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 materialBockelée-Morvan, D., Lis, D., Wink, J.E., et al. 2000A&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 Haser’s 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]/[SO2]~1.6 - is SO2 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)

• 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 it’s 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 speciesin 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: http://deepimpact.jpl.nasa.gov/home/index.html

• Rosetta: http://sci.esa.int/science-e/www/area/index.cfm?fareaid=13

• Stardust: http://stardust.jpl.nasa.gov/home/index.html

• Info on Hale-Bopp: http://www2.jpl.nasa.gov/comet/

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