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 Bockelée-Morvan, D., Lis, D., Wink, J.E., et al 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 production rates Schleicher et al comet properties Greenberg interstellar origin theory Lewis & Prinn 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: HC 3 N, SO, HCOOH, SO 2, NH 2 CHO, HCOOCH 3 HNCO and OCS also confirmed Fig. 1

Molecular Production Rates and Abundances Molecular column density: Assumes optical thinness n u calculated using local thermal equilibrium (LTE) approximation Local densities of parent and daughter species found with Haser’s model For some species, more in-depth modeling was done, yielding similar results to LTE

Sulfur Species [SO]: 0.3% and [SO 2 ]: 0.2% relative to water SO abundance could vary from comet to comet [SO]/[SO 2 ] ~ is SO 2 only parent?

Sulfur Species (cont.) [OCS]: 0.4% relative to water In agreement with value found for Hyakutake (Gérard 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) HC 3 N, NH 2 CHO and other species have low abundances with respect to NH 3 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 –H 2 CO 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 it’s unlikely Ionic content too small (photoionization takes too long), formation requires two subsequent reactions and more complex ions

Is Hale-Bopp Typical? C 2 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: ml Rosetta: e/www/area/index.cfm?fareaid=13 Stardust: Info on Hale-Bopp:

Conclusion New species were detected using spectra Comparison of abundances to interstellar sources showed similarities There is a strong link between comets and interstellar ices Comets give clues about the origins of life, despite their historical role as omens of death and destruction