N. R. dello Russo

Remote infrared observations of parent volatiles in comets: A window on the early solar system

Abstract Organic volatiles and water in Oort Cloud comets were investigated at infrared wavelengths. The detected species include H 2 O, CO, CH 3 OH, CH 4 , C 2 H 2 , C 2 H 6 , OCS, HCN, NH 3 , and H 2 CO. Several daughter fragments (CN, OH, NH 2 , etc.) are also measured, and OH prompt emission provides a proxy for water. Long-slit spectra are taken at high spectral dispersion and high spatial resolution, eliminating several sources of systematic error. The resulting parent volatile production rates are highly robust, permitting a sensitive search for compositional diversity among comets. Here, seven OC comets are compared. Six (including Halley) exhibit similar compositions (excepting CO and CH 4 ). Their low formation temperatures (∼30 K) suggest this group probably formed beyond 30 AU from the young sun. However, C/1999 S4 is severely depleted in hypervolatiles and also in methanol, and it likely formed near 5–10 AU. C/2001 A2 is discussed briefly to illustrate future prospects.

A Survey of Organic Volatile Species in Comet C/1999 H1 (Lee) Using NIRSPEC at the Keck Observatory

The organic volatile composition of the long-period comet C/1999 H1 (Lee) was investigated using the —rst of a new generation of cross-dispersed cryogenic infrared spectrometers (NIRSPEC, at the Keck Observatory atop Mauna Kea, HI). On 1999 August 19¨21 the organics spectral region (2.9¨3.7 km) was completely sampled at both moderate and high dispersion, along with the CO fundamental region (near 4.67 km), revealing emission from water, carbon monoxide, methanol, methane, ethane, acetylene, and hydrogen cyanide. Many new multiplets from OH in the 1¨0 band were seen in prompt emission, and numerous new spectral lines were detected. Several spectral extracts are shown, and global production rates are presented for seven parent volatiles. Carbon monoxide is strongly depleted in comet Lee relative to comets Hyakutake and Hale-Bopp, demonstrating that chemical diversity occurred in the giant

Water Production Rates, Rotational Temperatures, and Spin Temperatures in Comets C/1999 H1 (Lee), C/1999 S4, and C/2001 A2

Water hot-band lines were detected in comets C/1999 H1 (Lee), C/1999 S4 (LINEAR), and C/2001 A2 (LINEAR) in the 2.9 � m spectral region using high-dispersion (k=� k � 2 ; 10 4 ) infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory. The density of H2O emissions in this spectral region, the spectral coverage and resolution of NIRSPEC, and fluorescence models developed for these hot bands enabled the determination of H2O production rates, rotational temperatures, and ortho-to-para ratios (OPRs) in these comets. Previous studies revealed clear diversity in the volatile organic chemistries of these comets, suggesting that they may have formed in different regions of the early solar nebula. The nuclear spin temperature of H2O as derived from its OPR is another possible indicator of cometary formation temperature and region. Nuclear spin temperatures for H2O were derived on one date in comet S4 and two dates in Lee and A2. Derived spin temperatures for H2O in these comets are � 30, 30 þ15 � 6 ,a nd 23 þ4 � 3 K for S4, Lee, and A2, respectively. Measurements are consistent with a possible link between nuclear spin temperatures and volatile abundances, but studies of more comets and continued improvements in water hot-band fluorescence models are needed to more stringently test this. Subject headingg comets: general — comets: individual (C/1999 S4, C/2001 A2, Lee (C/1999 H1)) — infrared: solar system — techniques: spectroscopic

Detection of CO and Ethane in Comet 21P/Giacobini-Zinner: Evidence for Variable Chemistry in the Outer Solar Nebula

Ethane and carbon monoxide were detected in a short-period comet of probable Kuiper Belt origin. Ethane is substantially less abundant compared with Hyakutake and Hale-Bopp, two comets from the giant-planet region of the solar nebula, suggesting a heliocentric gradient in ethane in precometary ices. It is argued that processing by X-rays from the young Sun may be responsible.