Other
Scientific paper
Nov 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010mwac.meet..p05j&link_type=abstract
"Midwest Astrochemistry Meeting 2010, held 5-6 November at the University of Illinois at Urbana-Champaign. http://midwest.astroc
Other
Scientific paper
The isotopic abundances of hydrogen and deuterium in comets are an important marker for early solar nebula processes and the formation of the solar system. To this end, comet C/2002 C1 (Ikeya-Zhang) was studied using high resolution (λ/Δλ ~ 25,000) near infrared spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea, Hawai'i. After reduction and subtraction of atmospheric spectral lines, a chi squared minimization method was used to determine a three sigma upper limit of Q(HDO) < 1.028 x 1030 molecules/second. The significance of this finding and further investigation is discussed. CCD images were taken using the CSHELL instrument at the NASA Infrared Telescope Facility (IRTF) in Mauna Kea, Hawaii on March 22-23 and April 12 of 2002 (C/2002 C1/Ikeya-Zhang). The section of the spectrum examined was the domain 2625-2635 cm-1 (HDO-C). This domain was selected based on the emission spectrum model of HDO. A 1" (5-pixel) wide slit was employed with a resolving power (λ/Δλ) of 25000. Exposures were taken for 60 seconds. The comet was nodded on chip in an ABBA pattern to reduce instrumental noise and time moving the telescope. Standard stars were imaged using a 4" (0.800 nm) slit, with 5 second exposure and 2 coadditions per image.
Results. HDO was not detected in the HDO-C setting for Ikeya-Zhang. A 3-sigma upper limit of 1.03 x 1030 molecules/second was calculated.
Discussion. To date HDO has been detected in three comets: Halley, Hyakutake, and Hale-Bopp. The ratio was found to be roughly twice the Standard Mean Ocean Water (SMOW) value in these three. These results are often used to reject cometary contribution to the Earth's oceans of any appreciable amount, and bolster the common perception that the D/H ratio in comets is universally around 2 SMOW. We believe this conclusion is premature for several reasons. Only the measurement in Halley is highly reliable, being based on in situ mass spectroscopy. The measurements in Hyakutake and Hale-Bopp are based on a single emission line in each, leading to large uncertainties in the HDO production rates and in turn the D/H ratio. When the reported uncertainties are considered along with the ratios, only the enrichment in Halley's comet is significantly different from the SMOW value. For Hyakutake, D/H(H2O) = (2.9+/-1.0) x 10-4, only < 1.5 sigma from SMOW. For Hale-Bopp, D/H(H2O) = (3.3+/-0.8) x 10-4, only < 2 sigma from SMOW. More recently, an upper limit for another emission line in Hale-Bopp was reported where D/H(H2O) < 1.8 x 10-4, a result that is often not mentioned, but is remarkably close to SMOW. Ultimately, our study and others are preliminary, and highlight a need for more data before the D/H ratio in comets is quantified.
Gibb Erika Lynne
Janini Philip
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