Transcript Infrared Spectroscopy of comet Hale-Bopp

Infrared spectroscopy of Hale-Bopp comet
Rassul Karabalin, Ge/Ay 132 Caltech
March 17, 2004
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.
• 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.
Observed spectrum
Investigation using ground based observatories is
more 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).
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 ν3 - ν2 band.
Production rates and g-factors were estimated
from this data.
Molecular Abundances and Production ratios
• Frequency ν 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
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.
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.
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.