples of planetary disk material as it was when it formed 4.6 billion years ago. Assuming that material within the crusted outer layer of a comet does not get exposed to sunlight or cosmic radiation, it has remained unaltered since formation. Thus, when an Oort Cloud comet makes its first trip into the inner solar system (an occurrence thought by some astronomers to be triggered by gravitational perturbations from nearby stars), it delivers this unprocessed material within visible range of Earth. As the comet approaches the sun, its temperature rises, causing outgassing of the volatiles within, forming the familiar comae and tails. This provides prime opportunities for observation and analysis. Analysis of comet spectra can tell us many things about the comet’s history. For instance, cometary spectra can show abundances of molecules (as compared to water, the most common molecule found in comets), which may provide information about chemical conditions in the early stellar cloud, as well as a record of reactions that have taken place in the coma due to interaction with the sun's radiation. In addition, measuring the spin temperature of a comet’s water ices is thought to indicate the temperature at which the material in the comet formed. Water can form molecules in two different spin states: ortho and para. Each spin state emits at a slightly different frequency, allowing measurement of the ratio of ortho to para to be calculated. Water molecules that formed at roughly 50K-60K and above are considered to be in “statistical equilibrium”: they form at a steady ratio of spin states (3 ortho for every 1 para).[2] Below this temperature range, the ratio varies. Thus, by measuring the actual ratio of spin states found within the comet, the temperature range at which that comet's ices formed can be determined. This paper will report abundances (compared to water) and spin temperatures of several volatile compounds in Comet C/2007 N3 (Lulin), such as methane (CH4) and hydrogen cyanide (HCN). These results will contribute to a growing library of information about comet composition, which will lead to a further understanding of the origin of these icy bodies, and perhaps to the origin of organic compounds on Earth. Data Acquisition and Analysis Using the Keck II 10-meter telescope on Mauna Kea, Hawaii, Near Infrared Spectrograph (NIRSPEC) data were obtained between Jan 30 and Feb 1, 2009.[3] Spectra were extracted after the data were flat fielded, dark subtracted, and spatially and spectrally aligned. The LBLRTM atmospheric model was used to calibrate the frequency of the data and subtract atmospheric lines.[4] A bright standard star was observed on the same dates and was reduced in the same way to provide a flux calibration. Table 1 records the settings used during observation. Table 2 shows wave number ranges that were examined for spectra. Table 1. Instrument settings CROSS INTEGRATION FILTER ECHELLE SLIT (nm) DISPERSER TIME (s) KL2 62.34 32.80 0.432 x 24 60 KL3 61.38 33.15 0.432 x 24 60 Table 2. Targeted spectral regions, by molecule MOLECULE FILTER ORDER WAVE NUMBER RANGE (cm-1) H2O (Water) KL2 26 3455-3410 C2H2 (acetylene) KL2 25 3280-3320 HCN (Hydrogen Cyanide) KL2 25 3320-3270 CH4 (Methane) KL2 23 3050-3000 Each detected molecule was compared to a theoretical fluorescence emission spectrum for different rotational temperatures until the best fit was obtained using algorithms produce by Villanueva et. al.[5] Results Figures 1-4 show the extracted spectra of four volatiles detected in comet Lulin. In each figure, the extracted spectra were compared to a fluorescence emission model of the molecule. The methane R0 and R1 lines in the 3050-3000 wave number spectral region were particularly strong. Typically, these methane lines are difficult to discern from Earth-based observations, because they lie within very deep telluric lines. During the data collection taken on Comet Lulin, conditions were favorable, in that the comet’s Doppler shift (~-55 km/s) moved the methane lines out of the telluric interference, allowing for better transmittance. Figure 1. CH4 (Methane) Spectrum of Comet Lulin, order 23 showing CH4 emission features (black). The theoretical model (for a rotational temperature of 68K) is overlaid on the observed data above (red). The extracted methane and OH spectra are shown in red and green. The bottom graph shows residual data left over after the best-fit model is extracted. The bright green lines represent the “noise envelope.” Lines outside the noise envelope were strong enough emissions to be used in calculations. Figure 2. Volatiles Several volatile molecules are analyzed simultaneously, as in Figure 1. The actual data is shown at the top, overlaid with the theoretical model. The last row is the residual. Extracting the spectra as a group reduces the error caused when multiple molecules fluoresce at the same frequency. Figure 3. H2O Spectra for the two spin configurations of water are shown simultaneously. The ratio of ortho to para states indicate that Lulin’s water ices were in statistical equilibrium, meaning it formed at a temperature above 50K. Table 3 below shows results for some of the molecules detected in Comet Lulin. Production rate is a measure of how many molecules per second are being ejected into space from the comet’s surface. Abundances of each molecule (in relation to water) can be determined from the production rate if the production rate of water is known. Table 3. Production rate, abundances (compared to water), and spin temperatures, by molecule PRODUCTION RATE T ABUNDANCE UNCERTAINTY IN MOLECULE rot (molecules/sec) (K) (%) ABUNDANCE (+/- %) -- H2O 1.29E+29 +/- 1.29e+27 68 -- HCN (Hydrogen 0.01 1.46E+26 +/- 3.18e+24 68 0.11 Cyanide) 5.74E+25 +/- 1.34e+25 0.007 C2H2 68 0.04 (Acetylene) CH4 (Methane) 1.39E+27 +/- 9.79e+24 68 1.08 0.04 Discussion Infrared spectral observation of comets is still a relatively new field of study. Early in its inception, only rudimentary information about comet behavior and composition could be determined. Technology has improved over the years, but to further our knowledge of the formation of solar systems and the origins of organic compounds on Earth, better and more accurate observations will need to be made. For these observations to be carried out, new methods will need to be devised. Studies to date have shown comets to be very diverse in composition, meaning that only very broad generalizations can be made about the comet population of our solar system as a whole. Further observation and cataloging of chemical composition of comets will contribute to the ever-growing cometary knowledge base. Acknowledgements I’d like to thank the NASA-Missouri Space Grant Consortium for allowing me to participate in this project. I gratefully acknowledge support from NSF’s Planetary Astronomy program (NSF 0807939). Thanks also to Dr. Erika Gibb for giving me the opportunity to get a hands- on view of scientific research. And finally I’d like to thank Dr. Sonya Bahar and Dr. Gibb for supporting me in my journey towards my degree. Biography Matt Dennis grew up in and around the Greater St. Louis Area in Missouri. After a brief but very eventful time as a music major, and a brief but very uneventful time as a drafter, he decided that the time was right to fulfill a life-long desire: to earn a PhD in science.
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