学年度第一学期工作总结撰写.ppt

The Chemistry of Comet Hale-Bopp Wendy Hawley Journal Club April 6, 2006 Paper New molecules found in comet C/1995 O1 (Hale-Bopp). Investigating the link between cometary and interstellar material Bockele-Morvan, D., Lis, D., Wink, J.E., et al. 2000 A&A 353, 1101 What is a Comet? A small (50km nucleus) icy body orbiting the Sun in a highly elliptical orbit Anatomy 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 material Debate over origin of comets: Interstellar material? Solar nebula? Chemical composition of a comet gives clues about solar system formation Previous Work Biver et al. 1999 - production rates Schleicher et al. 1998 - comet properties Greenberg 1982 - interstellar origin theory Lewis & Prinn 1980 - solar nebula affected composition Several papers about production rates in other comets, most notably Hyakutake and Halley Comet Hale Bopp Discovered in 1995 by Alan Hale and Thomas Bopp Named “the Great Comet of 1997” Perihelion: April 1, 1997, 0.91AU Scope of Paper Use spectroscopy to determine chemical composition of Hale-Bopp Compare to composition of interstellar clouds Answer questions about the origin of comets, and the origin of our solar system Observations Taken during February to April 1997 Identified six new species from spectra: HC3N, SO, HCOOH, SO2, NH2CHO, HCOOCH3 HNCO and OCS also confirmed Fig. 1 Molecular Production Rates and Abundances Molecular column density: Assumes optical thinness nu calculated using local thermal equilibrium (LTE) approximation Local densities of parent and daughter species found with Hasers model For some species, more in-depth modeling was done, yielding similar results to LTE Sulfur Species SO: 0.3% and SO2: 0.2% relative to water SO abundance could vary from comet to comet SO/SO21.6 - is SO2 only parent? Sulfur Species (cont.) OCS: 0.4% relative to water In 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 other species have low abundances with respect to NH3 New CHO species have low abundances, do not contribute much to C or O abundance Volatile Composition Abundances are assumed to be abundance of cometary ices Does not account for extended sources Both CO and OCS have additional 50% contribution from distributed source H2CO abundance uses extended source calculation Volatile Composition (cont.) Could coma chemical reactions cause minor species? A model of HCN formation cannot reproduce the abundance Ion-molecule interactions could produce new species, but its unlikely Ionic content too small (photoionization 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 Comparison to Interstellar Medium Theories on comet formation range from pure interstellar origin to pure solar condensate origin Major constituents were studied, minor species left out Compare to hot cores and bipolar flows Hot Cores dense clumps of gas heated to above 100K by UV radiation and shocks from recently formed stars Gases may form from sublimation of icy particles Typically 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 species in good agreement, but S- bearing species there is scatter Future Work Further data on interstellar clouds needed Space missions to comets will provide more information about nucleus and composition For more Information Deep Impact: /home/index.ht ml Rosetta: /science- e/www/area/index.cfm?fareaid=13 Stardust: /home/index.html Info on Hale-Bopp: /comet/ Conclusion New species were detected using spectra Comparison of a