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 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 H 2 0, CH 4, C 2 H 6, C 2 H 2, 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, H 2 O, C 2 H 6, C 2 H 2 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 H 2 O 1 11 – 1 10 in ν 3 - ν 2 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 photons s -1 molecule -1 Flux is in W m -2 Production ratios (Q) in molecules s -1 Abundances is percentage relative to H 2 O

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 H 2 O an CO was observed. As well as traces of C 2 H 6, C 2 H 2, CH 4 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.