Infrared spectroscopy of HaleBopp comet

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Infrared spectroscopy of Hale-Bopp comet
Rassul Karabalin, Ge/Ay 132 Caltech March 17,
2004
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Why is it interesting?

• Comet is a few kilometers in size object that
  reaches 50000 AU while orbiting Sun, originate
  from the so called Oort cloud.
• Comet consist of ice and small particles of
  gravel or larger rocks, that is believed to be
  the most primitive solar system material.
• Gas cloud of volatile molecules that evaporate
  from nucleus appears around the comet while it
  approaches Sun.
• Comet C/1995 O1 (Hale-Bopp) is one of the
  brightest comets passed within time scale of
  recorded human observations of the sky.
• Many cometary parent molecules have strong
  rotational-vibrational transitions in near
  infrared region.

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• Observation of emission lines of H20, CH4, C2H6,
  C2H2, HCN and CO at 2-5 µm region using
  ground based facility IRTF on Mauna Kea.
• High spatial resolution allowed to construct the
  spatial profiles of molecular distributions.
• Estimate of g-factors and molecular production
  rates from these data.
• Example of lower resolution data for 6-12 µm
  region obtained using the space based ISO
  facility.

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Observed spectrum
Investigation using ground based observatories
ismore complicated due to atmospheric
effects. Examples of spectra are obtained using
Infrared Telescope Facility on Mauna Kea.
Heliocentric and geocentric distances were 1.1
and 1.5 AU. Clearly seen emission lines from
CO, H2O, C2H6, C2H2 and HCN. Continuum emitted
by cometary grains with terrestrial atmospheric
effects was modeled (dashed lines).
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Corrected spectrum
Subtraction of calculated cometary continuum with
correction from terrestrial atmosphere give
emission line spectra plotted on graph. Clearly
seen vibration-rotation lines. For H2O 111 110
in ?3 - ?2 band. Production rates and g-factors
were estimated from this data.
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Molecular Abundances and Production ratios

• Frequency ? is in cm-1
• g-factors in 10-5 photons s-1 molecule-1
• Flux is in 10-17 W m-2
• Production ratios (Q) in 1029 molecules s-1
• Abundances is percentage relative to H2O

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Spatial profiles
Resolution of the experiment allowed to create
spatial profile of cometary nucleus. 2 plots
with linear and semilog scale. All plots are
normalized and centered to the peak. As
expected all cometary emissions have broader
spatial profile than that of the star. Clearly
seen emission at least partly from non-nuclear
source . Spatial brightness distribution can
reflect change in molecular excitation level or
in column density.
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Data at higher wavelength.
The other example of data from the space based
Infrared Space Observatory (ISO), with lower
resolution. Heliocentric distance is 4.6 AU.
Close to 162K black body radiation in 6-9
µm. Above 9 µm well known silicate band is
clearly seen.
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Summary

• Recent approach of extremely bright comet
  Hale-Bopp provided unique observational
  opportunities in near infrared spectrum region.
• As expected large abundances of H2O an CO was
  observed. As well as traces of C2H6, C2H2,
  CH4 and HCN.
• No significant differences of physical
  properties or chemical composition from
  P/Halley comet.
• Molecular production rates dependence on
  heliocentric distance provide understanding of
  physical and chemical composition of comet
  nuclei.
• Space missions to short period comets can
  provide deeper understanding of comets
  structure.