Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007phdt........26m&link_type=abstract
Proquest Dissertations And Theses 2007. Section 0009, Part 0494 281 pages; [Ph.D. dissertation].United States -- Arizona: The U
Astronomy and Astrophysics
Astronomy
Astrochemistry, Astrobiology, Radio Astronomy, Isotope Ratios, Interstellar Medium, Carbon, Solar System
Scientific paper
Studies of carbon's evolutionary path have been conducted via millimeter and submillimeter observations of circumstellar envelopes (CSEs), planetary nebulae (PNe), molecular clouds and comets. The 12 C/ 13 C isotope ratio was measured in Galactic molecular clouds using the CN isotopologs. A gradient of 12 CN/13 CN was determined to be 12 C/ 13 C = 6.01 D GC +12.28, where D GC is distance from the Galactic center. The results of CN are in agreement with those of CO and H 2 CO indicating a true ratio not influenced by fractionation effects or isotope-selective photodissociation. The 12 C/ 13 C isotope ratios in the envelopes of various types of stars were also measured from both CO and CN isotopologs. Such objects as carbon and oxygen-rich asymptotic giant branch (AGB) stars, supergiants, planetary nebulae, and S-type stars were observed. Results from this study indicate 12 C/ 13 C values for supergiants ~ 10 and AGB stars 12 C/ 13 C ~ 20-76. Theory would suggest a lower ratio for objects undergoing third dredge-up, though this is seemingly not the case. Multiple carbon-bearing species including CO, HCN, HNC, CN, CS, and HCO + have also been observed towards the oxygen-rich supergiant, VY CMa. This object has recently revealed a unique chemistry where carbon is not solely contained in CO, and may play a more important role in the chemical network of oxygen-rich circumstellar envelopes. Additionally, observations of species with carbon-carbon bonds, such as CCH and c-C 3 H 2 , have been conducted towards evolved planetary nebulae, such as the Helix and Ring nebulae. There is a close agreement in the inventories of species found in PNe and diffuse clouds, suggesting a potential molecular precursor to the interstellar medium. Observations of carbon-bearing species (H 2 CO and CO) in comets C/1995 O1 (Hale-Bopp), C/2001 Q4 (NEAT), and C/2002 T7 (LINEAR) have been conducted. Formaldehyde is known to have an extended distribution in these objects, likely arising from silicate-organic grains. Evidence of cometary fragmentation was also obtained for H 2 CO in comet T7 LINEAR as well as for HNC and HCO + in Hale-Bopp. Such events could contribute to planetary distribution of organics.
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