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. 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
Fig. 1
• Taken during February
to April 1997
• Identified six new
species from spectra:
HC3N, SO, HCOOH,
SO2, NH2CHO,
HCOOCH3
• HNCO and OCS also
confirmed
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 species
in good agreement, but Sbearing 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.ht
ml
• Rosetta: http://sci.esa.int/sciencee/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