Infrared Observations of Oxidized Carbon in Comet C/2002 T7 (LINEAR)
Cometary nuclei are generally recognized as the most primitive remnants of the early Solar System. Their physical and chemical attributes allow a glimpse into the conditions under which icy bodies formed. Parent volatiles in comets are now routinely studied, and a significant diversity in composition among the comets sampled to date has been demonstrated. This forms the foundation of an emerging cometary taxonomy based on chemical composition. In spring 2004, comet C/2002 T7 (LINEAR) was observed using the facility echelle spectrometer (CSHELL) at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. CSHELL offers seeing-limited spatial resolution and sufficiently high spectral resolving power (R = λ /δλ ~ 2.5 x 104) to permit line-by-line intensities to be measured along its 30 arc-second slit. Its small pixels favor measurement of molecules released from ices housed in cometary nuclei ("native" ices) over those released from spatially extended sources in the coma. Emission lines from multiple molecular species were targeted in the 3 to 5 µm wavelength region. The observations revealed an extremely rich volatile chemistry in C/2002 T7 (LINEAR). I present the chemical composition of oxidized carbon in C/2002 T7 (LINEAR). Carbon monoxide (CO), formaldehyde (H2CO), and methyl alcohol (CH3OH) were detected simultaneously or nearly simultaneously with H2O on multiple UT dates spanning 2004 May 3-9 (heliocentric distance Rh = 0.66 - 0.71 AU) and May 30 - June 2 (Rh = 0.99 - 1.03 AU). I will discuss native production rates, rotational temperatures, and mixing ratios (abundances relative to H2O) for oxidized carbon. My results illustrate that C/2002 T7 (LINEAR) is enriched in CH3OH, while CO is borderline depleted compared to other Oort cloud comets that have been measured. I tested for chemical heterogeneity in C/2002 T7 (LINEAR), both diurnal, presumably associated with rotation of the nucleus, and serial (i.e., over a range in Rh). However, no evidence was seen for either short term or long term changes in the mixing ratios of CO, H2CO, or CH3OH, suggesting a homogeneous composition that did not vary with location on or depth in the nucleus. The gas production rates of C/2002 T7(LINEAR) did however reveal short term variability having a rotational period of 2.32 days. This periodicity is consistent with an elongated nucleus having uniform activity per unit surface area, or alternatively with a more nearly spherical nucleus having distinct active regions, or some combination of these. This represents the first measure of a rotational light curve in a comet based on multiple parent volatiles.
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