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

Abstract

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