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1、The Chemistry of Comet Hale-Bopp,Wendy HawleyJournal ClubApril 6,2006,Paper,New molecules found in comet C/1995 O1(Hale-Bopp).Investigating the link between cometary and interstellar materialBockele-Morvan,D.,Lis,D.,Wink,J.E.,et al.2000A&A 353,1101,What is a Comet?,A small(50km nucleus)icy body orbi
2、ting the Sun in a highly elliptical orbitAnatomy of a comet:,Historical Significance,Bayeux Tapestry(11th century)image of comet Halley,1857 drawing of a comet hitting Earth,Scientific Significance,Comets thought to contain pristine materialDebate over origin of comets:Interstellar material?Solar ne
3、bula?Chemical composition of a comet gives clues about solar system formation,Previous Work,Biver et al.1999-production ratesSchleicher et al.1998-comet propertiesGreenberg 1982-interstellar origin theoryLewis&Prinn 1980-solar nebula affected compositionSeveral papers about production rates in other
4、 comets,most notably Hyakutake and Halley,Comet Hale Bopp,Discovered in 1995 by Alan Hale and Thomas BoppNamed“the Great Comet of 1997”Perihelion:April 1,1997,0.91AU,Scope of Paper,Use spectroscopy to determine chemical composition of Hale-BoppCompare to composition of interstellar cloudsAnswer ques
5、tions about the origin of comets,and the origin of our solar system,Observations,Taken during February to April 1997Identified six new species from spectra:HC3N,SO,HCOOH,SO2,NH2CHO,HCOOCH3HNCO and OCS also confirmed,Fig.1,Molecular Production Rates and Abundances,Molecular column density:Assumes opt
6、ical thinnessnu calculated using local thermal equilibrium(LTE)approximationLocal densities of parent and daughter species found with Hasers modelFor some species,more in-depth modeling was done,yielding similar results to LTE,Sulfur Species,SO:0.3%and SO2:0.2%relative to waterSO abundance could var
7、y from comet to cometSO/SO21.6-is SO2 only parent?,Sulfur Species(cont.),OCS:0.4%relative to waterIn agreement with value found for Hyakutake(Grard et al.1998)S/O:0.02(solar system value),N-and CHO-bearing Species,In agreement with upper limits for other comets(Crovisier et al.1993)HC3N,NH2CHO and o
8、ther species have low abundances with respect to NH3New CHO species have low abundances,do not contribute much to C or O abundance,Volatile Composition,Abundances are assumed to be abundance of cometary icesDoes not account for extended sourcesBoth CO and OCS have additional 50%contribution from dis
9、tributed sourceH2CO abundance uses extended source calculation,Volatile Composition(cont.),Could coma chemical reactions cause minor species?A model of HCN formation cannot reproduce the abundanceIon-molecule interactions could produce new species,but its unlikelyIonic content too small(photoionizat
10、ion takes too long),formation requires two subsequent reactions and more complex ions,Is Hale-Bopp Typical?,C2 and CN abundances are typical(Schleicher et al.1998)Parent molecules similar to those in other comets(Crovisier 1993)Hale-Bopp is assumed to be a representative for all comet-kind,Compariso
11、n to Interstellar Medium,Theories on comet formation range from pure interstellar origin to pure solar condensate originMajor constituents were studied,minor species left outCompare to hot cores and bipolar flows,Hot Cores,dense clumps of gas heated to above 100K by UV radiation and shocks from rece
12、ntly formed starsGases may form from sublimation of icy particlesTypically have large hydrogenated molecule abundance and organic material,Bipolar Flows,Produced by young protostars,also help investigate icy particles sublimated in the presence of low-mass stars,N-and CHO-bearing speciesin good agre
13、ement,but S-bearing species there is scatter,Future Work,Further data on interstellar clouds neededSpace missions to comets will provide more information about nucleus and composition,For more Information,Deep Impact:Rosetta:Stardust:Info on Hale-Bopp:http:/www2.jpl.nasa.gov/comet/,Conclusion,New species were detected using spectraComparison of abundances to interstellar sources showed similaritiesThere is a strong link between comets and interstellar icesComets give clues about the origins of life,despite their historical role as omens of death and destruction,
